Philippines' Best Destinations

BORACAY ISLANDS

Among all destinations in the Philippines, Boracay obviously is the most visited and the most popular. This butterfly-shaped island off the northwest coast of Panay have moved the island’s image from innocent to one of the most known islands in the world. A small island surrounded with white powdery sand, similar to that of icing sugar, uniquely made its way to worldwide recognition. Along with Boracay’s white-sand beaches, dramatic sunset showering the whole island with golden silhouettes, crystal clear water, and serene coves are as well bars, restaurants and hotels which tropical and international ideas blended together to satisfy the necessities of all tourists.

The kept promises of Boracay

Considering over a hundred side by side gatherings of hotels, resorts and cottages all over the island, resorts range from plush airconditioned rooms to simple thatched huts cooled only by electric fans. There are banks equipped with automated teller machines for instant cash needs, and credit cards are accepted in almost all restaurants and resorts.

A wide range of sports activities, nightlife and social affairs regularly happens from December to May. Regatta- a sailing competition, Bikini Open, a competition of sexy bodies, and other fun-gearing activities famed Boracay holiday an addictive tropical delight.

For water activities, Boracay’s western coast is ideal for windsurfing, wakeboarding, waterskiing and diving. Every establishment in the island has diving gears for rent. In Boracay, locals alternate source of living as tourist guides leaves you no worries for touring around. Bancas or pump-boats are visibly lined up along the stretching coast of most tourist-packed areas. Snorkeling and fishing to quiet coves at the other side of the island are their best offered activities. Other than that, discovering the interiors of the island by motorbike, horseback riding or simply walking barefoot around the sandy streets of the coast are daily routines most tourists pass their Boracay holidays. For little necessities, although small, and somewhat far from the industrialized metropolis, Boracay provides its visitors every little necessities within few meters away.

If your energy is still up for the night, Boracay is a trendy island filled with discos and loud bars ideal for dancing, drinking and to dropping your head at the beach. No one leaves Boracay without trying the famous shooters of cocomangas bar or dancing barefoot at open discos by the beach. These are the hypes not a single tourist failed to do when in Boracay. "So much to do for such limited time." Visitors of Boracay taste the fun and get addicted to it. They come back and explore more but most of the time they choose to stay...

BAGUIO - SUMMER CAPITAL OF THE PHILIPPINES

Originally, Baguio City was a settlement camp of American troops during their occupation in 1900s where they patterned the architecture of houses and buildings after their homes in the United States. And to easily access the neighboring towns, they have constructed Kennon Road; a highway known for its narrow and challenging turns. Apparently, this changed its peaceful face to a city of merchants and uncontrolled migrants from surrounding towns. When Americans left, more settlers invaded the thriving city and soon turned into a crowded district of mixed locals and vacationing tourists who come mainly for its temperate climate.

Wright Park

The city’s economy moved progressively despite the absence of the founding Americans. Parks, Gardens, Museums, markets and shopping malls were established everywhere and perhaps elevated the rapid rise of tourism. Big growth on hotels, lodging houses, restaurants and bars is increasing every year. Aside from farming, locals shifted to handicrafts to sustain the rising demand of market coming from tourism. Because of this Baguio famed itself as the best place to go for cheap wood carvings and fabric weavings. These shops are all over the city market and nearby town markets of Asin and Trinidad.

Fresh Flowers of Baguio

Baguio illustrates an ambiance of people wearing warm outfits all year around. If you only see Philippines as a tropical country, visiting Baguio somehow changes that view. Filipinos regarded this city amidst the towering peaks of the Cordillera, as the summer capital of the country, where they can move away from the irritating heat during summer.

Massive malls, congestion of population, and real state developments, nowadays, reduced Baguio’s appeal as the greenest mountain city of the north. For people who have known Baguio twenty years ago and seeing how congested it is now with unsightly views of houses instead of trees on top of the mountains can be terribly disappointing. For sure back then, less cars, less houses, less people, but with lots and lots of pine trees harmonized the simple and natural living of Baguio. Within Camp John Hay luckily pine-covered forests have been preserved but almost all hills nearby La Trinidad have been fully constructed. Although Baguio has moved this far, it is still the largest commercial district in the Cordillera that is chartering its neighboring towns an ample assurance of economic progression.













Burnham Park

TAGAYTAY

Because of its cooler climate and fresher environment, Tagaytay is considered the closest alternative to the temperate climate of Baguio. It is frequented by Visitors from Manila who are mainly reatreating from the polluted air of the city. The town’s focal attraction is the presence of this miniature-like volcano that is sitting in the middle of Taal lake. According to history, its bizarre location evolved from its huge eruptions that created a crater lake then another crater lake within that lake and another lake within that crater and it, as well has a small island. Widely known as the smallest volcano in the world, Taal Volcano is a scenic view that can be seen from the stretching slopes of Tagaytay highways elevated at 2500 ft. above sea level.


Taal Volcano

Trekking up to the volcano crater is a worthy experience. Many guided tours are oftenly operated by resorts nearby. Though if you want to go without them, the best access is from Talisay. Bancas are available for transfer up to the island where horses also are chartered for tourists who may hesitate to walk up to the crater. The uphill hike takes about1 hour. Bring enough water, wear comfortable shoes and expect strong dusty winds throught out the hike. The water can be rough in the afternoon, so it is best to depart in the morning. Be aware, however, of those people that may run after your cars offering boat rides or trekking guides. They are nuisance and may only cause you troubles and lose of time. It is always advised to go there with ample information of the place and activities to do.

For experienced mountaineers planning to head further deep into the volcano crater, be cautious of the hot surroundings and its sulfuric emissions from the steam vents. The hot water of the crater can cause severe skin irritation due to its high concentration of harmful chemicals. And last but most importantly, the volcano may look peaceful from a far but don’t forget that it can erupt anytime.


Taal Lake

Tagaytay is a peaceful getaway from the fast lanes of urban life. Its cool natural ambiance yields an abundant display of fruits and vegetables mostly seen along the highway or at the busy market of the town.


ALBAY and the looming threats of Mount Mayon

Albay Province is home to the majestic Mayon Volcano. Named after a beautiful maiden, "Magayon", which means beautiful, this scenic beauty is sighted to have the most perfect cone among volcanoes in the world. It has an exquisite appearance that disguises an startling behavior known only to residents of Albay. This volcano unpredictably behaves like a raging dragon, chasing the jumbling locals with flaming lava and layers of ashes out of their houses. Undecided whether to desert their hometown or to live with the dreadful presence of its shadow, they, however, choose to stay and consider the presence of Magayon a lingering phantom that is periodically haunting them with fearful visits.


Mayon Volcano

Despite these discouraging facts about Mt. Mayon, it is still regarded an ideal destination for an ultimate climb. Many aspire to reach its summit but because of its intimidating nature, Philvocs advise those who only come for sightseeing to stay at the view point located at the mid part of the volcano. There are skilled mountaineers who have already reached its crater but only a few encourages others to go. It is no doubt a thrilling experience but the climb is tough and may require strong endurance, courage, and knowledge on volcanic topography. Interested climbers should first consider their skills before going. Consulting PHILVOLCS for its precise volcanic activity before heading and taking an experienced guide is a must, or climb with someone who had been there. The climb and descent takes about 2 days.

For passers by, the ruins of a community buried by Mayon’s raging eruption in 1814 whereas boulders of lava submerged the whole town and ran over the Cagsawa church filled with townspeople is undoubtedly informative and somewhat nostalgic. It is located along the national highway; just few kilometers out of the city.

Other points of interest start from the city to the hot spring town of Tiwi where Mayon is at her photographic best. Try visiting ancient churches of albay, caves around Camalig and beaches within the area. On the way back to Legaspi City, handicrafts and local delicacy stores are found near the market, where bus terminals are also located. Transportation is well organized and very safe. However, nothing else is here unless you get out and look for a more natural getaway. Moving down south brings you to the lovely quiet town of Sorsogon.

BULUSAN

Mt. Bulusan may not appear as majectic as Mt. Mayon but mountaineers will experience a more challenging climb than struggling with the rocky terrains of Mayon. It has abundant rain forest that is maintaining the presence of some of the wildest animals in the country. Wildlife is rich in the area where nights can be filled with roaring sounds of owls and presence of deers and boars at daytime. At night, darkness is brightened by the sprinkling lights of fireflies. Thus climbing Mt. Bulusan does not only invigorates a mountaineer’s obsession for heights, it also promises surprising encounters with wild animals and peculiar vegetation.

Bulusan Volcano

Mt Bulusan is located in the inner district of Sorsogon where a jeepney takes you to its town and a trycicle to its entry site, Bulusan Lake- a lake oftenly mistaken as the volcano’s main crater. It is in fact a body of emerald green water by the peripheral crater surrounded by a densed gathering of trees and lush tropical plants. If not planning to hike farther to the volcano’s summit, a refreshing one hour walk around this lake is a worth-trying activity. And consider it as the beginning of a more interesting adventure if moving farther ahead. Expect challenging sights of thicker dense forest on the way that surprisingly emmerge to an open field of towering grass carpeting the surroundings towards the old crater valley called Agingay. This hike promises an extreme jungle experience but without the threat of distressing incidents.

Trekkers can camp overnight or leave the same day but caution should be maintained, this volcano is still active. Be sure to wear thick hiking boots, long pants and jackets to protect from unexpected presence of leeches and stingy insects. Taking at least one guide is highly advised.

On the way back, you will pass by tiny roads heading to some falls and hot springs with varying temperatures for a short healing bath.

DONSOL, home of Whale sharks


Whaleshark

This place at the southern tip of luzon recently made a huge break on the country’s falling tourism when hundreds of whale sharks were spotted congregating in the muddy but plankton-rich waters of Donsol. Whale shark spotting in sorsogon became a word of mouth passed on to every tourist exploring the Philippines. As thousands of visitors arrive each season to swim and witness their friendly behavior, the unknown town of Donsol, in which its economy was initially reliant to fishing and cottage industry alone, now benefits from higher revenue brought by the presence of these giant fish locally called "Butanding".

These gentle giants arrive from as early as November but the official season starts from February and runs until end of May, where the sea of Donsol is at its clearest and calmest. Local tourism officials and professional divers have established guidelines on proper whale shark watching intended to maintain safety and to defend the creatures from over exploitation.
The town of Donsol is 1 hr drive from Sorsogon and slightly similar if you would be coming from Legazpi City. These two cities are jumping-off points to Donsol: both have decent accomodations, and transporting facilities. Lodging in Donsol can be fussy and rental shops for snorkeling equipments are still scarse. It is recommended to bring your own gears and packed lunch if staying there for the day.

The calmer the water, the more whale sharks get close to the surface and there are more chances of interaction.


A whaleshark close encounter

Best time to go is in March or April when these creatures are most present. It is guaranteed to witness minimum of 8 to 10 whale sharks in two hours. So take your time, swim with them and enjoy more of their sociable behaviour. Only cameras without flashes are allowed to use.

Because of their mysterious congregation, the once "never heard" town of Donsol became a common word of mouth relayed to every tourist arriving in the country.

BATANES GROUP OF ISLANDS

Batanes is a windswept province gathering 14 islands 310 km. north of Luzon. Batang, Sabtang, Ibuhos and Ibayat are the main islands inhabited by not more than 20,000 people. These natives are called Ivatans, which refers to "a place where boats are cast ashore". Basco is the capital and the center of

Batanes is geographically volcanic, which is evidently shown in its rugged and rocky land formations. The wide and rough ocean between Philippines and Taiwan isolated the islands from the mainland of the country, providing its people with skills, unique and tough characters for survival. Because of the frequent visit of destructive typhoons, houses are built of stone and tiles designed primarily for blocking strong winds. Natives rely basically on farming, while fishing is a supplementary activity.

Mt. Iraya, a dormant volcano standing 1,517 meters above sea level and Mt. Mutarem, with a height of 495 meters are two major peaks considered as hiker’s paradise for mountaineering visitors. There are interesting routes that feature variety of terrains from flat to rough signifying levels of difficulty. Despite the peaceful environment Batanes is famous for, it is still advisable to take at least one local guide when heading for a hike. Sabtang has six minor peaks, all of which stand from 200 - 350 meters, running down the backbone of the island and best recommended for easy hikes.

The Batan shoreline has sheer limestone cliffs, great boulder beaches, white sand beaches and incredibly fresh environment enough to unwind one’s exhausted spirit. Aside from the above mentioned luxury, your entire visit will also have remarkable opportunities for photography, water sports, and serene encounter with nature.

SAGADA
Sagada is a town out of 10 municipalities, nestling in the middle of a valley at the upper end of malitep tributary of the Chico river , 1,500 meters above sea level. Its lofty little town, dirt-free air, and sights of towering pine trees, for every visitor, represents an ambiance of tranquility and peaceful life. There are no televisions, phone lines and aggressive merchants that are yet invading the quiet neighborhood of Sagada. At nine in the evening people are sent home to follow the curfew carried out by local authorities. Quite negative if done in the frantic metropolis but for Sagadans, this system helps them preserve their town’s organized life and to avoid strangers go beyond their traditional culture. When in Sagada, don’t be surprised by the locals’ distant behaviour. For them it is a simple way to avoid opportunists who have plans to exploit their land and peaceful living.

Because of its temperate weather, undisturbed environment and the presence of towering pine trees, foreign visitors simply describe Sagada as their home in the Philippines. For Filipinos from the lowlands, Sagada is beautiful but still appears an unfamiliar backdrop away from their usual tropical soil.

Sagada is not just a place for sightseeing. Things to do and activities are plentiful right from the center of the town. To start with, follow the sights proposed by the tourism office. You will be given instructions and a map indicating places of interest. These are subterrenean caves used as burial grounds by the natives, eminent limestone cliffs and nearby scenic falls. Some caves like Sumaging, the deepest and the biggest, requires strong endurance and enthusiasm.

Exploring Sumaging Cave
Sumaging Cave

Other stunning destinations are Bomod-ok and Bokong Falls, Danom Lake and Weaving Shops. Trekking the nearby mountains is an ultimate experience although must be done with local guides. There are simple trails for short hikes which are leading to some picturesque summits, and can be easily done in one day, or longer for those who may want to camp out. Those who came with motorbikes and mountainbikes, all trails inside Sagada are excellent courses for hi-adrenalin trips. Sagada may not have the luxury of hi-tech society but it has the abundance not even a well traveled man have ever experienced anywhere in the Philippines.

Sagada may be far from being invaded by lowland merchants, however, food and relevant needs are bountiful. Green products are good, fresh, juicy and green. Food is served generously and reasonably cheap in all restaurants. Try the homemade yoghurt covered with granola and a choice of fresh strawberries, bananas and mangoes at the Yoghurt House. It is fresh and creamy; good enough to supply a long energetic day. Before going at least prepare enough cash calculated according to your duration of stay. There are no banks or any financial institution that can help in case you run out of cash. For accomodation, inns and lodging houses though with simple facilities are plenty in Sagada- all situated together at the center of the town. Alfredo’s Inn, Ganduyan Rest House, the Green House, and St. Joseph Resthouse are among the most visited.

Sagada - St. Mary’s Church

PALAWAN

Palawan is popularly known as the “Last Frontier” pointing to Borneo and is also located just north of Malaysia’s Sabah island. It is the largest province in the Philippines with an area of 1.5 million hectares and has 1,769 islands and islets.

This huge island yet still unexplored, possesses the largest rainforest, unusual plants and animal species. These are

the Palawan Peacock, the sea cow or Dugong, the mouse deer, tabon birds, colorful butterflies and some of most beautiful orchids in the country. In short, the island has the biggest reservation of wildlife among other places in the country. Aside from these treasures, Palawan has exceptional landscapes consisting of mountainous ranges, unusual rock formations & hidden caves. It is as well an island known for its numerous fishing villages and dive sites.


Crystal-clear water of Palawan The main tourist destination is the group of Calamian Islands. It has the richness of white sand beaches, clear water and a wide range of marine resources perfect for scuba diving. Some places of the main island of Palawan are recommended for trekking particularly in the deep dense forests of the central district. But be aware, heading further south where Malaria is common should be avoided. Other attractions are St. Paul subterranean National Park that has underground river, Tabon Caves, where fossils of 50,000 years old ancient man were found and El Nido Village, the home of Nido Soup that is made out of bird’s nest. This is place known for its world-class resorts, powdery whitesand beaches, limestone hills, scattered islets and marine-rich dive sites.

The Calamian Group is a cluster of stunning islets dominated by three main islands in the north of Palawan: the islands of Busuanga, Coron and Culion. They all have abundant marine life, impressive rock formations and crystal clear tropical seas. Diving and kayaking are main activities mostly based in Coron.

Beaches of Palawan

Palawan is home to several tribes named Tagbanuas, Batac, and the cave dwellers Tau’t Bato. They inhabit mostly the interiors of Calamian Islands and some parts of the mainland. To avoid any exploitation of their cultural traits the government limited outsiders from exploring their occupied territories. Even without this law, some of these tribes as expected also restricted themselves to have close contact with anyone unfamiliar to them. Others such as Batac tribe have frequent contact with tourists working as guides. They are more sociable and easily approached.

Limestone Cliffs of El Nido

El Nido

MOALBOAL IN CEBU

Located at the west coast of Cebu, nestles a humble little town of Moalboal, and three kilometers from the town center, is the sleepy coastal village of Panagsama beach, homing to a small community of unexploited locals and friendly foreign divers. Moalboal is known for diving yet accomodation and food is affordable.

Moalboal’s exciting dive sites made it an extraordinary diving destination in the Visayas. Apart from its unique location and its lovely people, visiting divers usually get magnetized by the staggering life form decking out the inclined wall right by the shore of Panagsama beach. The dive starts at the beach, heading straight down to a recepting beauty of stunning hard and soft corals, anemones and sponges hosting thousands of inhabiting tropical fishes.

Another point of interest is Pescador Island Marine park. It is a shallow reef that shelters lively dancing gorgonians, fan corals and sharks such as whitetip and hammerheads. These are stunning sights that famed Moalboal to divers from around the world.

BOHOL

Located in Central Visayas, Bohol is an island famous for its well preserved natural environment. The island’s reigning pride are the world-famous Chocolate Hills, its little islands surrounded by white sand beaches, and Tarsier- the world tiniest primate. Considering its features Tarsier has been calculated a confusion of normal primates’ evolution. Characterized by its bizarre looks, this timid creature with big bright round eyes unfortunately faces the threat of extinction. Their habitat are destroyed and have these animals sold to mostly unwitting foreigners for souvenirs. Other distinct animals that also are suffering from this local ignorance are the lovely flying lemurs.

Apart from strips of white sand beaches, chocolate hills and Tarsier, touring the interiors of Bohol will give you sights of historic edifices built with strong European influence, lavishly designed with
carvings and painted ceilings. With a few efforts to exert, these magnifique sights of Bohol are enough to comprise a worthwhile touring vacation.It has an area of 3,864 sq/km with mainly coastal settlement except the town of Carmen, which is surrounded by a low central plateau. Its volcanic core is mostly covered by coralline limestone. The rivers are short and there are few good anchorages. The spectacular Chocolate Hills or Haycock Hills are named for their brown appearance during dry season. They are limestone remnants of an earlier erosion cycle, a phenomenon known to be present in only two or three other places in the world.

Motor biking is the easiest and the most exciting way to see these classic landmarks. If you are staying in Panglao Island, resorts’ staff are best source of information regarding bike rentals. When you reach Loboc town, don’t miss to pass by "nuts huts", a hidden restaurant resort up in the mountains overlooking Loboc River and Busay Falls. The ride can be tough and tricky but as soon as you get there, you will see what its worth. Then follow the road to Chocolate Hills. This trip gives you sights of sleepy towns, rolling hills, green fields, rivers and dark roads amidst thick forest.

White-sand beaches and nearby reefs inhabited by thousands of tropical fishes obviously account for the island’s invincible fame for water activities. The richness of Bohol’s marine life sited nearby Panglao Island is world-known destination both for beginners and advance divers. Even for swimming in its prestine water or simply walking on its sugar-like sand bars are just enough reasons to visit Bohol. Among these known destinations is Panglao Island, which has nice resorts, lively bars and delightful restaurants scattered along its wide strips of whitesand beaches. See Points Of Interest for details on adventure activities in Bohol.











Panglao Island

CAMIGUIN

Camiguin is a small island lying in the north-west coast of Mindanao. It is a volcanic island hosting seven volcanoes of which one of them remains unpredictably active. Mt Hibok-hibok submerged Camiguin’s old capital, Bombon, in 1871 and suffocated almost 2000 inhabitants from its raging eruption in 1951. Since then, the volcano rested quietly giving its inhabitants an assurance of normal living. And due to its volcanic debris from previous eruptions, most of Camiguin’s beaches are covered with pebbles and dark sand.

Katibawasan Falls, Camiguin

It’s capital, Mambajao, has a friendly small community of mostly Visayan and Chinese origin. Their source of livelihood comes from the produce of their land and the surrounding ocean. Every October, Lanzones Festival transform the island into an explicit sight of towsfolk dressed up in their traditional costumes to celebrate the harvest of this small, sweet tasting, light-yellow fruit that grows in bunches at the foot of Mt. Hibok-hibok. The festival is remarkably good-natured, filled with colors and properly organized.




Camiguin Lanzones Fruit

Aside from these natural landforms huddling in the island, Camiguin is an area of forested mountains mixed with coconut trees along its coasts. Some stunning waterfalls are also good sights that should not be missed when touring the island.

The best way to enjoy the sights of Camiguin is by motor biking along the 64 km coastal road. Piercing into the dense forests of the island, plunging into the waters of natural pool down of Katibawasan falls and enjoying the warm waters of Ardent hot spring are worthwhile activities. Climbing the famous Mt. Hibok-hibok is tolerated but somehow requires strong endurance and trekking skills. It takes about 6 hours for skilled climbers to reach the summit and it is always advisable to take at least one guide for a hassle-free climb.












View of Mt. Hibok Hibok, Camiguin from White Island

Most Puzzling Ancient Artifacts

World's wonders abound our daily lives..here are some amazing artifacts!

The Bible tells us that God created Adam and Eve just a few thousand years ago, by some fundamentalist interpretations. Science informs us that this is mere fiction and that man is a few million years old, and that civilization just tens of thousands of years old. Could it be, however, that conventional science is just as mistaken as the Bible stories? There is a great deal of archeological evidence that the history of life on earth might be far different than what current geological and anthropological texts tell us. Consider these astonishing finds:

The Grooved Spheres
Over the last few decades, miners in South Africa have been digging up mysterious metal spheres. Origin unknown, these spheres measure approximately an inch or so in diameter, and some are etched with three parallel grooves running around the equator. Two types of spheres have been found: one is composed of a solid bluish metal with flecks of white; the other is hollowed out and filled with a spongy white substance. The kicker is that the rock in which they where found is Precambrian - and dated to 2.8 billion years old! Who made them and for what purpose is unknown.

The Dropa Stones
In 1938, an archeological expedition led by Dr. Chi Pu Tei into the Baian-Kara-Ula mountains of China made an astonishing discovery in some caves that had apparently been occupied by some ancient culture. Buried in the dust of ages on the cave floor were hundreds of stone disks. Measuring about nine inches in diameter, each had a circle cut into the center and was etched with a spiral groove, making it look for all the world like some ancient phonograph record some 10,000 to 12,000 years old. The spiral groove, it turns out, is actually composed of tiny hieroglyphics that tell the incredible story of spaceships from some distant world that crash-landed in the mountains. The ships were piloted by people who called themselves the Dropa, and the remains of whose descendents, possibly, were found in the cave.

The Ica Stones
Beginning in the 1930s, the father of Dr. Javier Cabrera, Cultural Anthropologist for Ica, Peru, discovered many hundreds of ceremonial burial stones in the tombs of the ancient Incas. Dr. Cabrera, carrying on his father's work, has collected more than 1,100 of these andesite stones, which are estimated to be between 500 and 1,500 years old and have become known collectively as the Ica Stones. The stones bear etchings, many of which are sexually graphic (which was common to the culture), some picture idols and others depict such practices as open-heart surgery and brain transplants. The most astonishing etchings, however, clearly represent dinosaurs - brontosaurs, triceratops (see photo), stegosaurus and pterosaurs. While skeptics consider the Ica Stones a hoax, their authenticity has neither been proved or disproved.

The Antikythera Mechanism
A perplexing artifact was recovered by sponge-divers from a shipwreck in 1900 off the coast of Antikythera, a small island that lies northwest of Crete. The divers brought up from the wreck a great many marble and and bronze statues that had apparently been the ship's cargo. Among the findings was a hunk of corroded bronze that contained some kind of mechanism composed of many gears and wheels. Writing on the case indicated that it was made in 80 B.C., and many experts at first thought it was an astrolabe, an astronomer's tool. An x-ray of the mechanism, however, revealed it to be far more complex, containing a sophisticated system of differential gears. Gearing of this complexity was not known to exist until 1575! It is still unknown who constructed this amazing instrument 2,000 years ago or how the technology was lost.

The Baghdad Battery
Today batteries can be found in any grocery, drug, convenience and department store you come across. Well, here's a battery that's 2,000 years old! Known as the Baghdad Battery, this curiosity was found in the ruins of a Parthian village believed to date back to between 248 B.C. and 226 A.D. The device consists of a 5-1/2-inch high clay vessel inside of which was a copper cylinder held in place by asphalt, and inside of that was an oxidized iron rod. Experts who examined it concluded that the device needed only to be filled with an acid or alkaline liquid to produce an electric charge. It is believed that this ancient battery might have been used for electroplating objects with gold. If so, how was this technology lost... and the battery not rediscovered for another 1,800 years?

The Coso Artifact
While mineral hunting in the mountains of California near Olancha during the winter of 1961, Wallace Lane, Virginia Maxey and Mike Mikesell found a rock, among many others, that they thought was a geode - a good addition for their gem shop. Upon cutting it open, however, Mikesell found an object inside that seemed to be made of white porcelain. In the center was a shaft of shiny metal. Experts estimated that it should have taken about 500,000 years for this fossil-encrusted nodule to form, yet the object inside was obviously of sophisticated human manufacture. Further investigation revealed that the porcelain was surround by a hexagonal casing, and an x-ray revealed a tiny spring at one end. Some who have examined the evidence say it looks very much like a modern-day spark plug. How did it get inside a 500,000-year-old rock?

Ancient Model Aircraft
There are artifacts belonging to ancient Egyptian and Central American cultures that look amazingly like modern-day aircraft. The Egyptian artifact, found in a tomb at Saqquara, Egypt in 1898, is a six-inch wooden object that strongly resembles a model airplane, with fuselage, wings and tail. Experts believe the object is so aerodynamic that it is actually able to glide. The small object discovered in Central America (shown at right), and estimated to be 1,000 years old, is made of gold and could easily be mistaken for a model of a delta-wing aircraft - or even the Space Shuttle. It even features what looks like a pilot's seat.

Giant Stone Balls of Costa Rica
Workmen hacking and burning their way through the dense jungle of Costa Rica to clear an area for banana plantations in the 1930s stumbled upon some incredible objects: dozens of stone balls, many of which were perfectly spherical. They varied in size from as small as a tennis ball to an astonishing 8 feet in diameter and weighing 16 tons! Although the great stone balls are clearly man-made, it is unknown who made them, for what purpose and, most puzzling, how they achieved such spherical precision.

Impossible Fossils
Fossils, as we learned in grade school, appear in rocks that were formed many thousands of years ago. Yet there are a number of fossils that just don't make geological or historical sense. A fossil of a human handprint, for example, was found in limestone estimated to be 110 million years old. What appears to be a fossilized human finger found in the Canadian Arctic also dates back 100 to 110 million years ago. And what appears to be the fossil of a human footprint, possibly wearing a sandal, was found near Delta, Utah in a shale deposit estimated to be 300 million to 600 million years old.

Out-of-Place Metal Objects
Humans were not even around 65 million years ago, never mind people who could work metal. So then how does science explain semi-ovoid metallic tubes dug out of 65-million-year-old Cretaceous chalk in France? In 1885, a block of coal was broken open to find a metal cube obviously worked by intelligent hands. In 1912, employees at an electric plant broke apart a large chunk of coal out of which fell an iron pot! A nail was found embedded in a sandstone block from the Mesozoic Era. And there are many, many more such anomalies.

What are we to make of these finds? There are several possibilities:

• Intelligent humans date back much, much further than we realize.
• Other intelligent beings and civilizations existed on earth far beyond our recorded history.
• Our dating methods are completely inaccurate, and that stone, coal and fossils form much more rapidly than we now estimate.

In any case, these examples - and there are many more - should prompt any curious and open-minded scientist to reexamine and rethink the true history of life on earth.

CHINESE ZODIAC SIGNS

...what does await you on this year of the Ox?..May the insights on your Chinese Zodiac sign guide you along your way in fulfilling your goals!!! Good luck!!!

	Rat			Ox		Tiger
01/31/1900 - 02/18/1901 02/18/1912 - 02/05/1913 02/05/1924 - 01/24/1925 01/24/1936 - 02/10/1937 02/10/1948 - 01/28/1949 01/28/1960 - 02/14/1961 02/15/1972 - 02/02/1973 02/02/1984 - 02/19/1985 02/19/1996 - 02/06/1997		02/19/1901 - 02/07/1902 02/06/1913 - 01/25/1914 01/25/1925 - 02/12/1926 02/11/1937 - 01/30/1938 01/29/1949 - 02/16/1950 02/15/1961 - 02/04/1962 02/03/1973 - 01/22/1974 02/20/1985 - 02/08/1986 02/07/1997 - 01/27/1998			02/08/1902 - 01/28/1903 01/26/1914 - 02/13/1915 02/13/1926 - 02/01/1927 01/31/1938 - 02/18/1939 02/17/1950 - 02/05/1951 02/05/1962 - 01/24/1963 01/23/1974 - 02/10/1975 02/09/1986 - 01/28/1987 01/28/1998 - 02/15/1999
	Rabbit			Dragon		Snake
01/29/1903 - 02/15/1904 02/14/1915 - 02/02/1916 02/02/1927 - 01/22/1928 02/19/1939 - 02/07/1940 02/06/1951 - 01/26/1952 01/25/1963 - 02/12/1964 02/11/1975 - 01/30/1976 01/29/1987 - 02/16/1988 02/16/1999 - 02/04/2000		02/16/1904 - 02/03/1905 02/03/1916 - 01/22/1917 01/23/1928 - 02/09/1929 02/08/1940 - 01/26/1941 01/27/1952 - 02/13/1953 02/13/1964 - 02/01/1965 01/31/1976 - 02/17/1977 02/17/1988 - 02/05/1989 02/05/2000 - 01/23/2001			02/04/1905 - 01/24/1906 01/23/1917 - 02/10/1918 02/10/1929 - 01/29/1930 01/27/1941 - 02/14/1942 02/14/1953 - 02/02/1954 02/02/1965 - 01/20/1966 02/18/1977 - 02/06/1978 02/06/1989 - 01/26/1990 01/24/2001 - 02/11/2002
	Horse			Sheep		Monkey
01/25/1906 - 02/12/1907 02/11/1918 - 01/31/1919 01/30/1930 - 02/16/1931 02/15/1942 - 02/04/1943 02/03/1954 - 01/23/1955 01/21/1966 - 02/08/1967 02/07/1978 - 01/27/1979 01/27/1990 - 02/14/1991 02/12/2002 - 01/31/2003		02/13/1907 - 02/01/1908 02/01/1919 - 02/19/1920 02/17/1931 - 02/05/1932 02/05/1943 - 01/24/1944 01/24/1955 - 02/11/1956 02/09/1967 - 01/29/1968 01/28/1979 - 02/15/1980 02/15/1991 - 02/03/1992 02/01/2003 - 01/21/2004			02/02/1908 - 01/21/1909 02/20/1920 - 02/07/1921 02/06/1932 - 01/25/1933 01/25/1944 - 02/12/1945 02/12/1956 - 01/30/1957 01/30/1968 - 02/16/1969 02/16/1980 - 02/04/1981 02/04/1992 - 01/22/1993 01/22/2004 - 02/08/2006
	Rooster			Dog		Boar
01/22/1909 - 02/09/1910 02/08/1921 - 01/27/1922 01/26/1933 - 02/13/1934 02/13/1945 - 02/01/1946 01/31/1957 - 02/17/1958 02/17/1969 - 02/05/1970 02/05/1981 - 01/24/1982 01/23/1993 - 02/09/1994		02/10/1910 - 01/29/1911 01/28/1922 - 02/15/1923 02/14/1934 - 02/03/1935 02/02/1946 - 01/21/1947 02/18/1958 - 02/07/1959 02/06/1970 - 01/26/1971 01/25/1982 - 02/12/1983 02/10/1994 - 01/30/1995			01/30/1911 - 02/17/1912 02/16/1923 - 02/04/1924 02/04/1935 - 01/23/1936 01/22/1947 - 02/09/1948 02/08/1959 - 01/27/1960 01/27/1971 - 02/14/1972 02/13/1983 - 02/01/1984 01/31/1995 - 02/18/1996

Zodiac Animal Signs and Dates of Birth

ORIGIN OF CHINESE ZODIAC

In ancient times, our ancestors counted the years with 10 celestial stems and 12 terrestrial branches. Although this was scientific, most people were illiterate and could not memorize or calculate easily. Thus the animals that influenced people's lives were chosen to symbolize the terrestrial branches: the rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog and pig.

Legend has it that one day the gods ordered that animals be designated as the signs of each year with the twelve who arrived first selected. At that time, the cat and mouse were good friends and neighbors. When they heard of this news, the cat said to mouse: 'We should arrive early to sign up, but I usually get up late.' The mouse then promised to awaken his friend and to go together. However, on the morning when he got up, he was too excited to recall his promise, and went directly to the gathering place. On the way, the mouse encountered the tiger, ox, horse, and other animals that ran much faster. In order not to fall behind them, he thought up a good idea. He made the straightforward ox carry him on condition that he sang for the ox. The ox and mouse arrived first. The ox was happy thinking that he would be the first sign of the years, but the mouse had already slid in front, and became the first lucky animal of the Chinese zodiac. Meanwhile his neighbor the cat was too late; when it finally arrived, the selections were over. That's why other animals appear behind the little mouse and why the cat hates mice so much that every time they meet, the cat will chase and kill the mouse.

It is also said that this order of the zodiac originated in the Han Dynasty (206 - 220) according to the 12 time periods of a day:

The first period is from 11 p.m. to 1 a.m., the time rats actively seek food;
The second one is from 1 to 3 a.m., a time that oxen regurgitate;
From 3 to 5 a.m. tigers hunt prey and display their fiercest nature;
From 5 to 7 a.m., based on tales, the jade rabbit on the moon was busy pounding medicinal herb with a pestle;
From 7 to 9 a.m. dragons were said to hover in the sky to give people rainfall;
From 9 to 11 a.m. snakes start to leave their burrows;
From 11 a.m. to 1 p.m. the day is flourishing, as vigorous as an unconstrained horse;
From 1 to 3 p.m. it was said that if sheep ate grass at this time, they would grow stronger;
From 3 to 5 p.m. monkeys become lively;
From 5 to 7 p.m. roosters return to their roost as it is dark;
From 7 to 9 p.m. dogs begin to carry out their duty to guard entrances;
From 9 to 11 p.m. all is quiet and pigs are sleeping soundly.

For a long time there has been a special relationship between humans and the 12 zodiacal animals. Humans admired them, took them as totems, and in the artistic field, those animal signs were among the features of those themes. This can be reflected in artisans' paper-cut works, New-Year pictures, pottery and bronze wares, especially on the bronze mirrors before the appearance of glass ones, on which elaborate forms showed creativity and passion as well as wishes for a good life.

CHARACTER OF PERSONS UNDER EACH SIGN

Year of the Rat –1912, 1924, 1936, 1948, 1960, 1972, 1984, 1996, 2008, 2020, 2032, 2044

Though in people's eyes, the rat is not adorable, and even some Chinese sayings that related to it have almost derogatory meanings, it ranged as the head of the Chinese zodiac. It was recognized as an animal with spirit, wit, alertness, delicacy, flexibility and vitality.

People under this rat sign are usually smart and willing to accumulate wealth and to take efforts to be successful. Throughout their life, there will be many other people who can bring great fortune to them. Thus despite timidity, most of them are happy and harmonious with others.

People under the rat sign are usually smart and willing to accumulate wealth and to make efforts to be successful. Throughout their lives, there will be many other people who can bring great fortune to them. Thus despite timidity, most of them are happy and harmonious with others.

Best match: dragon, monkey, ox; Avoid: sheep, horse, rabbit, rooster

Year of the Ox - 1913, 1925, 1937, 1949, 1961, 1973, 1985, 1997, 2009, 2021, 2033, 2045

Ranked as the second zodiac, the ox has a large volume. In the life, people often used the ox to indicate something big or large in number. The ox is sedulous, simple, honest, and straightforward. Fractious people are said to have an 'oxen temper'.

People born in this year are probably honest, laborious, patient, obstinate, and poor at communication. Leaders in their career may not discover their abilities. In their old age, they would be bestowed with happiness. Women are usually good wives who pay attention to children's education, but are likely to believe others' cajolery, so should be cautious.

Best match: rat, snake, rooster; Avoid: dragon, horse, sheep, dog, rabbit

Year of the Tiger - 1914, 1926, 1938, 1950, 1962, 1974, 1986, 1998, 2010, 2022, 2034, 2046

Tigers, considered to be brave, cruel, forceful and terrifying, are the symbol of power and lordliness. In ancient times, people usually compared emperors or grandees with the tiger. Court officials often said that 'accompanying the emperor is just like being at the side of a tiger'. There are also many legends on hunting tigers dealing with struggling against evil might.

People born in the year of the tiger are tolerant, staunch, valiant, and respected. In their middle age, their fate may be uneven, but afterwards will enjoy a bright prospect. Their shortcoming is to project themselves before others. But most women under the tiger sign are intelligent, faithful and virtuous.

Best match: horse, dog; avoid: snake, monkey

Year of the Rabbit - 1915, 1927, 1939, 1951, 1963, 1975, 1987, 1999, 2011, 2023, 2036, 2047

The rabbit has represented hope of the Chinese people for a long time. It is tender and lovely. The moon goddess Chang'e in Chinese legend had a rabbit as her pet, which stimulated the thought that only the rabbit was amiable enough to match her noble beauty. The Chinese character 'Tu' (rabbit) is part of 'Yi' (escape or leisure) indicating speed and distance. The Han people have a custom that a pregnant woman is not allowed to eat rabbit meat for fear that the child will be born with a harelip. The newborn is given paintings of children and rabbits representing that the child will have a peaceful and happy life.

People born under the sign of the rabbit are gentle, sensitive, modest, and merciful and have strong memory. They like to communicate with others in a humorous manner. They cannot bear dull life, so they are good at creating romantic or interesting spice. But they lack meditative abilities and often sink money into ideas that may cause failures in their career.

Best match: sheep, dog, pig; avoid: rat, ox, dragon, rooster, horse

Year of the Dragon - 1916, 1928, 1940, 1952, 1964, 1976, 1988, 2000, 2012, 2024, 2036, 2048

The dragon enjoys a very high reputation in Chinese culture. It is the token of authority, dignity, honor, success, luck, and capacity. In ancient China, a dragon was thought to speed across the sky with divine power. Emperors entitled themselves exclusively as 'dragon'; their thrones were called 'dragon thrones', their clothes 'dragon gowns'.

People under the sign of the dragon are lively, energetic and fortunate. They often can be leaders and try to go for perfection. When they meet with difficulties, they are not discouraged. But they are a little arrogant, and impatient, and women are over- confident. If they overcome these defects, they can have a brighter future.

Best match: rat, monkey, rooster; avoid: dog, ox, dragon, rabbit

Year of the Snake - 1917, 1929, 1941, 1953, 1965, 1977, 1989, 2001, 2013, 2015, 2037, 2049

In the Chinese zodiac, snake is listed after the dragon, but its place and the significance as the symbol of worship is far behind the dragon's. The snake carries the meanings of malevolence, cattiness and mystery, as well as acumen, divination and the ability to distinguish herbs. In some places, people believe that a snake found in their court can bring delight. In Spring Festival, people like to paste onto their doors and windows the paper-cut 'Fu' character (happiness), combined with the snake twisting around a rabbit as a popular pattern indicating wealth.

People born in the year of the snake often have a good temper, a skill of communicating, and a gracious morality, but they are likely to be jealous and suspicious. They should be cautious about discussion with others, as that might cause them to lose friendship and opportunities. Women under the sign of the snake do well in housework but are irritable. They might gradually enjoy happiness in their old age.

Best match: ox, rooster; avoid: tiger, monkey, pig

Year of Horse - 1918, 1930, 1942, 1954,1966, 1978, 1990, 2002, 2014, 2026, 2038, 2050

The spirit of the horse is recognized to be the Chinese people's ethos – making unremitting efforts to improve themselves. It is energetic, bright, warm-hearted, intelligent and able. Ancient people liked to designate an able person as 'Qianli Ma' (a horse that covers a thousand li a day).

People born in the year of the horse have ingenious communicating techniques and in their community they always want to be in the limelight. They are active, clever, kind to others, and like to join in a venture career. They cannot bear too much constraint. However they are interested in only the superficial level of an object, neglecting the essence. Once they suffer from failure, they become pessimistic.

Best match: tiger, sheep, dog; Avoid: rat, ox, rabbit, horse

Year of the Sheep - 1919, 1931, 1943, 1955, 1967, 1979, 1991, 2003, 2015, 2027, 2039, 2051

The sheep is among the animals that people like most. It is gentle and calm. Since ancient times, people have learned to use its fleece to make writing brushes and skin to keep warm. As it is white, people describe delicate and precious white jade to be 'suet jade'. Thus it is close to the meaning of good things.

People under the sign of the sheep are tender, polite, filial, clever, and kind-hearted. They have special sensitivity to art and beauty, faith in a certain religion and a special fondness for quiet living. They cope with business cautiously and circumspectly. In their daily life, they try to be economical. Women born in this year are willing to take good care of others, but they should avoid pessimism and hesitation.

Best match: rabbit, horse, pig; avoid: rat, ox, dog

Year of the Monkey - 1920, 1932, 1944, 1956, 1968, 1980, 1992, 2004, 2016, 2028, 2040, 2052

The monkey is a clever animal. People used to compare it to a smart person. During the Spring and Autumn Period (770 BC – 476 BC), the dignified official title, marquis with the pronunciation 'Hou', was the same as the monkey's. The monkey was thereby bestowed with auspicious meaning. It appeared on pictures pasted on the walls and doors to predict good fortune in officialdom.

Most people born in the Year of the Monkey are lively, flexible, and versatile. They love moving and sports. To help others they put their own business aside. When communicating, they do not like to be controlled and have a strong desire to present themselves. In their work, they will show amazing creativity. If they are not impatient and mouthy, they can gain more achievement.

Best match: rat, dragon; avoid: tiger, snake, pig

Year of the Rooster - 1921, 1933, 1945, 1957, 1969, 1981, 1993, 2006, 2017, 2029, 2041, 2053

The rooster is almost the epitome of fidelity and punctuality. For ancestors who had no alarm clocks, the rooster's crowing was significant, as it could awaken people to get up and start to work. Another symbolic meaning the rooster carries is exorcising evil spirits. People used to worship ancestors and believed in fortune telling for a long time.

Roosters are considered to be honest, bright, communicative, ambitious and warm-hearted. They might be enthusiastic about something quickly, but soon might be impassive. They have strong self-respect and seldom rely on others. As most roosters are born pretty or handsome, they might have several loves in their lives, treating each lover seriously. If they can overcome their arrogance, they will make more progress.

Best match: ox, dragon, snake; avoid: rooster, dog, rabbit

Year of the Dog - 1922, 1934, 1946, 1958, 1970, 1982, 1994, 2006, 2018, 2030, 2042, 2054

The dog is the human's friend who can understand the human's spirit and obey its master, whether he is wealthy or not. The Chinese regard the dog as an auspicious animal. If a dog happens to come to a house, usually the host will be very glad to adopt it, for it symbolizes the coming of fortune. Poodles, especially black poodles, bring more luck. The masters of dogs are not only humans, but the immortal in legends as well. The invincible God Erlang used a wolf-dog mix to help him capture monsters.

One born in the Year of Dog has a straightforward character. In their career and love, they are faithful, courageous, dexterous, smart and warm-hearted. Most women under this sign are appealing but lack stability. Correct this defect and good fortune will come to them.

Best match: tiger, rabbit, horse; avoid: ox, dragon, sheep, rooster

Year of the Pig - 1923, 1935, 1947, 1959, 1971, 1983, 1995, 2007, 2019, 2020, 2031, 2043

A pig is not as smart as a dog in understanding human thought. It likes sleeping and eating and becomes fat. Thus it usually features laziness and clumsiness. On the positive side, it behaves itself, has no calculation to harm others, and can bring affluence to people. Consequently, pigs were once regarded as wealth.

People born in the Year of the Pig are honest and frank. They have a calm appearance and strong heart, but they lack patience and independence. As they do not like to talk in a roundabout way, they are thought to be unsociable. Luckily, they are tolerant and optimistic, so not until they become your friends can their virtue, advantages and fidelity to friendship be found.

Best match: sheep, rabbit; avoid: snake, pig, monkey

YEAR OF BIRTH (BENMING NIAN)

The distinctive zodiacal way of calculating years decides that every 12 years is a circulation. People will meet the year of their birth (Benming Nian) considered in relation to the 12 Terrestrial Branches which starts with the Chinese lunar New Year.

It is said that in one's year of birth, he will offend 'Taisui', a mysterious power or celestial body that could control people's fortune. That is, he will meet either exultation or misery during that year. The best way to avoid miserable events is to wear clothes, a waistband, or decorations such as a red bracelet and necklace as a talisman that must be purchased by others. If the person himself buys them, the function is greatly lessened.

This may be from Chinese people's special affection for red since the Han Dynasty (206 BC - 220). In the Han people's wedding ceremony, the bride was picked up by her bridegroom's red sedan, the new couple wore red clothes, and their rooms were decorated with red candles, red carpet and red lanterns. When an army won a battle, victory would be reported with a red flag; and when a candidate passed the imperial examination, he would wear red flowered clothes. Thus redness has become the token of festivities, success, bravery, rightness, and exorcism.

Currently there are new viewpoints about one's birth year and offending 'Taisui': there are just more and greater changes in the birth year than in other years, which are not always ominous. If he makes an effort, holds his belief firmly, and does not harm others, he will change his fate to predominance, so there is no need to be anxious.

Light - a source of life!

The Light that completes and nourishes us...where do they come from and how are they formed? Here's article I have reserched to share with you...

Sunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. This is usually during the hours known as day. Near the poles in summer, sunlight also occurs during the hours known as night and in the winter at the poles sunlight may not occur at any time. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and heat. Radiant heat directly produced by the radiation of the sun is different from the increase in atmospheric temperature due to the radiative heating of the atmosphere by the sun's radiation. Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer. The World Meteorological Organization defines sunshine as direct irradiance from the Sun measured on the ground of at least 120 W·m⁻².^[1]

Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultra-violet light. Bright sunlight provides luminance of approximately 100,000 candela per square meter at the Earth's surface.

Sunlight is a key factor in the process of photosynthesis.

Calculation

To calculate the amount of sunlight reaching the ground, both the elliptical orbit of the earth and the earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (E_ext), corrected for the elliptical orbit by using the day number of the year, known as the Julian date (Jd), is: E_ext=E_sc(1 + 0.034 * cos(2pi(Jd − 2) / 365))

The solar illuminance constant (E_sc), is equal to 128 Klux. The direct normal illuminance, (E_dn), corrected for the attenuating effects of the atmosphere is given by: E_dn=E_ext*e^-cm

Where c is the atmospheric extinction coefficient and m is the relative optical air mass.

Solar constant

A 1903 Langley bolograph with an erroneous solar constant of 2.54 calories/minute/square centimeter.

Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber.

The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter (W/m²),^[2] though this fluctuates by about 6.9% during a year (from 1412 W/m² in early January to 1321 W/m² in early July) due to the earth's varying distance from the Sun, and typically by much less than one part per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×10¹⁷ W, plus or minus 3.5%. The solar constant does not remain constant over long periods of time (see Solar variation). The approximate average value cited,^[2] 1366 W/m², is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute.

The Earth receives a total amount of radiation determined by its cross section (π·R_E²), but as it rotates this energy is distributed across the entire surface area (4·π·R_E²). Hence the average incoming solar radiation (sometimes called the solar irradiance), taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 342 W/m²). At any given moment, the amount of Solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.

The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is linked to the apparent magnitude of the Sun, −26.8, in that the solar constant and the magnitude of the Sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun.

In 1884, Samuel Pierpont Langley attempted to estimate the Solar constant from Mount Whitney in California. By taking readings at different times of day, he attempted to remove effects due to atmospheric absorption. However, the value he obtained, 2903 W/m², was still too great. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m². Abbott proved that one of Langley's corrections was erroneously applied. His results varied between 1.89 and 2.22 calories (1318 to 1548 W/m²), a variation that appeared to be due to the Sun and not the Earth's atmosphere.^[3]

The angular diameter of the Earth as seen from the Sun is approximately 1/11,000 radians, meaning the solid angle of the Earth as seen from the sun is approximately 1/140,000,000 steradians. Thus the Sun emits about two billion times the amount of radiation that is caught by Earth, in other words about 3.86×10²⁶ watts.^[4]

Sunlight intensity in the Solar System

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from Sun. A rough table comparing the amount of light received by each planet on the Solar System (and the dwarf planets Ceres and Pluto) follows (from data in [1]):

Planet	Perihelion - Aphelion distance (AU)	Solar radiation maximum and minimum (W/m²)
Mercury	0.3075 - 0.4667	14446 - 6272
Venus	0.7184 - 0.7282	2647 - 2576
Earth	0.9833 - 1.017	1413 - 1321
Mars	1.382 - 1.666	715 - 492
Jupiter	4.950 - 5.458	55.8 - 45.9
Saturn	9.048 - 10.12	16.7 - 13.4
Uranus	18.38 - 20.08	4.04 - 3.39
Neptune	29.77 - 30.44	1.54 - 1.47

The actual brightness of sunlight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example Venus' thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 5000-10000 lux, comparable to that of Earth during a dark, very cloudy day.

Sunlight on Mars would be more or less like daylight on Earth wearing sunglasses, and as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus it would give perceptions and "feel" very much like Earth daylight.

For comparison purposes, sunlight on Saturn is somewhat slightly brighter than Earth sunlight on the average sunset or sunrise. Even on Pluto the Sun would be still bright enough to almost match the average living room. To see the Sun shine as dim as the full Moon on the Earth, a distance of about 500 AU (~69 light-hours) is needed: there is only a handful of objects in the solar system known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

Composition

Solar irradiance spectrum above atmosphere and at surface

The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K. About half that lies in the visible short-wave part of the electromagnetic spectrum and the other half mostly in the near-infrared part. Some also lies in the ultraviolet part of the spectrum.^[5] When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in human skin pigmentation.

The spectrum of electromagnetic radiation striking the Earth's atmosphere is 100 to 10⁶ nanometer (nm). This can be divided into five regions in increasing order of wavelengths:^[6]

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface (Lithosphere). This spectrum of radiation has germicidal properties, and is used in germicidal lamps.
• Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the photochemical reaction leading to the production of the Ozone layer.
• Ultraviolet A or (UVA) spans 315 to 400 nm. It has been traditionally held as less damaging to the DNA, and hence used in tanning and PUVA therapy for psoriasis.
• Visible range or light spans 400 to 700 nm. As the name suggests, it is this range that is visible to the naked eye.
• Infrared range that spans 700 nm to 10⁶ nm [1 millimeter (mm)]. It is largely responsible for the warmth or heat that the sunlight carries. It is also divided into three types on the basis of wavelength:
  • Infrared-A: 700 nm to 1400 nm
  • Infrared-B: 1400 nm to 3000 nm
  • Infrared-C: 3000 nm to 1 mm.

Climate effects

On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, combining the perception of bright white light (sunlight in the strict sense) and warming. The warming on the body and surfaces of other objects is distinguished from the increase in air temperature.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies.^[7] For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages

Life on Earth

The existence of nearly all life on Earth is fueled by light from the sun. Most autotrophs, such as plants, use the energy of sunlight to turn air into simple sugars—a process known as photosynthesis. These sugars are then used as building blocks and in other synthetic pathways which allow the organism to grow.

Heterotrophs, such as animals, use light from the sun indirectly by consuming the products of autotrophs, either directly or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as respiration.

In prehistory, humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the Neolithic Revolution, the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and nutrients for future harvests. Animals which had previously only provided humans with meat and tools once they were killed were now used for labour throughout their lives, fueled by grasses inedible to humans.

The more recent discoveries of coal, petroleum and natural gas are modern extensions of this trend. These fossil fuels are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in these fossil fuels has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of primary energy. As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g. alternative fuels, Malthusian catastrophe, new urbanism, peak oil).

Cultural aspects

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear sunglasses. Cars, many helmets and caps are equipped with visors to block the sun from direct vision when the sun is at a low angle.

Prism splitting light

In colder countries many people prefer sunnier days and often avoid the shade. In hotter countries the converse is true; during the midday hours many people prefer to stay inside to remain cool. If they do go outside, they seek shade which may be provided by trees, parasols, and so on.

Sunshine is often blocked from entering buildings through the use of walls, window blinds, awnings, shutters or curtains.

Sunbathing

Sunbathing is a popular leisure activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include beaches, open air swimming pools, parks, gardens, and sidewalk cafés. Sunbathers typically wear limited amounts of clothing or some simply go nude. An alternative some use to sunbathing is to use a sunbed that generates ultraviolet light and can be used indoors regardless of outdoor weather conditions and amount of sun light.

For many people with pale or brownish skin, one purpose for sunbathing is to darken one's skin color (get a sun tan) as this is considered in some cultures to be beautiful, associated with outdoor activity, vacations or holidays, and health. Some people prefer nude sunbathing so that an "all-over" or "even" tan can be obtained.

Skin tanning is achieved by an increase in the dark pigment inside skin cells called melanocytes and it is actually an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the sun or from artificial sunlamps. Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

Effects on health

The body produces vitamin D from sunlight (specifically from the UVB band of ultraviolet light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet.

Excessive sunlight exposure has been linked to all types of skin cancer caused by the ultraviolet part of radiation from sunlight or sunlamps.^{[citation needed]} Sunburn can have mild to severe inflammation effects on skin; this can be avoided by using a proper sunscreen cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called skin photodamage), which produces a difficult to treat cosmetic effect. Some people are concerned that ozone depletion is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer.^[8]

A lack of sunlight, on the other hand, is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) involve replicating sunlight via sunlamps tuned to specific (visible, not ultra-violet) wavelengths of light or full-spectrum bulbs.

A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS) later in life.^[9]

The Amazing Stars

Stars that continuously amaze our eyes...how are they formed? I will share you some facts about them as I have researched over the net.

A star is a massive, luminous ball of plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.

Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion.^[1] The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun^[2] expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.^[3]

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.^[4] Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.

Observation history

Historically, stars have been important to civilizations throughout the world. They have been used in religious practices and for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.^[5] The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.^[6] The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to the nearest star, the Sun.

The oldest accurately dated star chart appeared in Ancient Egypt in 1,534 BCE.^[7] Islamic astronomers gave to many stars Arabic names which are still used today, and they invented numerous astronomical instruments which could compute the positions of the stars. In the 11th century, Abū Rayhān al-Bīrūnī described the Milky Way galaxy as multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.^[8]

In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.^[9] Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae), suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have other planets, possibly even Earth-like, in orbit around them,^[10] an idea that had been suggested earlier by such ancient Greek philosophers as Democritus and Epicurus.^[11] By the following century the idea of the stars as distant suns was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on the solar system, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.^[12]

The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions from the time of the ancient Greek astronomers Ptolemy and Hipparchus. The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.^[10]

William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he performed a series of gauges in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.^[13] In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems.

The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere. In 1865 Secchi began classifying stars into spectral types.^[14] However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.

Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius, and inferred a hidden companion. Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 day period. Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.^[15]

The twentieth century saw increasingly rapid advances in the scientific study of stars. The photograph became a valuable astronomical tool. Karl Schwarzschild discovered that the color of a star, and hence its temperature, could be determined by comparing the visual magnitude against the photographic magnitude. The development of the photoelectric photometer allowed very precise measurements of magnitude at multiple wavelength intervals. In 1921 Albert A. Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope.^[16]

Important conceptual work on the physical basis of stars occurred during the first decades of the twentieth century. In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.^[17]

With the exception of supernovae, individual stars have primarily been observed in our Local Group of galaxies,^[18] and especially in the visible part of the Milky Way (as demonstrated by the detailed star catalogues available for our galaxy^[19]). But some stars have been observed in the M100 galaxy of the Virgo Cluster, about 100 million light years from the Earth.^[20] In the Local Supercluster it is possible to see star clusters, and current telescopes could in principle observe faint individual stars in the Local Cluster—the most distant stars resolved have up to hundred million light years away^[21] (see Cepheids). However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away^[22]—ten times the distance of the most distant star cluster previously observed.

Star designations

The concept of the constellation was known to exist during the Babylonian period. Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths. Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.

As well as certain constellations and the Sun itself, stars as a whole have their own myths.^[23] They were thought to be the souls of the dead or gods. An example is the star Algol, which was thought to represent the eye of the Gorgon Medusa.

To the Ancient Greeks, some "stars," known as planets (Greek πλανήτης (planētēs), meaning "wanderer"), represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken.^[23] (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. Their names were assigned by later astronomers).

Circa 1600, the names of the constellations were used to name the stars in the corresponding regions of the sky. The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation. Later the English astronomer John Flamsteed came up with a system using numbers, which would later be known as the Flamsteed designation. Numerous additional systems have since been created as star catalogues have appeared.

The only body which has been recognized by the scientific community as having the authority to name stars or other celestial bodies is the International Astronomical Union (IAU).^[24] A number of private companies (for instance, the "International Star Registry") purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them,^[24] and many in the astronomy community view these organizations as frauds preying on people ignorant of star naming procedure.^[25]

Units of measurement

Most stellar parameters are expressed in SI units by convention, but CGS units are also used (e.g., expressing luminosity in ergs per second). Mass, luminosity, and radii are usually given in solar units, based on the characteristics of the Sun:

solar mass:	kg[26]
solar luminosity:	watts[26]
solar radius:	m[27]

Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit (AU)—approximately the mean distance between the Earth and the Sun (150 million km or 93 million miles).

Formation and evolution

Stars are formed within extended regions of higher density in the interstellar medium, although the density is still lower than the inside of an earthly vacuum chamber. These regions are called molecular clouds and consist mostly of hydrogen, with about 23–28% helium and a few percent heavier elements. One example of such a star-forming region is the Orion Nebula.^[28] As massive stars are formed from molecular clouds, they powerfully illuminate those clouds. They also ionize the hydrogen, creating an H II region.

Protostar formation

The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae (massive stellar explosions) or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.

Artist's conception of the birth of a star within a dense molecular cloud. NASA image

As the cloud collapses, individual conglomerations of dense dust and gas form what are known as Bok globules. These can contain up to 50 solar masses of material. As a globule collapses and the density increases, the gravitational energy is converted into heat and the temperature rises. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core.^[29] These pre-main sequence stars are often surrounded by a protoplanetary disk. The period of gravitational contraction lasts for about 10–15 million years.

Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.^[30]

Main sequence

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence and are called dwarf stars. Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity.^[31] The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.^[32]

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the amount of mass lost is negligible. The Sun loses 10⁻¹⁴ solar masses every year,^[33] or about 0.01% of its total mass over its entire lifespan. However very massive stars can lose 10⁻⁷ to 10⁻⁵ solar masses each year, significantly affecting their evolution.^[34] Stars that begin with more than 50 solar masses can lose over half their total mass while they remain on the main sequence.^[35]

An example of a Hertzsprung-Russell diagram for a set of stars that includes the Sun (center). (See "Classification" below.)

The duration that a star spends on the main sequence depends primarily on the amount of fuel it has to fuse and the rate at which it fuses that fuel. In other words, its initial mass and its luminosity. For the Sun, this is estimated to be about 10¹⁰ years. Large stars consume their fuel very rapidly and are short-lived. Small stars (called red dwarfs) consume their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer.^[2] However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no such stars are expected to exist yet.

Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. In astronomy all elements heavier than helium are considered a "metal", and the chemical concentration of these elements is called the metallicity. The metallicity can influence the duration that a star will burn its fuel, control the formation of magnetic fields^[36] and modify the strength of the stellar wind.^[37] Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. (Over time these clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.)

Post-main sequence

As stars of at least 0.4 solar masses^[2] exhaust their supply of hydrogen at their core, their outer layers expand greatly and cool to form a red giant. For example, in about 5 billion years, when the Sun is a red giant, it will expand out to a maximum radius of roughly 1 AU (150,000,000 km), 250 times its present size. As a giant, the Sun will lose roughly 30% of its current mass.^[32][38]

In a red giant of up to 2.25 solar masses, hydrogen fusion proceeds in a shell-layer surrounding the core.^[39] Eventually the core is compressed enough to start helium fusion, and the star now gradually shrinks in radius and increases its surface temperature. For larger stars, the core region transitions directly from fusing hydrogen to fusing helium.^[40]

After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. The star then follows an evolutionary path that parallels the original red giant phase, but at a higher surface temperature.

Massive stars

Betelgeuse is a red supergiant star approaching the end of its life cycle

During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium.

The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.^[41]

The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.^[39] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

Collapse

An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf.^[42] The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.

The Crab Nebula, remnants of a supernova that was first observed around 1050 AD

In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.^[43]

Most of the matter in the star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula^[43]) and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole.^[44] In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.

The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.^[43]

Distribution

A white dwarf star in orbit around Sirius (artist's impression). NASA image

In addition to isolated stars, a multi-star system can consist of two or more gravitationally bound stars that orbit around each other. The most common multi-star system is a binary star, but systems of three or more stars are also found. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of co-orbiting binary stars.^[45] Larger groups called star clusters also exist. These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.

It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems. This is particularly true for very massive O and B class stars, where 80% of the systems are believed to be multiple. However the portion of single star systems increases for smaller stars, so that only 25% of red dwarfs are known to have stellar companions. As 85% of all stars are red dwarfs, most stars in the Milky Way are likely single from birth.^[46]

Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (10¹¹) galaxies in the observable universe.^[47] While it is often believed that stars only exist within galaxies, intergalactic stars have been discovered.^[48] Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the observable universe.^[49]

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (10¹²) kilometres, or 4.2 light-years away. Light from Proxima Centauri takes 4.2 years to reach Earth. Travelling at the orbital speed of the Space Shuttle (5 miles per second—almost 30,000 kilometres per hour), it would take about 150,000 years to get there.^[50] Distances like this are typical inside galactic discs, including in the vicinity of the solar system.^[51] Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos.

Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare. In denser regions such as the core of globular clusters or the galactic center, collisions can be more common.^[52] Such collisions can produce what are known as blue stragglers. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity in the cluster .^[53]

Characteristics

The Sun is the nearest star to Earth

Almost everything about a star is determined by its initial mass, including essential characteristics such as luminosity and size, as well as the star's evolution, lifespan, and eventual fate.

Age

Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old—the observed age of the universe. The oldest star yet discovered, HE 1523-0901, is an estimated 13.2 billion years old.^[54]

The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.^[55][56]

Chemical composition

When stars form they are composed of about 70% hydrogen and 28% helium, as measured by mass, with a small fraction of heavier elements. Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure. Because the molecular clouds where stars form are steadily enriched by heavier elements from supernovae explosions, a measurement of the chemical composition of a star can be used to infer its age.^[57] The portion of heavier elements may also be an indicator of the likelihood that the star has a planetary system.^[58]

The star with the lowest iron content ever measured is the dwarf HE1327-2326, with only 1/200,000th the iron content of the Sun.^[59] By contrast, the super-metal-rich star μ Leonis has nearly double the abundance of iron as the Sun, while the planet-bearing star 14 Herculis has nearly triple the iron.^[60] There also exist chemically peculiar stars that show unusual abundances of certain elements in their spectrum; especially chromium and rare earth elements.^[61]

Diameter

Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.^[62]

The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required in order to produce images of these objects. Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon (or the rise in brightness when it reappears), the star's angular diameter can be computed.^[63]

Stars range in size from neutron stars, which vary anywhere from 20 to 40 km in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter approximately 650 times larger than the Sun—about 0.9 billion kilometres. However, Betelgeuse has a much lower density than the Sun.^[64]

Kinematics

The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy. The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.

Radial velocity is measured by the doppler shift of the star's spectral lines, and is given in units of km/s. The proper motion of a star is determined by precise astrometric measurements in units of milli-arc seconds (mas) per year. By determining the parallax of a star, the proper motion can then be converted into units of velocity. Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.^[65]

Once both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed. Among nearby stars, it has been found that population I stars have generally lower velocities than older, population II stars. The latter have elliptical orbits that are inclined to the plane of the galaxy.^[66] Comparison of the kinematics of nearby stars has also led to the identification of stellar associations. These are most likely groups of stars that share a common point of origin in giant molecular clouds. ^[67]

Magnetic field

Main article: Stellar magnetic field

Surface magnetic field of SU Aur (a young star of T Tauri type), reconstructed by means of Zeeman-Doppler imaging

The magnetic field of a star is generated within regions of the interior where convective circulation occurs. This movement of conductive plasma functions like a dynamo, generating magnetic fields that extend throughout the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation. This surface activity produces starspots, which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic fields that reach out into the corona from active regions. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.^[68]

Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, however, functioning as a brake to gradually slow the rate of rotation as the star grows older. Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods.^[69] During the Maunder minimum, for example, the Sun underwent a 70-year period with almost no sunspot activity.

Mass

One of the most massive stars known is Eta Carinae,^[70] with 100–150 times as much mass as the Sun; its lifespan is very short—only several million years at most. A recent study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe.^[71] The reason for this limit is not precisely known, but it is partially due to the Eddington luminosity which defines the maximum amount of luminosity that can pass through the atmosphere of a star without ejecting the gases into space.

The reflection nebula NGC 1999 is brilliantly illuminated by V380 Orionis (center), a variable star with about 3.5 times the mass of the Sun. NASA image

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more,^[72] due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core.^[73] For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter.^[74][75] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter.^[76][75] Smaller bodies are called brown dwarfs, which occupy a poorly defined grey area between stars and gas giants.

The combination of the radius and the mass of a star determines the surface gravity. Giant stars have a much lower surface gravity than main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs. The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.^[17]

Rotation

The rotation rate of stars can be approximated through spectroscopic measurement, or more exactly determined by tracking the rotation rate of starspots. Young stars can have a rapid rate of rotation greater than 100 km/s at the equator. The B-class star Achernar, for example, has an equatorial rotation velocity of about 225 km/s or greater, giving it an equatorial diameter that is more than 50% larger than the distance between the poles. This rate of rotation is just below the critical velocity of 300 km/s where the star would break apart.^[77] By contrast, the Sun only rotates once every 25 – 35 days, with an equatorial velocity of 1.994 km/s. The star's magnetic field and the stellar wind serve to slow down a main sequence star's rate of rotation by a significant amount as it evolves on the main sequence.^[78]

Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum—the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin. A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind.^[79] In spite of this, the rate of rotation for a pulsar can be very rapid. The pulsar at the heart of the Crab nebula, for example, rotates 30 times per second.^[80] The rotation rate of the pulsar will gradually slow due to the emission of radiation.

Temperature

The surface temperature of a main sequence star is determined by the rate of energy production at the core and the radius of the star and is often estimated from the star's color index.^[81] It is normally given as the effective temperature, which is the temperature of an idealized black body that radiates its energy at the same luminosity per surface area as the star. Note that the effective temperature is only a representative value, however, as stars actually have a temperature gradient that decreases with increasing distance from the core.^[82] The temperature in the core region of a star is several million kelvins.^[83]

The stellar temperature will determine the rate of energization or ionization of different elements, resulting in characteristic absorption lines in the spectrum. The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star (see classification below).^[17]

Massive main sequence stars can have surface temperatures of 50,000 K. Smaller stars such as the Sun have surface temperatures of a few thousand degrees. Red giants have relatively low surface temperatures of about 3,600 K, but they also have a high luminosity due to their large exterior surface area.^[84]

Radiation

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind^[85] (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star’s outer layers) and as a steady stream of neutrinos emanating from the star’s core.

The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element, gamma ray photons are released from the nuclear fusion reaction. This energy is converted to other forms of electromagnetic energy, including visible light, by the time it reaches the star’s outer layers.

The color of a star, as determined by the peak frequency of the visible light, depends on the temperature of the star’s outer layers, including its photosphere.^[86] Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotational velocity of a star. If the distance of the star is known, such as by measuring the parallax, then the luminosity of the star can be derived. The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. (Mass can be measured directly for stars in binary systems. The technique of gravitational microlensing will also yield the mass of a star.^[87]) With these parameters, astronomers can also estimate the age of the star.^[88]

Luminosity

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star is determined by the radius and the surface temperature. However, many stars do not radiate a uniform flux—the amount of energy radiated per unit area—across their entire surface. The rapidly rotating star Vega, for example, has a higher energy flux at its poles than along its equator.^[89]

Surface patches with a lower temperature and luminosity than average are known as starspots. Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. Larger, giant stars have much bigger, much more obvious starspots,^[90] and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.^[91] Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.^[92]

Magnitude

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star’s luminosity, distance from Earth, and the altering of the star’s light as it passes through Earth’s atmosphere. Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years), and it is directly related to a star’s luminosity.

Number of stars brighter than magnitude

Apparent magnitude	Number of Stars[93]
0	4
1	15
2	48
3	171
4	513
5	1,602
6	4,800
7	14,000

Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times^[94] (the 5th root of 100 or approximately 2.512). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star. The faintest stars visible to the naked eye under good seeing conditions are about magnitude +6.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; the larger the magnitude number, the fainter. The brightest stars, on either scale, have negative magnitude numbers. The variation in brightness (ΔL) between two stars is calculated by subtracting the magnitude number of the brighter star (m_b) from the magnitude number of the fainter star (m_f), then using the difference as an exponent for the base number 2.512; that is to say:

Δm = m_f − m_b

2.512^Δm = ΔL

Relative to both luminosity and distance from Earth, absolute magnitude (M) and apparent magnitude (m) are not equivalent for an individual star;^[94] for example, the bright star Sirius has an apparent magnitude of −1.44, but it has an absolute magnitude of +1.41.

The Sun has an apparent magnitude of −26.7, but its absolute magnitude is only +4.83. Sirius, the brightest star in the night sky as seen from Earth, is approximately 23 times more luminous than the Sun, while Canopus, the second brightest star in the night sky with an absolute magnitude of −5.53, is approximately 14,000 times more luminous than the Sun. Despite Canopus being vastly more luminous than Sirius, however, Sirius appears brighter than Canopus. This is because Sirius is merely 8.6 light-years from the Earth, while Canopus is much farther away at a distance of 310 light-years.

As of 2006, the star with the highest known absolute magnitude is LBV 1806-20, with a magnitude of −14.2. This star is at least 5,000,000 times more luminous than the Sun.^[95] The least luminous stars that are currently known are located in the NGC 6397 cluster. The faintest red dwarfs in the cluster were magnitude 26, while a 28th magnitude white dwarf was also discovered. These faint stars are so dim that their light is as bright as a birthday candle on the Moon when viewed from the Earth.^[96]

Classification

Surface Temperature Ranges for
Different Stellar Classes^[97]

Class	Temperature	Sample star
O	33,000 K or more	Zeta Ophiuchi
B	10,500–30,000 K	Rigel
A	7,500–10,000 K	Altair
F	6,000–7,200 K	Procyon A
G	5,500–6,000 K	Sun
K	4,000–5,250 K	Epsilon Indi
M	2,600–3,850 K	Proxima Centauri

The current stellar classification system originated in the early 20th century, when stars were classified from A to Q based on the strength of the hydrogen line.^[98] It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at around 9000 K, and is weaker at both hotter and cooler temperatures. When the classifications were reordered by temperature, it more closely resembled the modern scheme.^[99]

There are different single-letter classifications of stars according to their spectra, ranging from type O, which are very hot, to M, which are so cool that molecules may form in their atmospheres. The main classifications in order of decreasing surface temperature are: O, B, A, F, G, K, and M. A variety of rare spectral types have special classifications. The most common of these are types L and T, which classify the coldest low-mass stars and brown dwarfs. Each letter has 10 sub-divisions, numbered from 0 to 9, in order of decreasing temperature. However, this system breaks down at extreme high temperatures: class O0 and O1 stars may not exist.^[100]

In addition, stars may be classified by the luminosity effects found in their spectral lines, which correspond to their spatial size and is determined by the surface gravity. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). Most stars belong to the main sequence, which consists of ordinary hydrogen-burning stars. These fall along a narrow, diagonal band when graphed according to their absolute magnitude and spectral type.^[100] Our Sun is a main sequence G2V yellow dwarf, being of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. For example, an "e" can indicate the presence of emission lines; "m" represents unusually strong levels of metals, and "var" can mean variations in the spectral type.^[100]

White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.^[101]

Variable stars

The asymmetrical appearance of Mira, an oscillating variable star. NASA HST image

Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties. Of the intrinsically variable stars, the primary types can be subdivided into three principal groups.

During their stellar evolution, some stars pass through phases where they can become pulsating variables. Pulsating variable stars vary in radius and luminosity over time, expanding and contracting with periods ranging from minutes to years, depending on the size of the star. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.^[102]

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events.^[102] This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variables undergo a dramatic change in their properties. This group includes novae and supernovae. A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova.^[4] The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion.^[103] Some novae are also recurrent, having periodic outbursts of moderate amplitude.^[102]

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots.^[102] A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days.

Structure

The interior of a stable star is in a state of hydrostatic equilibrium: the forces on any small volume almost exactly counterbalance each other. The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star. The pressure gradient is established by the temperature gradient of the plasma; the outer part of the star is cooler than the core. The temperature at the core of a main sequence or giant star is at least on the order of 10⁷ K. The resulting temperature and pressure at the hydrogen-burning core of a main sequence star are sufficient for nuclear fusion to occur and for sufficient energy to be produced to prevent further collapse of the star.^[104][105]

As atomic nuclei are fused in the core, they emit energy in the form of gamma rays. These photons interact with the surrounding plasma, adding to the thermal energy at the core. Stars on the main sequence convert hydrogen into helium, creating a slowly but steadily increasing proportion of helium in the core. Eventually the helium content becomes predominant and energy production ceases at the core. Instead, for stars of more than 0.4 solar masses, fusion occurs in a slowly expanding shell around the degenerate helium core.^[106]

In addition to hydrostatic equilibrium, the interior of a stable star will also maintain an energy balance of thermal equilibrium. There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior. The outgoing flux of energy leaving any layer within the star will exactly match the incoming flux from below.

This diagram shows a cross-section of a solar-type star. NASA image

The radiation zone is the region within the stellar interior where radiative transfer is sufficiently efficient to maintain the flux of energy. In this region the plasma will not be perturbed and any mass motions will die out. If this is not the case, however, then the plasma becomes unstable and convection will occur, forming a convection zone. This can occur, for example, in regions where very high energy fluxes occur, such as near the core or in areas with high opacity as in the outer envelope.^[105]

The occurrence of convection in the outer envelope of a main sequence star depends on the mass. Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers.^[107] Red dwarf stars with less than 0.4 solar masses are convective throughout, which prevents the accumulation of a helium core.^[2] For most stars the convective zones will also vary over time as the star ages and the constitution of the interior is modified.^[105]

The portion of a star that is visible to an observer is called the photosphere. This is the layer at which the plasma of the star becomes transparent to photons of light. From here, the energy generated at the core becomes free to propagate out into space. It is within the photosphere that sun spots, or regions of lower than average temperature, appear.

Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This is surrounded by a transition region, where the temperature rapidly increases within a distance of only 100 km. Beyond this is the corona, a volume of super-heated plasma that can extend outward to several million kilometres.^[108] The existence of a corona appears to be dependent on a convective zone in the outer layers of the star.^[107] Despite its high temperature, the corona emits very little light. The corona region of the Sun is normally only visible during a solar eclipse.

From the corona, a stellar wind of plasma particles expands outward from the star, propagating until it interacts with the interstellar medium. For the Sun, the influence of its solar wind extends throughout the bubble-shaped region of the heliosphere.^[109]

Nuclear fusion reaction pathways

Overview of the proton-proton chain

The carbon-nitrogen-oxygen cycle

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition, as part of stellar nucleosynthesis. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is converted into energy, according to the mass-energy equivalence relationship E = mc².^[1]

The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. As a result the core temperature of main sequence stars only varies from 4 million K for a small M-class star to 40 million K for a massive O-class star.^[83]

In the Sun, with a 10 million K core, hydrogen fuses to form helium in the proton-proton chain reaction:^[110]

4¹H → 2²H + 2e⁺ + 2ν_e (4.0 MeV + 1.0 MeV)

2¹H + 2²H → 2³He + 2γ (5.5 MeV)

2³He → ⁴He + 2¹H (12.9 MeV)

These reactions result in the overall reaction:

4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV)

where e⁺ is a positron, γ is a gamma ray photon, ν_e is a neutrino, and H and He are isotopes of hydrogen and helium, respectively. The energy released by this reaction is in millions of electron volts, which is actually only a tiny amount of energy. However enormous numbers of these reactions occur constantly, producing all the energy necessary to sustain the star's radiation output.

Minimum stellar mass required for fusion

Element	Solar masses
Hydrogen	0.01
Helium	0.4
Carbon	5[111]
Neon	8

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon—the carbon-nitrogen-oxygen cycle.^[110]

In evolved stars with cores at 100 million K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process that uses the intermediate element beryllium:^[110]

⁴He + ⁴He + 92 keV → ^8*Be

⁴He + ^8*Be + 67 keV → ^12*C

^12*C → ¹²C + γ + 7.4 MeV

For an overall reaction of:

3⁴He → ¹²C + γ + 7.2 MeV

In massive stars, heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process. The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron-56. Fusion can not proceed any further except through an endothermic process, and so further energy can only be produced through gravitational collapse.^[110]

The example below shows the amount of time required for a star of 20 solar masses to consume all of its nuclear fuel. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.^[112]

Fuel material	Temperature (million kelvins)	Density (kg/cm³)	Burn duration (τ in years)
H	37	0.0045	8.1 million
He	188	0.97	1.2 million
C	870	170	976
Ne	1,570	3,100	0.6
O	1,980	5,550	1.25
S/Si	3,340	33,400	0.0315[113]

LAUGHTER

There is one kind of a laugh that I always did recommend; it looks out of the eye first with a merry twinkle, then it creeps down on its hands and knees and plays around the mouth like a pretty moth around the blaze of a candle, then it steals over into the dimples of the cheeks and rides around in those little whirlpools for a while, then it lights up the whole face like the mellow bloom on a damask rose, then it swims up on the air, with a peal as clear and as happy as dinner-bell, then goes back again on gold tiptoes like an angel out for an airing, and lies down on its little bed of violets in the heart where it came from...Josh Billings

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