Star



Stars are massive, luminous balls of plasma that are held together by gravity. Stars vary greatly in shape, colour, size, and temperature, with the closest (andd most important) star to humanity being the Sun. Stars have massive gravitational pulls, powerful enough to draw in planets, asteroids, and other celestial bodies, into elliptical orbits around them. Stars are extremely long-lived, with smaller ones living about 9 billion years, and larger ones living for many trillions of years. Stars often group together to form a globular cluster or, in extremely large groups, even a galaxy.

Life of a Star
Stars form in clouds of dust and gas known as nebulae. Gas, dust, and rock gather to form a rocky ball, which grows in size as more and more dust and rock collects onto it. While many of these balls become comets or asteroids, when more and more dust and gas piles on, and the ball of rock increases in size, fusion at the centre of the ball causes it to combust into plasma; it is at this point that the object technically becomes a star. Stars generally start small, from 1,000 kilometers to 20,000 kilometers across. The young stars quickly absorb the rest of the nebula, using its energy to grow. This begins the longest period of a star's life: its main or regular sequence. To create energy, stars convert hydrogen into helium; this process varies in length based on the size of the star. When the last of the hydrogen fuel is consumed, the stars swell rapidly, reaching their "giant" stage. Giant stars quickly use up their other elements, experiencing increasingly large energy outbursts, until, the star explodes in a supernova, forming a new nebula from which new stars can be formed.

Size classification
The size is the primary deciding factor in classifying a star.


 * Subdwarf stars are the smallest stars, ranging from 1,000 to 200,000 kilometers in diameter. Subdwarf stars have little energy and as a result are fairly cool. They are quite common; about 5 out of 10 stars are subdwarf stars.

Examples: Sun, Beta Cygni B, Sirius B


 * 250px-RedDwarfNASA.jpgDwarf stars are larger than subdwarf stars but are less common. Dwarf stars have more energy than subdwarfs. Red dwarf stars are some of most commmon of all stars; about 3 out of 10 stars are dwarf stars.

Examples: Sirius, Beta Cygni, Alpha Centauri A and B


 * Subgiant stars are about 10 to 35 times bigger than the Sun and give off the strongest hydrogen emissions of all stars. They are about 6 in 15 stars are subgiants.

Examples: Arcturus, Vega, Regulus


 * Giant stars (easily confused with the giant stage of a star's life) are massive, luminous stars that give off mild energy emissions, but weak hydrogen emissions. Ranging from 5 to 450 times bigger than the Sun, about 1 in 20 stars is considered a giant star.

Examples: Aldebaran, S Doradus, T Cephei, Rho Cassiopiae


 * Supergiant stars are extremely large stars, ranging from 50 to 3000 times the size of the sun. Most supergiants are in the "giant" stage of their life, with a few exceptions such as Antares. About 1 in 40 stars is considered a supergiant.

Examples: Rigel, Antares, Wezen


 * Hypergiant stars are the most luminous stars, over 200,000 times brighter and more than 1000 times the size of the Sun. They experience major, but rare, energy outbursts that are a reflection of their enormous amount of energy. About 5 in 120 stars are considered hypergiants.

Examples: NML Cygni, VY Canis Majoris, V838 Monocerotis

Temperature/Colour classification
The secondary deciding factor in the classification of a star is its temperature and colour.


 * T-type stars are the coolest stars, ranging from 500 to 1,500 Celsius. T-type stars are quite rare and very dim. Their limited luminosity means that they can be hard to spot, even with telescopes. Their colour is dark carmine red.

Examples: Gliese 229 B


 * L-type stars are quite mysterious. This star gives massive energy outbursts, suggesting that L-type stars are unstable. L-stars are pink, or reddish-pink, and like T-stars, they are quite cool, ranging from 1,500 to 3,000 degrees.

Examples: V838 Monocerotis


 * M-stars are cool, fairly dim stars, and are some of the most common. They give off weak emissions, wespecially compared to L-stars, and have the least energy of stars their size. Their red colour is a result of their cool temperature, from 3,000 to 4,000 degrees.

Examples: Gliese 229 A, Gacrux, La Superba, KY Cygni, V354 Cephei


 * K-type stars are common, fairly calm stars, and are probably the best-known type of stars, if only because our Sun is an example. K-type stars give off virtually no emissions, and most are older stars, with exceptions such as Alpha Centauri B and Aldebaran. Coloured a bright orange, K-type stars are luminous and appear white from planets due to atmospheric scattering. K-type stars range in temperature from 4,000 to 5,000 degrees Celsius.

Examples: Alpha Centauri B, Arcturus, Aldebaran


 * G-type stars are the most common class of stars, slightly warmer than our Sun. The G-type stars are calm, but not as calm as K-type stars. G-type stars burn through their hydrogen fuel very quickly, and as a result do not last as long as other stars. It is estimated that at least 10% of the G-type stars visible from Earth have died already. The colour of G-type stars ranges from pale orange to bright yellow. G-type stars range in temperature from 5,000 to 6,000 Celsius.

Examples: Alpha Centauri A; Tau Ceti; Rho Cassiopeiae; V382 Carinae;


 * F-type stars are fairly rare; but the closest to Earth, Canopus, has shed some light on this type. F-type starss are quite luminous and give off frequent mild energy emissions. F-type stars are the second-shortest lived stars after G-type stars, and are slightly warmer. F-type stars have little unusual features; like G-type stars their energy emissions are mild. Most F-type stars known are dwarf or subgiant stars with a temperature ranging from 6,000 to 7,000 degrees Celsius. The less luminous F-type stars appear a pale yellow, while the more luminous examples may be near-white in colour, hence the informal name "yellow-white stars".

Examples: Canopus, Procyron


 * A-type stars are quite rare; but more common than the cooler F-type stars or the slightly hotter B-type stars. A-type stars are rather stable; they rarely give off energy emissions but have gravitational pulls that, for the most part, are powerful enough to elongate the star. A-type stars are the brightest of stars, mostly due to their pure white colour. They range in temperature from 7,000 to 9,000 degrees Celsius.

Examples: Sirius A and B; Vega; Deneb A;


 * B-type stars are very rare; only four examples are known. All of the known B-type stars are large and give off powerful emissions. Unlike most stars, they are neither elliptical nor near-perfect spheres; instead they are little more than "hot vacuums", with no sharp edge, and trail out into space. Their colour is white, or blue-white, and their temperature ranges from 9,000 to 20,000 degrees Celsius.

Examples: Spica, Alnitak, Regulus; Rigel, and the Pistol Star


 * O-type stars are more common than B or F-type stars, but rarer than A-type stars. O-type stars are extremely luminous; amongst the most luminous of all stars. O-type stars are the calmest stars, that give off virtually no energy emissions. The gravity of O-type stars moulds them into near-perfect spheres; the only exceptions are the smallest O-type stars, such as Regulus. O-type stars are highly radioactive; if one was put in the Sun's place, all life on Earth would be instantly eradicated. Like B-type stars, they are extremely hot stars, being over 20,000 degrees in temperature. Their cokour ranges from turquoise in the cooler examples to blue in the hotter ones.

Examples: LBV 1806-20; Eta Carinae; Cygnus OB2-12;