The Stars

Depending on the original mass of the gas ball, now star, the star can last from hundreds of billions of years to just a few tens of millions of years. The more massive the star originally, the hotter it runs (from core to surface), and the shorter is its lifetime. Blue stars die before yellow stars, which in turn die before red stars.

The primary core fusion mechanism of a star is the conversion of hydrogen nuclei into helium nuclei, with the release of tremendous amounts of energy. As long as there is hydrogen nuclei at a star's core, it is considered to be functioning normally, to be a so-called main-sequence star.

Once a star runs out of hydrogen nuclei fuel at its core, other fusion processes take over there, and the star starts to die. Fusion continues, but in a layered, outwardly expanding shell-like fashion.

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Stars are classified by spectral type, according to a system developed by Annie Jump Cannon. The spectral types are "O, B, A, F, G, K and M". A good way to remember this letter sequence is the mnemonic "Oh boy, [the] astronomony final's gonna kill me!" The letters are arranged according to how hot a star is at its surface. An "O" star can run 30,000 K at its surface, whereas an "M" star runs about 3000 K at its surface. Our Sun is a "G" star, with a surface temperature of about 5800 K.

Cannon's spectral classification was ultimately explained, brilliantly, by a colleague of hers, Cecilia Payne-Gaspochkin.

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Typically, the hotter a star, the more massive it is, and the more intrinsically bright it is. This second conclusion assumes that an observed star is not in a dying, giant phase.

The mass of a star can also be determined by Newton's form of Kepler's Third Law, provided that the star is part of a binary system (pair of stars orbiting each other). For example, if two stars in a binary system are the same spectral class and the same apparent brightness, then Newton's form of Kepler's Third Law can be used to approximate the combined mass of the two stars, where each star's mass is half that value.

A star's mass is typically given as a multiple of the Sun's mass, referred to as a solar mass. An "O' star can run as high as 150 solar masses, and a red star can run as little as 0.08 solar masses (about 76 times the mass of Jupiter).

A star's intrinsic brightness varies greatly as well. A large "O" star can be a million times intrinsically brighter than the Sun, and a small "M" star can be one ten thousandth as intrinsically bright as the Sun.

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When stars start to die, they typically enlarge as they shift to shell fusion. As the star enlarges, it becomes brighter, because it has greater surface area from which to "shine". These enlarged stars are called giants, or supergiants, and are no longer considered main-sequence stars.

Certain stars enlarge and contract on a regular basis. These stars are called variable stars. One common type of variable star are the Cepheids. Another colleague of Cannon's, Henrietta Leavitt, determined that the variation in the apparent brightness of a Cepheid could be used to determine how far away that star's host galaxy was.

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Most all of these star characteristics can be summarized in a single chart called the Hertzsprung-Russell diagram, or H-R diagram. Click here to learn more.