Stellar Classifications
Stars are classified using three catagories: size, spectral class, and brightness.
Putting it all togther
Size
Ia - Large Supergiant
Ib - Supergiant
II - Large Giant
III - Giant
IV - Subgiant
V - Dwarf or Main-sequence
VI - Subdwarf
Spectral Classes
Type A - White Stars
Some Type A stars have metal-rich gas envelopes, their planets may be richer in minerals.
Examples: Altair, Deneb, Sirius A, and Vega.
Type B - Blue-white Stars
Even small Type B stars mass 10 times the mass of Sol.
Examples: Rigel and Spica.
Type D - White Dwarf
They have no interior luminosity at all, but still posses a glowing hydrogen atmosphere.
Examples:
Type F - Yellow-white Stars
Even Type F dwarfs mass two times Sol, and their Class M planets orbit correspondingly twice as far out.
Examples: Canopus, Polaris and Procyon.
Type G - Yellow Stars
These stars emit a balance of radiation and energy ideal for human life.
Examples: Alpha Centauri A, Capella, Sol and Tau Ceti.
Type K - Orange Stars
Since they usually put out less radiation than Type G stars, humans need less radiation protection in Type K systems, and planets with thinner atmospheres are still considered Class M.
Examples: Aldebaran and Arcturus are Type K giants; Epsilon Eridani and Epsilon Indi are typical Type K dwarfs.
Type M - Red Stars
Over two thirds of all stars are Type M, possibly because these stars burn slowest of all.
Examples: Although Antares and Betelgeuse are Type M supergiants, red dwarfs like Barnard's Star and Wolf 359 are far more common.
Type O - Blue Stars
They burn very fast and die out in only a few million years. They tend to be found only in spiral arms or the Galactic Core. They come in only supergiant and dwarf sizes.
Examples:
Brightness
Relative brightness runs from 9 (the dimmest) to 0 (the brightest).
Although bluer stars are hotter, and thus normally brighter, than redder ones, this rule is by no means hard and fast: the red giant Betelgeuse is over three times brighter than the blue dwarf Spica.
Putting it all togther
When a star's classification is referred to, it is put together as: "spectral class""brightness" "size"
Examples:
- Earth's Sun (Sol) is a Type G2 V star: a yellow, bright, normal ("dwarf" or "main-sequence") star.
- Vulcan orbits a Type M0 III star: a red, very bright, giant star.
- Rigel is a Type B8 Ia star: a blue-white, relatively dim (but stilll much brighter than Sol), large supergiant star.
Final Note: Any star except Type O can have planets, although dwarfs are unlikely to have Class K, L, M, or N planets, which vaporize during the nova phase.