Stars are classified by the characteristics of the light they emit.
Oh, Be A Fine Girl, Kiss Me
The majority of stars are of one of the following seven classes: Class O, Class B, Class A, Class F, Class G, Class K, or Class M. That strange order corresponds to the temperatures of the stars. Class O stars are the hottest, and each class is cooler than the one before it, with M being the coldest. The mnemonic "Oh, Be A Fine Guy, Kiss Me" can be used to help you remember the order.
Why the strange order, you ask? Stars were once classified on an alphabetic scale, but eventually it was discovered that seven letter classes included the vast majority of the stars. The old scale was also made before we understood the implications of the spectrum and how hot stars burn. Once those discoveries were made, the choices were either to invent a new scale and throw out years of convention, or just use the same old scale but rearranged for greater utility. The rearrange won out.
The sooner the letter appears within the O B A F G K M sequence, the hotter and brighter the star. Stars from the front of that order are bluish (with lots of UV light), and tend to burn out after short lives. The later you get in the sequence, the cooler (temperature) and more common the star is likely to be.
- Class O stars are very large, hot and bright. Their power output is a million times that of our sun. They burn out "quickly" (on the astronomical scale). They emit UV light and have a bluish tinge to our eyes. They tend to not have planets, and other stars near O-class stars lack planets as well because of photoevaporation.
- Class B stars tend to cluster in Molecular Clouds, such as many of the brighter stars in the Orion_constellation.
- Class A stars are bluish white.
- Class F stars are white, and a little hotter than our sun.
- Class G stars are yellow stars. Our sun is a Class G star. Very few supergiant stars are class G, most Gs are main sequence or dwarfs. Of main sequence stars in our part of the galaxy, about 1 in 13 stars are class G.
- Class K stars are orange in color, and a little colder than our sun.
- Class M stars make up 76% of the stars in our stellar neighborhood. Most of them are Red Dwarf stars.
The classes also indicate what types of elements and chemicals are found in the stars, but it's pretty complicated stuff and lacks much gaming application.
Stars outside those categories exist, mostly being rare types that have been discovered since the updating of the classification system. These include:
- Class W star - dying blue supergiants
- Magnetic O stars - Class O's with strong magnetic fields
- Class L star - cool dwarf stars emiting red and infrared light
- Class T star - cool methane brown dwarfs
- Class Y star - a hypothetical class of ultra-cool brown dwarfs
- Class C star - carbon stars, red giants nearing the ends of their lives
- Class S star - giant carbon-monoxide-laden stars that otherwise have traits in common with Class M and Class C
- Class D star - white dwarf stars - low mass stars that have shrunk to planet size and are cooling down
- Class P star - not actually stars, but rather planetary nebula
- Class Q star - not actually stars, but nova
It's not as simple as a single letter. More meaningfully, stars are categories by a letter/number/roman-numeral system, such as the designation of Sirius as A1V. The "A" means it's a Class A star, as above. The "1" means it's only 1/10th of the way of cooling down to the next class - in this case, Class F. The "V" indicates is a relatively small star, a dwarf or main sequence star.
By comparison, our sun is a G2V. Sirius is two full categories hotter and brighter than our sun, but in the same size range.
Class Os tend to be very large, Class M's very small, and it's common for stars in the categories in between those extremes to fall in between in size. Some exist that don't follow that trend, however, which is part of the reason for the extra classifications, such as Class C, and for the Roman Numerals at the end of the Morgan-Keenan classifications. The larger the roman numeral, the smaller the star. Super Giants are a I, Giant Stars are III, and Main Sequence or Dwarf Stars are a V.
Game and Story Use
- Useful for designing aliens and space colonies.
- Don't put your colony near a Supergiant designated O2I (for example) unless you want it to boil away.
- Class O and Class B stars probably won't burn long enough to evolve intelligent life anywhere near them (especially since O's aren't expected to have planets). Should it somehow happen, though, the resulting life forms would have to be very tolerant of heat and radiation.
- Probably has some Technobabble applications.
- Superman's powers depended on the color of the sun he was closest too. So we know he's non-super near a Class M. Does he get even more potent if he goes to a class O or B? Too bad those classes aren't likely to have intelligent life near them.