Stars have many sizes, ages, and temperatures. Most stars do not stay in the same class for their entire life. As a star evolves, it's fuel changes and as a result, its average surface temperature and its size will change. Most stars are main sequence stars. These are the medium mass stars. They progress through similar stages as they evolve.
Their common names of stars, based on size, are hypergiants, supergiants, bright giants, giants, main sequence dwarfs (most stars), white dwarfs, and subdwarfs.
Spectral Classification
The most widely used system of stellar classification is spectral classification. Stars radiate light at different frequencies. As nuclear fusion occurs within a star, first hydrogen is used as fuel. The hydrogen is fused into helium, and eventually the star will begin to use helium as fuel. The helium becomes lithium, and so on, and each new, heavier fuel is used in succession. Each element radiates light in a different areas of the spectrum. Since each element radiates a unique set of frequencies, this is like a signature that astrophysicists can use to understand what makes up a particular star.
Common Star Classes
Here are the spectral classifications most commonly used. This system has been refined several times in the past century, so not all the letters are used sequentially as duplicate classifications where removed, and newer ones inserted. Temperature is an important factor (measured in degrees Kelvin).
- Class A - bluish white, main sequence, 7,500K to 10,000K
- Class B - very luminous, blue, short lived main sequence, 10,000K to 30,000K
- Class C - giant and super giant stars near the end of life, with lots of carbon in the stellar atmosphere 3,500K to 4,500K
- Class D - white dwarfs, at the end of their life, temperature ranges widely from near ambient to 100,000K
- Class F - white/yellow main sequence stars, 6,000K to 7,500K
- Class G - yellow main sequence stars (our Sun is a G), 5,000K to 6,000K
- Class K - cooler orange stars, usually main sequence, but sometimes giant and supergiant, 3,500K to 5000K
- Class M - the most common type of star, red dwarfs but also some giants and supergiants, 2,000K to 3,500K
- Class L - dark red dwarfs - some not capable of fusion but still radiate energy, 1,300K to 2,500K
- Class O - extremely hot, big, blue main sequence stars, very rare because stars don't stay in this stage for long, 30,000K to 60,000K
- Class P - non-stellar, planetary nebulae
- Class Q - non-stellar, novae (exploded stars)
- Class S - cooler range than M, but hotter than C, mainly giants and supergiants
- Class T - very cool methane dwarfs, they do not emit visible light, but emit infrared light (lower frequency), 1,000K
- Class W - Wolf-Rayet stars, a new classification for dying supergiants, up to 70,000K
- Class Y - ultra cool dwarfs, predicted but not currently detected
