| O | Mostly high ionized silicon, nitrogen, plus ionized helium. | Oh |
| B | Neutral helium, no ionized helium, Balmer lines strenghten. | Be |
| A | Balmer lines peak at A0, single ionized calcium and metals appear. | A |
| F | Balmer and ionized metall weaken, neutral metals strengthen. | Fine |
| G | Balmer lines continue to weaken, ionized calcium lines peak in intensity. | Girl |
| K | Many lines due to neutral metals, TiO starts to appear. | Kiss |
| M | TiO bands dominate. | Me |
The classification system with the letters O B A F G K M is a classification in terms of temperature, decreasing form O to M.
Every class is subdivided in decimal subclasses like B9, A4,
K3. Not all decimal subdivisions are used in every major class. We
won't discuss this here in detail. If we can determine the subclass
from our spectra this is explained in the spectral classes .
The Sun for example with
a surface temperature of 5700 K is classified as G2 star.
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