The overall color of a star is an indication of its surface temperature. Hotter stars emit more energy towards the blue end of the spectrum relative to the other colors and thus appear blue, while cooler stars emit more energy towards the red end.
Astronomers have devised a classification scheme based on letters
of the alphabet. The original scheme used the following letters:
Class | Color | Temperature (K) |
O | blue | 25,000 - 40,000 |
B | blue-white | 10,000 - 25,000 |
A | white | 7,500 - 10,000 |
F | yellow-white | 6,000 - 7,500 |
G | yellow | 5,000 - 6,000 |
K | orange | 3,500 - 5,000 |
M | orange-red | 2,000 - 3,500 |
These classes were subsequently divided into 10 divisions by appending a number from 0 to 9. Thus F0 is the hottest within the F class, followed by F1, then F2, and on eventually to F9, which is followed by G0. In this scheme our sun is designated as a G2 star.
Later this scheme was extended on the cool end with the addition
of three minor classes: R, N, and S. Recently the R and N
class have been replaced by the C class. There is also a W subclass
of O for extremely hot white stars with expanding gas shells.