Stellar Spectral Types

What is the most important thing to know about a star? Its brightness ( or apparent magnitude) might top the list, but right behind would be its spectral type. Without its spectral type, a star is a meaningless dot. Add a few letters and letters such as "O" or "G" and some numbers, the star suddenly gains personality and character. And such letters in a star's name, tells its spectral code and much other information related to it. To those who understand its meaning, the spectral code tells at a glance just what kind of object the star really is: its color, size, and luminosity compared to the Sun and stars of all other types, even its history and its future.

Specifically, the spectral classification system is based on two physical properties that imprint themselves on the spectrum: a star's surface temperature and atmospheric pressure. Stars can be classified by their surface temperatures as determined from Wien's Law, but this poses practical difficulties for distant stars. Spectral characteristics offer a way to classify stars which gives information about temperature in a different way -- particularly absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. The standard classes are:

Temperature

O 30,000 - 60,000 K 

B 10,000 - 30,000 K

A7,500 - 10,000 K

F 6,000 - 7,500 K

G 5,000 - 6,000 K

K 3,500 - 5,000 K

M < 3,500 K

Furthermore, stars that are classified into different groups have various colors that match with their temperatures; hotter a star is, bluer a star will be, and inversely, cooler a star is, redder a star will be. In other words, the spectral sequence O-M represents stars of essentially the same chemical composition but of different temperatures and atmospheric pressures. Specifically, the temperature sets the star's color and determines its surface brightness: how much light comes from each square meter of its surface. The atmospheric pressure depends on the star's surface gravity and therefore, roughly, on its size. The size and surface brightness in turn yield the star's luminosity and often its evolutionary status ( young, nearing death, or in between). 

Micheal Briley, University of Wisconsin, Oshkosh

As the graph showcases, stellar spectra from hot to cool. A star's temperature largely determines which dark absorption lines appear in its spectrum. 

Citations

http://abyss.uoregon.edu/~js/glossary/spectral_types.html