Exploring the universe

How to Classify Stars

There are different types of stars. Stars are usually classified based on their spectral characteristics. There are seven different types O, B, A, F, G, K, and M. Researchers some times use mnemonic to help them rember the order like
Oh Be A Fine Girl (or Guy), Kiss Me




\fg_c6d4ff M\odot 

\fg_c6d4ff L\odot
O Blue > 25,000 K 60 1,400,000
B Blue 11,000-25,000 K 18 20,000
A White 7,500- 1,000 K 3.2 80
F Yellow 6,000-7,500 K 1.7 6
G Yellow 5,000-6,000 K 1.1 1.2
K Orange 3,500-5,000 K 0.8 0.4
M Red < 3,500 K 0.3 0.04

K = Kelvin 1 Kelvin = -273 ° C or -459.67 Fahrenheit, it is the absolute freezing point colder can it not be, atoms would stop moving. Stars are also classified by luminosity which is the amount of energy that a star emits per unit of time. Luminosity is measured in joules per second or watts just like power. It is a measure of the brightness of the star and is usually counted in comparison with the sun \fg_c6d4ff L\odot

The solar mass is a standard unit of mass in astronomy is  denoted \fg_c6d4ff M\odot where \fg_c6d4ff {\odot } is the sun symbol, 

Type Star
Ia luminous supergiants
Ib less luminous supergiants
II bright giants
III normal giants
IV subgiants
V dwarf
VI subdwarf
VII White dwarf

Hertzsprung-Russell diagram

A famous diagram in astronomy is the Hertzsprung-Russell diagram.
The diagram is a plot of luminosity against the temperature of the star.
Both luminosity and temperature are proportional to the star mass only. Therefore stars of different mass will lie on a line this is known as the main sequence. As the stars spectral characteristics is a good indicator of temperature it is used on the horizontal axis of the diagram


Our sun is a main sequence star of class G2V (yellow dwarf), which means it has a medium temperature and normal size.