Stellar classification

Stellar classification in simple terms refers to how stars are categorised. Stars are categorised based off of their spectral characteristics (elements they absorb) as well as temperature. There are 7 classes listed below in order of decreasing temperature :

- O
- B
- A
- F
- G
- K
- M

Stars that fall under the first two classes are quite infrequent but are very luminous. M stars, on the other hand, are common but not as bright as O and B stars.

Hertzsprung - Russel diagram

Also referred to as HRD or simply H-R diagram, this is a scatter plot of a stars' absolute magnitude (how bright they seem in the sky, obviously a quantitative value) versus their temperature (colour as well as spectral type). Astronomers commonly plot star's colour, temperature, spectral type, luminosity and evolutionary stage. The diagram shows the 3 distinct types of stars.

Supergiants - As stars approach the end of their life, they become giants and supergiants. These types of stars have diminished all hydrogen and are extremely old. This eventually leads to an event known as a supernova, where the star explodes and eventually the star either becomes a neutron star, black hole or white dwarfs depending on it's mass.

Main sequence stars : Most stars including the sun are main sequence stars and are driven by nuclear fusion, where nuclei of atoms merge to make a larger and different element. The hotter they are, the brighter they are. Stars usually are a part of this stage for roughly 5 billion years; it is considered to be the most stable stage.

The Sun is a as a G2V type star (G type main sequence)

White dwarfs: White dwarfs are typically planet-sized stars that are therefore quite dense. These are formed when stars of low mass (such as our sun) have exhausted their hydrogen and no longer undergo nuclear fusion.

Spectral classes

Star type	Colour	Surface temperature (K)	Average mass (the sun = 1, i.e how many times greater)	Average luminosity (the sun = 1)
O	Blue	25000+	60	1400000
B	Blue	11000 - 25000	18	20000
A	Blue	7500 - 11000	3.2	80
F	Blue - White	6000 - 7500	1.7	6
G	White - Yellow	5000 - 6000	1.1	1.2
K	Orange - Red	3500 - 5000	0.8	0.4
M	Red	Less than 3500	0.3	0.04 (extremely faint)