1. Stellar Classification

2. Like living things, stars can be divided into different categories.
The categories are based mostly on their spectral lines (composition) and temperature.
There are 7 main classifications:
O, B, A, F, G, K, M (Oh be a fine girl, kiss me!) in order of decreasing temperature.
and more subclasses (I, II, III,…, VII) based on brightness and size

3. Spectral Lines and Temperature
When an atom is in a hot environment, it is more likely to lose electrons and become an ion.
As a result, ions of atoms that are more difficult to ionize can only be found in hotter stars.
For example, He is extremely difficult to ionize, so it is found in only the hottest stars (O, and hot B)
Ions have different absorption spectra than their neutral atoms, so they can be identified using the star’s absorption spectra.

4. Spectral Lines and Temperature
Metals are easier to ionize, so they are found in mid to low temperature stars (A, F, G, K)
Molecules can only be formed under low temperature conditions (cool K, and M)
Even neutral He atoms are only found in the spectral lines of hotter stars (cool O, B, hot A)
Notice that neutral Hydrogen is found in most star classes (except for O  why?)

5. Star class: O Colour = blue Surface Temperature = over 25 000 K
Average mass = 60 solar masses
Average radius = 15 solar radii
Contains ionized He
Very rare!

6. Star class: B Colour = blue Surface Temperature = 11 000 - 25 000 K
Average mass = 18 solar masses
Average radius = 7 solar radii
Contains neutral He
Rare
Example: Vega

7. Star class: A Colour = blue Surface Temperature = 7500 – 11 000 K
Average mass = 3.2 solar masses
Average radius = 2.5 solar radii
Strong in H
Example: Sirius

8. Star class: F Colour = blue to white
Surface Temperature = K
Average mass = 1.7 solar masses
Average radius = 1.3 solar radii
Strong in H, some Ca

9. Star class: G Colour = white to yellow
Surface Temperature = K
Average mass = 1.1 solar masses
Average radius = 1.1 solar radii
Neutral and ionized metals
Example: Capella

10. Star class: K Colour = orange to red
Surface Temperature = K
Average mass = 0.8 solar masses
Average radius = 0.9 solar radii
Strong in metals
Example: Arcturus

11. Star class: M Colour = red Surface Temperature = under 3500 K
Average mass = 0.3 solar masses
Average radius = 0.4 solar radii
Contains some molecules (titanium oxide)
Example: Betelguise