Stellar Classification

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

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.

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?)

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!

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

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

Star class: F Colour = blue to white Surface Temperature = 6000 -7500 K Average mass = 1.7 solar masses Average radius = 1.3 solar radii Strong in H, some Ca

Star class: G Colour = white to yellow Surface Temperature = 5000 - 6000 K Average mass = 1.1 solar masses Average radius = 1.1 solar radii Neutral and ionized metals Example: Capella

Star class: K Colour = orange to red Surface Temperature = 3500 - 5000 K Average mass = 0.8 solar masses Average radius = 0.9 solar radii Strong in metals Example: Arcturus

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