The spectral classes

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The following table is a summary of the main characteristics of stars, and their relative abundance within our galaxy.
The parameters of mass, radius and luminosity of stars are expressed in relation with the Sun.
These parameters are only relevant for main sequence stars, not for supergiant ones.
As the spectral class is only based upon the surface temperature of a star, this table can only define one point on the Hertzprung-Russel diagram.

The W class corresponds to the Wolf-Rayet stars.

Spectral
class
Surface
temperature
(degrees K)
Mass Radius Luminosity Life time
(million of years)
Relative abundance
(in %)
W 50.000 >40 20 1.000.000 <1 negligible
O5 40.000 32 18 600.000 1 0.00002
B0 28.000 16 7.4 16.000 10   
B5 15.500 6.5 3.8 600 100 0.1
A0 9.900 3.2 2.5 60 500  
A5 8.500 2.1 1.7 20 1.000 1
F0 7.400 1.75 1.4 6 2.000  
F5 6.600 1.25 1.2 3 4.000 3
G0 6.000 1.06 1.1 1.3 10.000  
G2 Sun 5.800 1 1 1 12.000  
G5 5.500 0.92 0.9 0.8 15.000 9
K0 4.900 0.80 0.8 0.4 20.000 (1)  
K5 4.100 0.69 0.7 0.1 30.000 14
M0 3.500 0.48 0.6 0.02 75.000  
M5 2.800 0.20 0.3 0.001 200.000 73
C ~2500 ~5/10 (2) (2) 0.1 negligible
S ~2000 ~5/10 (2) (2) 0.1 negligible

(1) The estimated lifetime of the universe is less than 20 billion years, so no K or M class star is yet dead.

(2) These stars are often variable. They can sometimes get up to 1000 solar radii. Similarly, their luminosity can vary between 30 and 30,000 times the Solar luminosity.

The majority of stars are small stars, in the classes M and K.
Giant and supergiant stars are very rare, and their short lifetimes emphasizes this rarity.

 


Besides these physical features, we must notice spectral characteristics : spectra of stars in the same class show the same radiation lines. In fact, two stars with the same spectral type are quite similar in physical features.

Class Prominent lines Other lines
W Ionized helium, hydrogen Carbon and Oxygen (class WC)
Nitrogen (class WN)
O Ionized Helium Neutral helium, weak hydrogen lines
B Neutral Helium Hydrogen lines growing in B6 to B9
A Strong hydrogen lines in A0, decreasing towards A9 ionized calcium lines stronger and stronger from A0 to A9
F Stronger ionized calcium lines  
G Strong ionized calcium lines Metals, specially iron
K Strong neutral metals lines CH and CN molecules
M Absorption lines of titanium oxyde Many metal lines
C Neutral metals Carbon
S Neutral metals Zirconium oxyde, Yttrium, Baryum

 


References :
The Harvard Spectral Sequence
The Classification of Stellar Spectra (J.S. Allen)