51-53] Spectral Type 53 
The characteristics of the various types of spectra are briefly indicated in 
the following table: 
Type 
Elements shewn 
Typical stars 
Probable 
effective 
Temp. 
0 
Ionised helium. Doubly and 
trebly ionised oxygen and 
nitrogen 
Oa BD + 35° 4013 
Öd X Cephei 
Oe 5 X Orionis 
Oe 8 Plaskett’s star 
) About 
j 30,000 (?) 
28,000 
B 
Hydrogen and Helium (strong) 
ionised silicon, oxygen, ni 
trogen, magnesium and 
calcium 
BO e Orionis 
Bl V Puppis 
B 8 Rigel 
23.000 
22.000 
15,000 
A 
Hydrogen (strong), ionised 
and neutral metals (weak) 
A 0 Sirius 
A 2 Deneb 
A 5 Altair 
11,200 
10,900 
8,600 
F 
Ionised calcium (very strong), 
hydrogen and metallic lines 
F 0 Canopus 
F 5 Procyon 
7,500 
8,000 
G 
Neutral metals and ionised 
calcium, hydrogen (weak) 
„ n f Capella A 
Cr0 \Sun 
5,650 
6,000 
K 
Neutral metals and ionised 
calcium (very strong), 
hydrogen (very feeble). At 
Kb titanium oxide bands 
begin to appear 
K 0 Arcturus 
K 5 Aldebaran 
Ä’7 61 Cygni 
4,200 
3,300 
4,000 
M 
Titanium oxide (strong). 
Continuous spectrum very 
weak at violet end 
M 0 Betelgeux 
Mi 3 Krüger 60 
J/6 a Herculis 
3,000 
3,200 
2,500 
The table shews that the sequence of spectral types can be regarded 
not only as one of decreasing temperature, but also as one of decreasing 
ionisation or increasing aggregation. In class 0 trebly and doubly ionised 
atoms are prominent; later we come to singly ionised atoms, then to complete 
atoms and finally to complete molecules exhibiting band-spectra. The complete 
molecular structures are built up as the temperature diminishes. 
53. One reason why a star’s spectrum cannot conveniently be specified 
simply as a temperature, is that the spectrum of a mass of hot gas does not- 
depend solely on its temperature; it depends to an appreciable degree on its 
density as well. The theoretical reason for this is provided at once by the work 
of Saha, R. H. Fowler and Milne, to which reference has already been made. We 
shall discuss the theory of ionisation more fully below (§ 137) when it will be 
seen that the ionisation temperature for the atoms of any particular element 
depends very largely on the density of the matter of which the atoms form 
part. Thus stars of the same spectral class have different effective tempera 
tures when their atmospheres have different densities, the differentiation 
being most marked when the effective temperature is low. As a consequence,