63 
57] The Temperature-Luminosity Diagram 
A general discussion by Adams, Joy and Humason*, in 1925 shews that 
the absolute magnitude of the faintest of the giant M -type stars is + 0*7, 
while that of the brightest of the dwarfs is 6'9, leaving a clear gap of 6’2 
magnitudes. For stars of type M 5 or later this interval increases to about 
irO magnitudes. 
Thus the red stars fall into two distinct groups—giants, such as Betelgeux 
and Antares, which emit a large amount of radiation and so require a very 
large surface to discharge it, and dwarfs such as the two components of 
Kruger 60, which emit but little radiation and so need only small surfaces. 
The Main Sequence. 
We notice that the majority of stars in figures 5 and 6 lie on a narrow 
slant-band which runs across, the diagram. This band is commonly called 
the “Main Sequence.” Various attempts have been made to fix its exact 
position. By comparing the magnitude-differences of the two components of 
binary systems with their differences of spectral type, H 0. Redman f has 
recently obtained the following determination of the central line of the main 
sequence: 
Spectral type 
BO 
B 5 
^0 
A 5 
F 0 
F 5 
GO 
G 5 
KO 
Kb 
MO 
Mb 
M vis 
-0-62 
0-20 
1-30 
2-52 
2-80 
3-32 
4-21 
5-09 
5-17 
6-79 
7-64 
11-80 
This method has the great advantage of being practically unaffected by 
errors in the determination of stellar distance. Redman further finds that in 
the region F 0 to K 0 the spread of the main sequence is about two-thirds of 
a magnitude on either side of the central line. 
The White Dwarfs. 
When a number of stars are entered in a temperature-luminosity diagram 
of the type shewn in fig. 5, practically all are found to lie either on the main 
sequence or to the right of it. Fig. 5 shews three stars occupying isolated 
positions to the left of the main sequence. 
Stars of this type are described as “white dwarfs.” Only four are known 
with certainty, namely, the three shewn in the diagram, and the companion 
to o Ceti, to which very possibly Procyon B ought to be added These stars 
are, however, of very low luminosity, so that white dwarfs are likely to escape 
detection except when they are near to the sun. Table IV (p. 33) shews that 
there are no fewer than three white dwarfs (counting Procyon B) within 
4 parsecs of the sun, and this suggests that the white dwarfs may be common 
objects in space, although not in star-tables. 
* Report of the Director of Mount Wilson Observatory, 1925-6, p. 128. 
| M.N ., R.A.S. lxxxviii. (1928), p. 722.