The Sizes and Masses of the Stars 73
space, give out respectively 1/20 and 1/40 of the Sun’s
light. Their spectra are K7 and K8, and their probable
surface brightness something like 1/30 and 1/35 of the
Sun’s, which would make their diameters about 1,100,000
and 800,000 miles. A typical dwarf М-star may be taken
as giving 1/150 the Sun’s light, which, with a surface
brightness of 1/50, would make its diameter 60 per cent,
of the Sun’s, or 500,000 miles. Finally, we may take
“Barnard’s star,” the nearest after the system of Alpha
Centauri, which is only 1/2700 as bright as the Sun. Its
spectrum is of an advanced M type, and we may perhaps
estimate its surface brightness as 1/300 of the Sun’s,
which would make its diameter some 300,000 miles. All
these are dwarf stars, and have been chosen as fairly
typical examples. The conclusion that such stars were
usually from a couple of million to half million miles in
diameter would be confirmed by further evidence.
For the giant stars, however, things must be very dif-
ferent. Take first the stars in Orion’s belt, which aver-
age 4000 times as bright as the Sun. They are of Class
B0, and their surface brightness may be estimated as
thirty times the Sun’s, which would correspond to a dia-
meter of just about ten million miles. For Rigel, of
Class B8, and 13,000 times the Sun’s luminosity, the
surface brightness may be estimated as 12 times the Sun’s
and the diameter about 30 million miles. Arcturus, a
good example of the ordinary run of the redder giants,
is about 90 times as bright as the Sun. Taking its sur-
face brightness as 1/7 of the Sun’s, since it is of Class
K0, we find a diameter of 25 million miles. At the ex-
treme of size come those stars which are at the same
time extremely bright and very red. Betelgeuse, in
Orion, is of Class M, and, according to the best meas-