SURVEY OF THE PROBLEM
from studies of absolute magnitude and spectral type and more directly
from the calculated densities of eclipsing variables. The confirmation that
is most easily grasped is afforded by the recent interferometer measure
ments of the angular diameters of stars at Mount Wilson. These show that
certain stars such as Betelgeuse, Antares and o Ceti, are of enormous bulk,
capable of containing the whole orbit of the earth inside them. We are
therefore compelled to extend our ideas of the nature of stars beyond
anything that would be suspected from knowledge of the sun.
The great bulk of these giant stars is due to low density rather than
great mass. Betelgeuse for example has a radius of the order 250 million
km. and a volume 50 million times greater than the sun. But the mass, or
amount of matter contained in it, is probably between 10 and 100 times
greater, so that the density is about a million times less. It is rather
interesting to notice that Einstein’s theory of gravitation has something
to say on this point. According to it a star of 250 million km. radius
could not possibly have so high a density as the sun. Firstly, the force of
gravitation would be so great that light would be unable to escape from
it, the rays falling back to the star like a stone to the earth. Secondly,
the red-shift of the spectral lines would be so great that the spectrum
would be shifted out of existence. Thirdly, the mass would produce so
much curvature of the space-time metric that space would close up round
the star, leaving us outside (i.e. nowhere). The second point gives a more
delicate indication and shows that the density is less than 0-001; for even
at that density there would be a red-shift of the spectrum too great to be
concealed by any probable Doppler effect.
Lest this argument should be regarded by our more conservative
readers as ultra-modern, we hasten to add that it is to be found in the
writings of Laplace—
A luminous star, of the same density as the earth, and whose diameter
should be two hundred and fifty times larger than that of the sun, would not,
in consequence of its attraction, allow any of its rays to arrive at us; it is
therefore possible that the largest luminous bodies in the universe may, through
this cause, be invisible*.
7 . For many years Lane’s discovery had little effect on the accepted
theories of stellar evolution. Sir Norman Lockyer accepted it and accord
ingly classified the stars in an ascending and descending temperature
sequence; but he was almost alone in his views. Astrophysicists in general
regarded the hottest stars as the earliest and the coolest stars as the latest
in order of development!. Probably they did not realise that any of the
I am indebted to Dr H. Jeffreys
* Laplace, Système du Monde, Book 5, Cp. vi.
for this reference.
t The expressions “early” and “late” type of spectrum are still commonly
employed for high-temperature types (B and A) and low-temperature types (K and
M) respectively.
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