Here we can detect only the lightest and simplest of
the elements. In the giant stars, also extremely tenuous (the density
of Betelgeuse can hardly exceed one-thousandth of an atmosphere) we
observe the spectra of iron, manganese, titanium, calcium, chromium,
magnesium, vanadium, and sodium, in addition to titanium oxide.
The outer part of these bodies, from which light reaches us, must
therefore be at a temperature of only a few thousand degrees, but
vastly higher temperatures must prevail at their centres. In passing
up the temperature curve more and more elements appear, the surface
temperature rises, and the internal temperature may reach millions
of degrees. At the same time the pressure within must also rise,
reaching enormous figures in the last stages of stellar life. Cook
has calculated that the pressure at the centre of the earth is
between 4,000 and 10,000 tons per square inch, and this must be
only a very small fraction of that attained within larger celestial
bodies. Jeans has computed the pressure at the centre of two colliding
stars as they strike and flatten, and finds it may be of the order
of 1,000,000,000 tons per square inch--sufficient, if their diameter
be equal to that of the sun--to vaporize them 100,000 times over.
Compare these pressures with the highest that can be produced on
earth.
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