on: Fig. 40. Spiral nebula in Ursa Major (Ritchey).

Luminous matter, in every variety of physical and chemical state,
is available for study in the most diverse celestial objects, from
the spiral and irregular nebulae through all the types of stars.
Doctor van Maanen's measures of the Mount Wilson photographs indicate
outward motion along the arms of spiral nebulae, while the spectroscope
shows them to be whirling at enormous velocities.]

COSMIC PRESSURES

This, it may fairly be said, is very speculative, but the fact
remains that celestial bodies appear to be the only places in which
the complex elements may be in actual process of formation from their
known source--hydrogen. At least we may see what a vast variety
of physical conditions these cosmic crucibles afford. At one end of
the scale we have the excessively tenuous nebulae, the luminosity of
which, mysterious in its origin, resembles the electric glow in our
vacuum tubes. 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. If the German gun that bombarded Paris were loaded with a
solid steel projectile of suitable dimensions, a muzzle velocity
of 6,000 feet per second could be reached. Suppose this