e it is an ocean of red-hot hydrogen, and outside this again is
an atmosphere stretching for some hundreds of thousands of miles into
space.

The significant feature, from our point of view, is the "sun-spot";
though the spot may be an area of millions of square miles. These areas
are, of course, dark only by comparison with the intense light of the
rest of the disk. The darkest part of them is 5000 times brighter than
the full moon. It will be seen further, on examining a photograph of
the sun, that a network or veining of this dark material overspreads the
entire surface at all times. There is still some difference of opinion
as to the nature of these areas, but the evidence of the spectroscope
has convinced most astronomers that they are masses of cooler vapour
lying upon, and sinking into, the ocean of liquid fire. Round their
edges, as if responding to the pressure of the more condensed mass,
gigantic spurts and mountains of the white-hot matter of the sun rush
upwards at a rate of fifty or a hundred miles a second, Sometimes they
reach a height of a hundred, and even two hundred, thousand miles,
driving the red-hot hydrogen before them in prodigious and fantastic
flames. Between the black veins over the disk, also, there rise domes
and columns of the liquid fire, some hundreds of miles in diameter,
spreading and sinking at from five to twenty miles a second. The surface
of the sun--how much more the interior!--is an appalling cauldron of
incandescent matter from pole to pole. Every yard of the surface is
a hundred times as intense as the open furnace of a Titanic. From the
depths and from the surface of this fiery ocean, as, on a small scale,
from the surface of the tropical sea, the vapours rise high into the
extensive atmosphere, discharge some of their heat into space, and sink
back, cooler and heavier, upon the disk.

This is a star in its yellow age, as are Capella and Arcturus and other
stars. The red stars carry the story further, as we should expect. The
heavier lines in their spectrum indicate more absorption of light, and
tell us that the vapours are thickening about the globe; while compounds
like titanium oxide make their appearance, announcing a fall of
temperature. Below these, again, is a group of dark red or "carbon"
stars, in which the process is carried further. Thick, broad, dark lines
in the red end of the spectrum announce the appearance of compounds
of carbon, and a still lower fall of tempe