They generally burst into fragments and fall without doing damage.

It is clear that "empty space" is, at least within the limits of our
solar system, full of these things. They swarm like fishes in the seas.
Like the fishes, moreover, they may be either solitary or gregarious.
The solitary bit of cosmic rubbish is the meteorite, which we have just
examined. A "social" group of meteorites is the essential part of a
comet. The nucleus, or bright central part, of the head of a comet (Fig.
19) consists of a swarm, sometimes thousands of miles wide, of these
pieces of iron or stone. This swarm has come under the sun's
gravitational influence, and is forced to travel round it. From some
dark region of space it has moved slowly into our system. It is not then
a comet, for it has no tail. But as the crowded meteors approach the
sun, the speed increases. They give off fine vapour-like matter and the
fierce flood of light from the sun sweeps this vapour out in an
ever-lengthening tail. Whatever way the comet is travelling, the tail
always points away from the sun.

A Great Comet

The vapoury tail often grows to an enormous length as the comet
approaches the sun. The great comet of 1843 had a tail two hundred
million miles long. It is, however, composed of the thinnest vapours
imaginable. Twice during the nineteenth century the earth passed through
the tail of a comet, and nothing was felt. The vapours of the tail are,
in fact, so attenuated that we can hardly imagine them to be white-hot.
They may be lit by some electrical force. However that may be, the comet
dashes round the sun, often at three or four hundred miles a second,
then may pass gradually out of our system once more. It may be a
thousand years, or it may be fifty years, before the monarch of the
system will summon it again to make its fiery journey round his throne.

[Illustration: _Photo: Harvard College Observatory._

FIG. 21.--TYPICAL SPECTRA

Six main types of stellar spectra. Notice the lines they have in common,
showing what elements are met with in different types of stars. Each of
these spectra corresponds to a different set of physical and chemical
conditions.] [Illustration: _Photo: Mount Wilson Observatory._

FIG. 22.--A NEBULAR REGION SOUTH OF ZETA ORIONIS

Showing a great projection of "dark matter" cutting off the light from
behind.]

[Illustration: _Photo: Astrophysical Observatory, Victoria, British
Columbia._

FIG. 23.--STAR CLUSTER I