The Outline of Science, Volume 1

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[Pg 37] the monarch of the system will summon it again to make its fiery journey round his throne. 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. 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. Photo: Astrophysical Observatory, Victoria, British Columbia. FIG. 23.—STAR CLUSTER IN HERCULES A wonderful cluster of stars. It has been estimated that the distance of this cluster is such that it would take light more than 100,000 years to reach us.