Page:Astronomy for Everybody.djvu/319

When the spectra of stars are carefully compared, it is always found that hardly any two are exactly alike. This shows that their atmospheres all differ in their physical constitution, or in the temperature of the substances which compose them. A great number of the dark lines of their spectra are found to be identical with those produced by known substances on earth. This shows that the substances of which the stars are made up are identical, in at least a great part, with those on the earth.

One of the most abundant of these substances is hydrogen. Several lines of hydrogen are found in nearly all the stars. Another substance which seems to be almost universal throughout the universe is iron. Yet another is calcium, the metallic base of lime. We all know that this substance abounds on the earth, and we have, in its diffusion among the stars, an example of the unity of nature in its widest extent.

Yet, variety is also the rule. Besides lines due to known substances, many stars show lines which have not yet been identified with those of any element that we know of. This is especially the case in the class known as Orion stars, because many of them are found in the constellation Orion. These stars are mostly very white or even blue in colour, and show a number of fine dark lines which are to a greater or less extent the same in all Orion stars, but are not those produced by any known chemical element. We therefore have reason to believe that there are in the stars other chemical elements than those with which we are acquainted.

There is a very curious case in which an element first