Now we can say more. Now the spectroscope, by breaking up and dividing for us the slender ray of light traveling from each star, has shown to us something of the nature of the stars. Now we know that the stars, like our sun, are burning bodies, surrounded by atmospheres full of burning gas. It has even been found out that in Sirius there are large quantities of sodium and magnesium, besides other metals, and an abundant supply of hydrogen gas. CHAPTER XXIV. DIFFERENT KINDS OF SUNS. Various in kind, various in size, various in color, various in position, various in motion, are the myriad suns scattered through space. So far are they from being formed on the same plan, turned out on the same model, that it may with reason be doubted whether any two stars could be found exactly alike. Why should we expect to find them so? No two oak-leaves, no two elm-leaves, precisely alike, are to be found upon earth. So some stars are large, some are small. Some are rapid in movement, some are slow. Some are yellow, some white, some red, green, blue, purple, or gray. Some are single stars; others are arranged in pairs, trios, quartets, or groups. Some appear only for a time, and then disappear altogether. Others are changeful, with a light that regularly waxes and wanes in brightness. We have now to give a little time and thought to variable stars and temporary stars; afterwards to double stars and colored stars. There are many stars which pass through gradual and steady changes--first brightening, then lessening in light, then brightening again. One such star is to be seen in the constellation of the Whale. It is named “Mira,” or “The Marvelous,” and the time in which its changes take place extends to eleven months. For about one fortnight it is a star of the second magnitude. Through three months it grows slowly more and more dim, till it becomes invisible, not only to the naked eye, but through ordinary telescopes. About five months it remains thus. Then again, during three months, it grows brighter and brighter, till it is once more a second-magnitude star, and after a fortnight’s pause begins anew to fade. At its maximum, this star is yellow; when it is faint, it is reddish. Spectrum analysis shows in it a striped spectrum of the third type, and when its light diminishes it preserves all the principal bright rays reduced to very fine threads. The most plausible explanation of this variability is to suppose that it periodically emits vapors similar to the eruptions observed in the solar photosphere. Instead of its periodicity being like that of the sun--eleven years and hardly perceptible--this variation of the sun of the Whale is three hundred and thirty-one days and very considerable. It is subject to oscillations and irregularities similar to those which we remark in our sun. Of all the variable stars, this is the easiest to observe, and it has been known for nearly three hundred years.