359. _New Star of 1866._--The most striking case of this kind in recent times was in May, 1866, when a star of the second magnitude suddenly appeared in _Corona Borealis_. On the 11th and 12th of that month it was observed independently by at least five observers in Europe and America. The fact that none of these new stars were noticed until they had nearly or quite attained their greatest brilliancy renders it probable that they all blazed up very suddenly. 360. _Cause of the Variability of Stars._--The changes in the brightness of variable and temporary stars are probably due to operations similar to those which produce the spots and prominences in our sun. We have seen (188) that the frequency of solar spots shows a period of eleven years, during one portion of which there are few or no spots to be seen, while during another portion they are numerous. If an observer so far away as to see our sun like a star could from time to time measure its light exactly, he would find it to be a variable star with a period of eleven years, the light being least when we see most spots, and greatest when few are visible. The variation would be slight, but it would nevertheless exist. Now, we have reason to believe that the physical constitution of the sun and the stars is of the same general nature. It is therefore probable, that, if we could get a nearer view of the stars, we should see spots on their disks as we do on the sun. It is also likely that the varying physical constitution of the stars might give rise to great differences in the number and size of the spots; so that the light of some of these suns might vary to a far greater degree than that of our own sun does. If the variations had a regular period, as in the case of our sun, the appearances to a distant observer would be precisely what we see in the case of a periodic variable star. The spectrum of the new star of 1866 was found to be a continuous one, crossed by bright lines, which were apparently due to glowing hydrogen. The continuous spectrum was also crossed by dark lines, indicating that the light had passed through an atmosphere of comparatively cool gas. Mr. Huggins infers from this that there was a sudden and extraordinary outburst of hydrogen gas from the star, which by its own light, as well as by heating up the whole surface of the star, caused the extraordinary increase of brilliancy. Now, the spectroscope shows that the red flames of the solar chromosphere (197) are largely composed of hydrogen; and it is not unlikely that the blazing-forth of this star arose from an action similar to that which produces these flames, only on an immensely larger scale. Distance of the Stars.