14. Let us consider, however, further, how far astronomy authorizes us to regard the Fixed Stars as being, like our Sun, the centres of systems of Planets. Those who hold this, consider them as having a permanent condition of brightness, as our Sun has had for an indefinite period, so far as we have any knowledge on the subject. Yet, as we have said, no small number of the stars undergo changes of brightness; and some of them undergo such changes, in a manner which is not discernibly periodical; and which must therefore be regarded as progressive. This phenomenon countenances the opinion of such a progress from one material condition to another; which, we have seen, is suggested by the analogy of the probable formation of our own solar system. The very star which is so often taken as the probable centre of a system, Sirius, has, in the course of the last 2,000 years, changed its light from red to white. Ptolemy notes it as a red star: in Tycho's time it was already, as it is now, a white one.[10] The star _Eta Argus_ changes both its degree of light and its color; ranging, in seemingly irregular intervals of time, from the fourth to the first magnitude,[11] and from yellow to red. Several other examples of the like kind have been observed. Mr. Hind[12] gives an example in which he has, quite recently, observed in two years a star change its color from very red to bluish. These variable unperiodical stars are probably very numerous. Also, some stars, observed of old, are now become invisible. "The lost Pleiad," by the loss of which the cluster, called the Seven Stars, offers now only six to the naked eye, is an example of a change of this kind already noted in ancient times. There are several others, of which the extinction is recognized by astronomers as proved.[13] In other cases, new stars have appeared, and have then seemed to die away and vanish. The appearance of a new star in the time of the Greek astronomer Hipparchus, induced him to construct his famous Catalogue of the Stars. Others are recorded to have appeared in the middle ages. The first which was observed by modern astronomers was the celebrated star seen by Tycho Brahe in 1572. It appeared suddenly in the constellation Cassiopeia, was fixed in its place like the neighboring stars, had no nebula or tail, exceeded in splendor all other stars, being as bright as Venus when she is nearest the earth. It soon began to diminish in brightness, and passing through various diminishing degrees of magnitude, vanished altogether after seventeen months. This star also passed through various colors; being first white, then yellow, then red. In like manner, in 1604, a new star of great magnitude blazed forth in the constellation Serpentarius; and was seen by Kepler. And this also, like that of 1572, after a few months, declined and vanished.