Variable Stars.— To ordinary observations, even to observations extending through genera tions, the light of the stars seem to be as per manent as the order of nature itself. But, although this is true of the great majority of stars which stud the heavens, it is now found that there are exceptions to the rule. So nu merous have these exceptions become that the study of variable stars has developed into one of the important branches of astronomy. The first star which was found to be variable in its light is known as Mira Ceti, or the wonder of the constellation Cetus, the Whale. This con stellation extends from two hours to three hours of right ascension, and its central regions are situated a little south of the equator. It is on the meridian at some hour in the evenings of the winter months. The star Mira is quite invisible to the naked eye the greater part of the time, but is found to blaze up at regular but slightly varying intervals of about 11 months, some times attaining to the second magnitude. After remaining visible to the naked eye for a few days, it gradually fades away into invisibility, but may always be seen in a telescope. Its greatest brightness varies markedly. Some times it brightens up only to the fifth magni tude, and is then not at all conspicuous.
There are two other remarkable variable stars. One is Algol in the constellation Per seus, which may be seen in some part of the northern sky at some hour on almost any night of the year except in April and May. Com monly this star is between the second and third magnitude, and is usually classified as of the second magnitude. At regular intervals of a little less than three days, it fades away to the fourth magnitude, and then, after a few hours, gradually increases again to its regular bright ness. It is now known that this diminution of light is a partial eclipse of the star; the brighter star is about 51 times as bright as our own sun, while the so-called "dark" companion is about three and one-third times as bright. Thus the companion is very far from being a dark body, but its interposition so greatly re duces the light which we receive from the brighter star that the effect of an eclipse is produced.
In another class of variable stars these two bodies are each of noticeable brightness so that the light which we receive from them varies twice in each revolution, once when the brighter star partially hides the fainter one, and once when it is itself partly hidden. Strictly speak ing, the system of Algol should he now included in this class since by the employment of the delicate selenium photometer the light of its companion can he definitely measured.
The most remarkable of this class is Beta Lyrae, quite near the bright star Vega. It may he seen in some part of the northern sky dur ing the spring, summer and autumn. Although the changes in its light are not striking, one who looks at it carefully night after night will notice them.
These three stars are only examples of a number of such objects now known to exceed 1,000, scattered over the whole sky. As obser vations become more and more accurate, it is found that quite a number of the stars visible to the naked eye are subject to slight fluctua tions of light, mostly a more or less regular period, but sometimes in a quite irregular way. Spectroscopic observations render it somewhat probable that in most cases the regular changes are produced by bright or dark bodies revolv ing round the star in very eccentric orbits. Sometimes they partially eclipse the star, as we have already mentioned; hut in many other cases it seems likely that they make the star blaze up by the disturbance they make when they approach close to it in their orbits.
The 'general rule is that a variable star goes through its series of changes in a regular period. At a certain day and hour it reaches its least brightness, which is called a minimum. Then it begins to increase in light. In the course of a certain number of hours or days it increases to its greatest brightness; then it be gins to decrease and once more fades away to its minimum. The length of time between two minima is a period. Commonly the successive periods of a variable star are nearly equal to each other, so that the stage of brightness at any future time can be predicted with confi dence. The periods range very widely from a few hours to about 600 days. It is very re markable that no period yet known exceeds two years. There are many periods less than a month; but there are only eight between 50 and 100 days. The number then increases up to 350 days. The greatest number of periods is just about one of our years, a coincidence which of course is a pure accident. Above this their frequency rapidly falls off. This limitation of the period strengthens the view that the vari ability of a great number of stars is produced by the disturbing action of an immense body revolving round the star in a very eccentric orbit.
New Stars.— In past generations, when the whole universe was supposed to be only a few thousand years old, it was quite natural to expect that new stars would from time to time be created to shine among their fellows. But this view has now to be modified. It is true that, from time to time, a star is seen to blaze out where none was known before. But the theory of a new creation is refuted by the fact that such stars invariably fade out in the course of a few weeks or months. On some very rare occasions they have continued visible to the naked eye for a year or more, and they can commonly be seen by the telescope for several years. The first remarkable case of this sort of which we have a scientific record occurred in 1752. On 11 November of that year, Tycho Brahe, the celebrated astronomer of Uraniburg, observed a star of the first magnitude in the constellation Cassiopeia. Before the cod of the month it was so bright as to be visible in full daylight. Then it began to fade away and finally disappeared from view in May following. This was before the invention of the telescope, so that Tycho was unable to trace the star any longer. Thirty-two years later Kepler observed a similar star in the constellation Ophiuchus. This star continued visible to the naked eye for more than a year, but it never became so bright as the star of Tycho. As the heavens were more carefully watched, and photographs of the stars were taken, these new stars were found to he more frequent than was formerly supposed. During the 19th century 11 were observed to blaze out, more than half of which were dur ing the last 14 years of the century. But the greatest number of them scarcely reached the limit of visibility to the naked eye, being dis covered principally by photography. On 20 Feb. 1901 the most wonderful of these objects which had been seen for 300 years blazed out in the constellation Perseus. In two or three days it was the third brightest star in the heavens. Then, following the example of its predecessor, it gradually faded away, though it still remains visible in the telescope. It is not yet certain whether this object was impressed on any photographs until within a few days of its first appearance.