Astronomy for Everybody/Part 6/Chapter 6

As a general rule the starry heavens may be taken as a symbol of eternal unchangeability. The proverb-makers have told us in all time how everything on the earth is subject to alternation and decay, while the stars of heaven remain as we see them, age after age. But it is now known that, although this is true of the great majority of the stars, there are some exceptions. These are so little striking that they were never noticed by the ancient astronomers.

The first person in history to observe a change in a star was one Daniel Fabritius, a diligent watcher of the heavens, who lived three centuries ago.

In August, 1596, he noticed a star of the third magnitude before unknown in the constellation Cetus, which soon faded away again, and disappeared from view in October. In subsequent years it was found to show itself at regular intervals of about eleven months.

Two centuries elapsed before another case of the kind was known. Then it was found that the star Algol, in Perseus, faded away from the second to the fourth magnitude for a few hours at intervals of a little less than three days.

Early in the nineteenth century other stars were found to be subject to a more or less regular variation of their ​light. As observers studied the heavens with greater care, more and more of such stars were found, until at the present time the list of them numbers four or five hundred, and is constantly increasing. Of these some vary in an irregular way, but a large majority go through a regular period.

The easiest of these objects to notice is Beta Lyra, which is marked B on the figure of that constellation already given. It can be seen at some hour of any clear evening, spring, summer, or autumn. If the reader as he takes his evening walk will, night after night, compare this star with the one nearest to it and nearly of the same magnitude, he will see that while on some evenings the two appear perfectly equal, on others Beta will be of a magnitude fainter than the other. Careful and continued watching will show that the change takes place in a period of about six days and a half. That is to say, if the two stars are equal on a certain evening, they will again appear equal at the end of six or seven days, and so on indefinitely. Midway between the two times of equality the variable one will be at its faintest. If the observer notes the magnitudes at this time with the greatest precision, a curious fact will be brought out. Every alternate minimum, as the phase of least light is called, is slightly fainter than that preceding or following. The actual period is therefore nearly thirteen days, during which time there are two maxima of equal brightness and two slightly different minima.

It is now known that the variation of light in this case is not really inherent in the star itself, but arises from ​the fact that the star is a double one, composed of two stars revolving around each other, and so near together as almost to touch. As they revolve, each one in succession wholly or partially hides the other. This fact is not brought out by the telescope, because the most powerful telescope that could be made would not show the two stars separately. It is the result of long and careful study of the spectrum of the star, which is found to be a double one, the lines in one of which alternately cover and recede from the lines of the other.

In the extent of variation of its light the most remarkable of the more conspicuous variable stars is Omicron Ceti, already mentioned as seen by Fabritius. It is now found to go through a regular period in three hundred and thirty days. During about two weeks of this time it is at its brightest, and is then sometimes of the second magnitude and sometimes much fainter—occasionally only of the fifth. After each maximum it gradually fades away for a few weeks and disappears from view to the naked eye. But with a telescope it can be seen all the year round.

The period of eleven months makes the maximum occur about a month earlier every year. During some years it will occur when the star is so near the sun that it cannot be easily observed. This will be the case during the years 1903-'05.

Algol, also called Beta Persei, being in northern declination, can be seen in our latitudes at some time on almost every night of the year. In autumn and winter it is visible in the early evening. The peculiarity of its ​variation is that it remains of the same brightness nearly all the time, but fades away for a few hours at intervals of about two days and twenty-one hours. It is now known that this is due to the partial eclipse of the star by a dark body nearly as large as itself, revolving round it. It is true that this body has never been seen by human eye and never will be. Its existence is made known by its causing the star to revolve in a small orbit. It is true that this motion of the bright star is too small to be observed with the telescope, but it is made certain by means of the spectroscope, which shows a change in the wave length of the light coming from the star.

Different variable stars differ very widely in the extent of their variation. In most cases the latter is so slight that only an expert observer would notice it. Frequently it cannot be determined until after a long study by various observers whether a "suspected variable" is really such.

These objects form a very interesting subject of observation for those who have at command little or no instrumental facilities. No telescope is needed unless the star is, at some of its phases, invisible to the naked eye. The points to be noticed and recorded are the exact magnitude of the star from minute to minute or hour to hour, as it is going through its most rapid change, in order to learn at what moment its brightness is greatest or least.

What adds to the interest of the astronomer in these objects is the evidence now being gathered that many, perhaps most of the stars, are not single bodies, but more ​or less complex systems of bodies having the widest diversity in their construction. Double stars have been familiar to every observer of the heavens since the time of the great Herschel. But it is only in the time of our generation that the spectroscope has begun to make known to us pairs of stars revolving round each other, of which the components are so close together that the most powerful telescope can never separate them. The history of science offers no greater marvel than the discoveries of invisible planets moving round many of the stars which are now being made, and in which the Lick observatory has recently taken the lead.

It now seems more or less probable that the changes of light in all stars having a regular and constant period is due to the revolution of large planets or other stars around them. Sometimes the variation is slight and is caused in the way we have described, by one body partially eclipsing the other as it passes across it. In this case there may be no real variation in the light; the star eclipsed shines just as bright behind the eclipsing body as when it is not eclipsed. But it now seems that, if the darker body revolves in a very eccentric orbit, so as to be much nearer the bright body at some times than at others, its attraction produces such a change in the other as to greatly increase its light. Just how this effect is produced it is as yet impossible to say.