Page:Astronomy for Everybody.djvu/356

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