Page:Sir William Herschel, his life and works (1881).djvu/155

Rh diminishing, others which regularly increased, one star whose light periodically varies (α Herculis), and at least one star (55 Herculis) which has utterly disappeared. On October 10, 1781, and April 11, 1782, he observed this latter star, but in May, 1791, it had totally vanished. There was no trace remaining.

The discovery of the variability of α Herculis was a more important one than would at first sight appear. Up to that time the only variable stars known were seven in number. Their periods were four hundred and ninety-four, four hundred and four, three hundred and thirty-four, seven, six, five, and three days. These periods seemed to fall into two groups, one of from three hundred to five hundred days, the other comparatively much shorter, of three to seven days. α Herculis came to occupy the middle place between these groups, its period being about sixty days.

The cause of these strange and regular variations of brightness was supposed by to be the rotation of the star bodily on an axis, by which revolution different parts