Page:A short history of astronomy(1898).djvu/419

§ 266] invisible to the naked eye and at other times to be conspicuous; a Dutch astronomer, Phocylides Holwarda (1618–1651), first clearly recognised its variable character (1639), and Ismaél Boulliau or Bullialdus (1605–1694) in 1667 fixed its period at about eleven months, though it was found that its fluctuations were irregular both in amount and in period. Its variations formed the subject of the first paper published by Herschel in the Philosophical Transactions (1780). An equally remarkable variable star is that known as Algol (or β Persei), the fluctuations of which were found to be performed with almost absolute regularity. Its variability had been noted by Geminiano Montanari (1632–1687) in 1669, but the regularity of its changes was first detected in 1783 by John Goodricke (1764–1786), who was soon able to fix its period at very nearly 2 days 20 hours 49 minutes, Algol, when faintest, gives about one-quarter as much light as when brightest, the change from the first state to the second being effected in about ten hours; whereas Mira varies its light several hundredfold, but accomplishes its changes much more slowly.

At the beginning of Herschel's career these and three or four others of less interest were the only stars definitely recognised as variable, though a few others were added soon afterwards. Several records also existed of so-called "new" stars, which had suddenly been noticed in places where no star had previously been observed, and which for the most part rapidly became inconspicuous again (cf. chapter, § 42; chapter , § 100; chapter , § 138); such stars might evidently be regarded as variable stars, the times of greatest brightness occurring quite irregularly or at long intervals. Moreover various records of the brightness of stars by earlier astronomers left little doubt that a good many must have varied sensibly in brightness. For example, a small star in the Great Bear (close to the middle star of the "tail") was among the Arabs a noted test of keen sight, but is perfectly visible even in our duller climate to persons with ordinary eyesight and Castor, which appeared the brighter of the two Twins to Bayer when he published his Atlas (1603), was in the 18th century (as now) less bright than Pollux.

Herschel made a good many definite measurements of the amounts of light emitted by stars of various magnitudes.