CHAPTER XVI
Variable Stars
In that interesting work A Cycle of Celestial Objects, Admiral Smyth says (p. 275), “Geminiano Montanari, as far back as 1670, was so struck with the celestial changes, that he projected a work to be intituled the Instabilities of the Firmament, hoping to show such alterations as would be sufficient to make even Aristotle—were he alive—reverse his opinion on the incorruptibility of the spangled sky: ‘There are now wanting in the heavens,’ said he, ‘two stars of the 2nd magnitude in the stem and yard of the ship Argo. I and others observed them in the year 1664, upon occasion of the comet that appeared in that year. When they first disappeared I know not; only I am sure that on April 10, 1668, there was not the least glimpse of them to be seen.’” Smyth adds, “Startling as this account is—and I am even disposed to question the fact—it must be recollected that Montanari was a man of integrity, and well versed in the theory and practice of astronomy; and his account of the wonder will be found—in good set Latin—in page 2202 of the Philosophical Transactions for 1671.”
There must be, I think—as Smyth suggests—some mistake in Montanari’s observations, for it is quite certain that of the stars mentioned by Ptolemy (second century A.D.) there is no star of the 2nd magnitude now missing. It is true that Al-Sufi (tenth century) mentions a star of the third magnitude mentioned by Ptolemy in the constellation of the Centaur (about 2° east of the star ε Centauri) which he could not find. But this has nothing to do with Montanari’s stars. Montanari’s words are very clear. He says, “Desunt in Cœlo duæ stellæ Secundæ Magnitudinis in Puppi Navis ejusve Transtris Bayero β et γ, prope Canem Majoris, à me et aliis, occasione præsertim Cometæ A. 1664 observatæ et recognitæ. Earum Disparitionem cui Anno debeam, non novi; hoc indubium, quod à die 10 April, 1668, ne vestigium quidem illarum adesse amplius observe; cæteris circa eas etium quartæ et quintæ magnitudinis, immotis.” So the puzzle remains unsolved.
Sir William Herschel thought that “of all stars which are singly visible, about one in thirty are undergoing an observable change.”[322] Now taking the number of stars visible to the naked eye at 6000, this would give about 200 variable stars visible at maximum to the unaided vision. But this estimate seems too high. Taking all the stars visible in the largest telescopes—possibly about 100 millions—the proportion of variable stars will probably be much smaller still.
The theory that the variation of light in the variable stars of the Algol type is due to a partial eclipse by a companion star (not necessarily a dark body) is now well established by the spectroscope, and is accepted by all astronomers. The late Miss Clarke has well said “to argue this point would be enforcer une porte ouverte.”
According to Dr. A. W. Roberts, the components of the following “Algol variables” “revolve in contact”: V Puppis, X Carinæ, β Lyræ, and υ Pegasi. Of those V Puppis and β Lyræ are known spectroscopic binaries. The others are beyond the reach of the spectroscope, owing to their faintness.
A very curious variable star of the Algol type is that known as R R Draconis. Its normal magnitude is 10, but at minimum it becomes invisible in a 7½-inch refracting telescope. The variation must, therefore, be over 3 magnitudes, that is, at minimum its light must be reduced to about one-sixteenth of its normal brightness. The period of variation from maximum to minimum is about 2·83 days. The variation of light near minimum is extraordinarily rapid, the light decreasing by about 1 magnitude in half an hour.[323]