It is very difficult to find an hypothesis which will explain satisfactorily all these characteristics, and attempts in this direction have not proved very successful. Mr. J. C. Duncan suggests the action of an absorbing atmosphere surrounding the component stars.
On March 30, 1612, Scheiner saw a star near Jupiter. It was at first equal in brightness to Jupiter’s satellites. It gradually faded, and on April 8 of the same year it was only seen with much difficulty in a very clear sky. “After that date it was never seen again, although carefully looked for under favourable conditions.”
An attempted identification of Scheiner’s star was made in recent years by Winnecke. He found that its position, as indicated by Scheiner, agrees with that of the Bonn Durchmusterung star 15°, 2083 (8½ magnitude). This star is not a known variable. Winnecke watched it for 17 years, but found no variation of light. From Scheiner’s recorded observations his star seems to have reached the 6th magnitude, which is considerably brighter than the Durchmusterung star watched by Winnecke.[348]
With reference to the colours of the stars, the supposed change of colour in Sirius from red to white is well known, and will be considered in the chapter on the Constellations. The bright star Arcturus has also been suspected of variation in colour. About the middle of the nineteenth century Dr. Julius Schmidt, of Athens, the well-known observer of variable stars, thought it one of the reddest stars in the sky, especially in the year 1841, when he found its colour comparable with that of the planet Mars.[349] In 1852, however, he was surprised to find it yellow and devoid of any reddish tinge; in colour it was lighter than that of Capella. In 1863, Mr. Jacob Ennis found it “decidedly orange.” Ptolemy and Al-Sufi called it red.
Mr. Ennis speaks of Capella as “blue” (classing it with Rigel), and comparing its colour with that of Vega![350] But the present writer has never seen it of this colour. To his eye it seems yellowish or orange. It was called red by Ptolemy, El Fergani, and Riccioli; but Al-Sufi says nothing about its colour.
Of β Ursæ Minoris, Heis, the eminent German astronomer said, “I have had frequent opportunities of convincing myself that the colour of this star is not always equally red; at times it is more or less yellow, at others most decidedly red.”[351]
Among double stars there are many cases in which variation of colour has been suspected. In some of these the difference in the recorded colour may possibly be due to “colour blindness” in some of the observers; but in others there seems to be good evidence in favour of a change. The following may be mentioned:—
η Cassiopeiæ. Magnitudes of the components about 4 and 7½. Recorded as red and green by Sir John Herschel and South; but yellow and orange by Sestini.
ι Trianguli. Magnitudes 5½ and 7. Secchi estimated them as white or yellow and blue; but Webb called them yellow and green (1862).
γ Leonis, 2 and 3½. Sir William Herschel noted them white and reddish white; but Webb, light orange and greenish yellow.