To this rule there are, however, like other rules, some notable exceptions. For instance, Aldebaran, Alpha Hydræ, Xi Cygni, and 31 Orionis, although distinctly reddish stars, show well-marked spectra of the second or solar type. On the other hand, Rho Ursæ Majoris and Omega Virginis, which, according to Dunér, are only slightly yellow, have well-marked spectra of the third type.
An apparent change of color seems in some cases to be well established. The supposed red color of Sirius in ancient times is well known. A certain established change is found in the case of the famous variable star Algol, which is distinctly described as red by Al-Sûfi in the Tenth Century. It is now pure white, or nearly so, and this is probably the best attested instance on record of change of color in a bright star.
Schmidt’s Nova Cygni of 1876 was noted as “golden yellow” on the night of its discovery. When it had faded to the eighth magnitude, Dr. Copeland called it “decided red,” but when examined at Lord Crawford’s observatory in September, 1877, its color was recorded as “faint blue”! The new star in the Andromeda nebula was considered to be yellowish or reddish by most observers when near its maximum, but about a month later its color was noted as “bluish.”
Among the red and variable stars, there are many suspected cases of color variation. Espin and other observers have noted that the wonderful variable Mira Ceti is much less red at maximum than at minimum. My own observations confirm this. When at its maximum brightness, Mira does not seem to me a very highly-colored star, while at one of its minima I noted it as “fiery red.” Possibly, however, the great difference between its maximum and minimum brilliancy may have an influence on estimations of its color. The remarkable variable Chi Cygni is said to be “strikingly variable in color.” Espin’s observations in different years show it “sometimes quite red, at others only pale orange red.” The star Birmingham 118 was described by Schjellerup in 1863 as “decided red,” but it was found yellow by Secchi in 1868; “bluish” by Birmingham, 1873-76; “no longer red” by Schjellerup in March, 1876; and “white” by Franks in 1885. Espin omits it from his revised edition of Birmingham’s Catalogue.
Birmingham 169 was found red by Struve, blue or bluish-white by Birmingham in 1874, and white at Greenwich in the same year. Espin also saw it white in March, 1888. The star Birmingham 30, which lies close to Phi Persei (54 Andromedæ), was described by Schweizer as a “red star with a little disk” in January, 1843; Birmingham noted it as “light red” in December, 1875; Copeland “deep red” in January, 1876; and Dreyer “reddish” in September, 1878; but Espin, in November and December, 1887, found it “certainly not red, and nothing peculiar in the star’s appearance.” It might be expected that these curious changes of color, if real, would be accompanied by corresponding changes in the star’s spectrum. Such may be the case, and observations in this direction would probably lead to some interesting results.
There seems to be some law governing the distribution of the colored stars. The white stars appear to be most numerous, as a rule, in those constellations where bright stars are most abundant, for instance, in Orion, Cassiopeia, and Lyra; yellow and orange stars in large and ill-defined constellations, such as Cetus, Pisces, Hydra, Virgo, etc. The very reddish stars are most numerous in or near the Milky Way, and one portion of the Galaxy—between Aquila, Lyra, and Cygnus—was termed by Birmingham “the red region in Cygnus.”
Many of the stars when examined with a good telescope are seen to be double, some triple, and a few quadruple, and even multiple. These when viewed with the naked eye, or even a powerful binocular, seem to be single, and show no sign of consisting of two components. These telescopic double stars should be carefully distinguished from those which appear very close together with the naked eye, and which in opera-glasses or telescopes of small power might be mistaken for wide double stars by the inexperienced observer. These latter stars, such as Mizar—the middle star in the tail of the Great Bear—and its small companion, Alcor, have been called “naked-eye doubles,” but they are not, properly speaking, double stars at all. Telescopic double stars are far closer, and even the widest of them could not possibly be seen double without optical aid, even by those who are gifted with the keenest vision. Of these so-called “naked-eye doubles,” we may mention Alpha Capricorni, which on a very clear night may be seen with the naked eye to consist of two stars. On a very fine night two stars may be seen in Iota Orionis, the most southern star in Orion’s Sword. The star Zeta Ceti has near it a fifth magnitude star, Chi, which may be easily seen with the unaided vision. The star Epsilon Lyræ (near Vega) is a severe test for naked-eye vision. Bessel, the famous German astronomer, is said to have seen it when thirteen years of age. Omicron Cygni (north of Alpha and Delta Cygni) forms another naked-eye double, and other objects of this class may be noticed by a sharp-eyed observer.
The star Mizar, already referred to, is itself a wide telescopic double, and it seems to have been the first double star discovered with the telescope (by Riccioli in 1650). It consists of two components, of which one is considerably brighter than the other. It will give an idea of the closeness of even a “wide” telescopic double when we say that the apparent distance between Mizar and Alcor is nearly forty times the distance which separates the close components of the bright star. From this it will be seen that even a powerful binocular field-glass would fail to show Mizar as anything but a single star. The components may, however, be well seen with a 3-inch telescope, or even with a good 2-inch. The colors of the two stars are pale green and white. Between Mizar and Alcor is a star of the eighth magnitude, and others fainter. Mizar was the first double star photographed by Bond.
The Pole Star has a small companion at a little greater distance than that which separates the components of Mizar, but owing to the faintness of this small star, the object is not so easy as Mizar.
The star Beta Cygni is composed of a large and small star, of which the colors are described as “golden yellow and smalt blue.” This is a very wide double, and may be seen with quite a small telescope. Another fine double star is that known to astronomers as Gamma Andromedæ. The magnitudes of the components are about the same as those of Mizar, but a little closer. Their colors are beautiful (“gold and blue”). This is one of the prettiest double stars in the heavens. It is really a triple star, the fainter of the pair being a very close double star; but this is beyond the reach of all but the largest telescopes. The star Gamma Delphini is another beautiful object, the components being a little more unequal in magnitude, but the distance between them about the same as in Gamma Andromedæ. I have noted the colors with a 3-inch telescope as “reddish yellow and grayish lilac.” Gamma Arietis, the faintest of the three well-known stars in the head of Aries, is another fine double star, a little closer than Gamma Delphini. This is an interesting object, from the fact that it was one of the first double stars discovered with the telescope—by Hooke, in 1664, when following the comet of that year.