Another very red star is No. 4 of Birmingham’s Catalogue, which will be found about 5° north, preceding the great nebula in Andromeda. It is of about the eighth magnitude, and may be well seen with a 3-inch refractor. Krüger describes it as “intensiv roth”; Birmingham as “fine red” and “crimson”; Franks as “fine color, almost vermilion”; and Espin as “intense red color, most wonderful.”
Another fine object is R Leporis, which forms roughly an equilateral triangle with Kappa and Mu Leporis. This is also variable from 6½ to 8½ magnitude. It was discovered by Hind in 1845, and described by him as “of the most intense crimson, resembling a blood-drop on the background of the sky; as regards depth of color, no other star visible in these latitudes could be compared with it.” Schönfeld called it “intensiv blutroth,” but Dunér, observing its spectrum in 1880, gives its color as a less intense red than that of other stars. Possibly it may vary in color as well as in light.
The variable star U Cygni, which lies between Omicron and Omega Cygni, is also very red. Webb described it as showing “one of the loveliest hues in the sky.” It varies from about the seventh to 11½ magnitude, with a period of about 461 days.
Another deeply colored star is the well-known variable R Leonis. Hind says: “It is one of the most fiery-looking variables on our list—fiery in every stage from maximum to minimum, and is really a fine telescopic object in a dark sky about the time of greatest brilliancy, when its color forms a striking contrast with the steady white light of the sixth magnitude a little to the north.” This latter star is 19 Leonis.
In the Southern Hemisphere there are some fine examples of red stars. Epsilon Crucis, one of the stars in the Southern Cross, is very red. Mu Muscæ is described by Dr. Gould as of “an intense orange red.” Delta2 Gruis is a very reddish star of about the fourth magnitude. Pi1 Gruis was observed by Gould as “deep crimson,” and forming a striking contrast with its white neighbor Pi2 Gruis, which he notes as “conspicuously white.” The variable L2 Puppis is described as “red in all its stages, and remarkably so when faint.” Miss Clerke, observing—at the Cape of Good Hope—R Doradûs, another southern variable, says: “This extraordinary object strikes the eye with the glare of a stormy sunset,” and with reference to the variable R Sculptoris, described by Gould as “an intense scarlet,” she says: “The star glows like a live coal in the field,” a description I have found myself very applicable to other small red stars.
An eighth magnitude star about 5° north of Beta Pictoris is noted by Sir John Herschel, in his Cape Observations, as “vivid sanguine red, like a blood-drop. A superb specimen of its class.” With reference to a star of about 8½ magnitude in the field with Beta Crucis, Herschel says: “The fullest and deepest maroon red; the most intense blood-red of any star I have seen. It is like a drop of blood when contrasted with the whiteness of Beta Crucis.”
Of stars of other colors, the asserted green tint of Beta Libræ has already been referred to. Among the brighter stars of the Southern Hemisphere, Theta Eridani, Epsilon Pavonis, Upsilon Puppis, and Gamma Tucanæ are said to be decidedly blue. The wonderful cluster surrounding the star Kappa Crucis contains several bluish, greenish and red stars, and is described by Sir John Herschel as resembling “a superb piece of fancy jewelry.”
Among the double stars we find many examples of colored suns. Of these may be mentioned Epsilon Boötis, of which the colors are “most beautiful yellow” and “superb blue,” according to Secchi; Beta Cephei, “yellow and violet”; Beta Cygni, “golden yellow and smalt blue”; Gamma Delphini, of which I noted the colors in 1874 as “reddish yellow and grayish lilac”; Alpha Herculis, “orange and emerald or bluish green,” and described by Admiral Smyth as “a lovely object, one of the finest in the heavens”; Zeta Lyræ, “pale yellow and lilac” (Franks); and Beta Piscis Australis, of which I observed the colors in India as white and reddish lilac.
Some distant telescopic companions to red stars have been described as blue. This may be in some case due, partly at least, to the effect of contrast. In others the blue color seems to be real. This has been shown spectroscopically to be the case with the bluish companions of Beta Cygni.
The physical cause of the difference of color is still more or less a matter of mystery. Although we can not consider it proved that the red stars are cooling and “dying out” suns, as has been suggested, we may, I think, conclude that their temperature, although doubtless very high, must be lower than that of the white stars. We know that a bar of iron when heated to redness is not so hot as when raised to “white heat,” and although the analogy between hot iron and stellar photospheres may not be a perfect one, it seems probable that the higher the temperature of a star, the whiter its color will be. Most of the white stars, as Sirius, Vega, and those only yellow or slightly colored, show spectra of Secchi’s first and second types, while the great majority of the red stars exhibit banded spectra of the third and fourth types.