There are, however, some remaining lines, the character of which has not yet been ascertained.
It would be a little premature to assert that there must be some substance in the Great Nebula not at present known to us on the earth. This would be, no doubt, one interpretation of the facts. We must, however, admit the possibility of another explanation. It is frequently found that the lines yielded by an incandescent material vary to some extent when the physical conditions of temperature and of pressure are modified. It is, therefore, not impossible that the unknown lines in the spectrum of the Great Nebula may be due to some element known to us, but which has not yet been tested under the conditions which would make it yield the particular rays we are speaking of.
The composition of a nebula as disclosed to us by these researches is very instructive. Here we are looking at an object which seems to lie at the very limits of the visible universe—an object so remote that our attempts to fathom its distance are quite unsuccessful; yet in this inconceivably distant part of our system we find at least one ingredient which we know well on the earth. Previous to actual trial no one would have expected, I think, to find the Great Nebula largely constituted from such a familiar element as hydrogen. This gas enters into the composition of water, and is thus an element of extreme abundance on the earth. That an element so common with us here should also be abundant in these awfully distant regions of the universe is one of the most astonishing facts that modern science has revealed.
As the eye follows these ramifications of the Great Nebula, ever fading away in brightness until it dissolves in the blackness of the sky; as we look at the multitudes of bright stars which sparkle out from the depths of the great glowing gas; as we ponder on the marvelous outlines of a portion of the nebula, we are tempted to ask what the true magnitude of this object must really be. Here, again, we have to confess that science is unable to satisfy this very legitimate curiosity. The only means of learning the true length and breadth of a celestial object depends upon our first having discovered the distance from us at which the object is situated. Unhappily we are, as I have said, entirely ignorant of what this distance may be in the case of the Great Nebula in Orion. Our ordinary methods of conducting such an inquiry are hardly applicable to such an object, and its position so near the Equator introduces fresh difficulties into the problem. We shall, however, certainly not err on the side of exaggeration if we assert that the Great Nebula must be many millions of times larger than that group of bodies which we call the Solar System.
COLORED, DOUBLE, MULTIPLE, BINARY, VARIABLE AND TEMPORARY STARS.—J. E. Gore
On a clear night a careful observer will notice a marked difference in the colors of the brighter stars. The brilliant white or bluish-white light of Sirius, Rigel, and Vega contrasts strongly with the yellowish color of Capella, the deeper yellow, or orange, of Arcturus, and the ruddy light of Aldebaran and Betelgeuse. These colors are, however, limited to various shades of yellow and red. No star of a decided blue or green color is known, at least among those visible to the naked eye in the Northern Hemisphere. The third magnitude star Beta Libræ is described by Webb as of a “beautiful pale green hue,” but probably such a tint in the light of this star will to most people prove quite imperceptible. Dr. Gould, observing it in the Southern Hemisphere—under, of course, more favorable conditions—says: “There is a decidedly greenish tinge to the light of Beta Libræ, although its color can not properly be called conspicuous.”
Among the ruddy stars visible to the naked eye, Mu Cephei, Herschel’s “garnet star,” is generally admitted to be the reddest, but it is not sufficiently bright to enable its color to be well distinguished without the aid of an opera-glass. With such an instrument, however, its reddish hue is striking and beautiful, and very remarkable when compared with other stars in its vicinity. Like so many of the red stars, Mu Cephei is variable in its light, but seems to have no regular period, and often remains for many weeks without perceptible change. It may be seen near the zenith in the early evening hours toward the end of October, and when in this position its ruddy color is very conspicuous.
Among the brightest stars, Betelgeuse is perhaps the reddest, and the contrast between its ruddy tint and the white color of Rigel in the same constellation (Orion) is very noticeable. Like Mu Cephei, Betelgeuse is irregularly variable in its light, but not to such an extent, and, like the “garnet star,” it frequently remains for protracted periods nearly constant in brightness. There are other cases of reddish color among the naked-eye stars. Among these may be mentioned Antares (Alpha Scorpii), Alphard (Alpha Hydræ), noted as red by the Persian astronomer Al-Sûfi, in the Tenth Century, and called by the Chinese “The Red Bird”; Eta and Mu Geminorum; Mu and Nu Ursæ Majoris; Delta and Lambda Draconis; Beta Ophiuchi; Gamma Aquilæ, and others in the Southern Hemisphere.
But it is among the stars below the limit of naked-eye vision that we meet with the finest examples of the red stars. Some of these are truly wonderful objects. The small star, No. 592 of Birmingham’s Catalogue of Red Stars (No. 713 of Espin’s edition), which lies a little south of the 5½ magnitude star 79 Cygni, was described as “splendid red” by Birmingham, “very deep red” by Copeland and Dreyer, and “orange vermilion” by Franks. The star 248 Birmingham, which lies about 5° south of Gamma Hydræ, is another fine specimen. Birmingham described it as “fine red” and “ruby”; Copeland as “brown red”; Dreyer as “copper red”; and Espin as “magnificent blood red.” This star is variable in light, as the estimates of magnitude range from 6.7 to below 9. About 3° to the northeast of this remarkable object is another highly-colored star, known as R Crateris. It is easily found, as it lies in the same telescopic field of view with Alpha Crateris, a 4½ magnitude star. Sir John Herschel described it as “scarlet, almost blood-color; a most intense and curious color.” Birmingham called it “crimson”; and Webb “very intense ruby.” Observing it with a 3-inch refractor in India in 1875, I noted it as “full scarlet.” It varies in light from above the eighth magnitude to below the ninth, and has near it a star of the ninth magnitude of a paler blue tint.