Star-Disks.—The stars as observed in telescopes are shorn of the false rays apparent to the naked eye, and they are seen with small spurious disks. That the disks are spurious is evident from the fact that the larger the telescope employed, the smaller the star-disks become. And moreover, when a star is occulted by the Moon, it disappears instantaneously. There is no gradual diminution of lustre; the star vanishes with great suddenness. Bright stars, like Aldebaran or Regulus, have been watched up to the Moon’s limb, and observers have been sometimes startled at the abruptness with which they were blotted out. An appreciable disk could not be withdrawn in this instantaneous manner; it would exhibit a perceptible decadence as the Moon increasingly overlapped it, but no such appearance is observed. On the occasion of the occultation of Jupiter on Aug. 7, 1889, the planet’s diameter was 41″·4, and the disappearance occupied 85 seconds. Now had Aldebaran or Regulus a real disk of only 1″ it would prevent their sudden extinctions, and their disappearances would be spread over perceptible though short intervals of time[56]. But there is every reason to conclude that the actual disks are to be represented by a small fraction of 1″, so that the largest instrument and the highest powers fail to reveal it. In this connection, Mr. Gore, in his ‘Scenery of the Heavens,’ p. 152, says:—“Let us take the case of α Centauri, which is, as far as is known at present, the nearest fixed star to the Earth. The distance of this star is about 25 billions of miles. From comparisons made between its light and the Moon, it has been found that its intrinsic brilliancy must be about four times that of the Sun. Supposing its greater lustre is due to its greater size—a not improbable supposition—it would subtend, if placed at the Sun’s distance, an angle twice as great, or about 1°, and hence we find that the angle subtended at its distance of 25 billions of miles would be about 1/76th of a second of arc, which the most powerful telescope yet constructed would be incapable of showing as a visible disk.”
Distance of the Stars.—The distances of the outer planets Uranus and Neptune, mentioned in an earlier chapter of this work, are sufficiently large to amaze us; but the distances of the stars may be said to be relatively infinite. For many years the problem of stellar distances defied all attempts to resolve it. At length, in 1838-39, Bessell, Henderson, and Struve obtained results for three stars—viz. 61 Cygni, α Centauri, and α Lyræ,—which practically settled the question. More recent measures of stellar parallax, while correcting the earlier values, have virtually corroborated them; though the figures adopted can only be regarded as approximations, owing to the difficult and delicate nature of the work. The binary star α Centauri appears to be the nearest of all; it has a parallax of 0″·75, and its distance from us is equal to 275,000 times the distance of the Sun. Light traversing space at the rate of 187,000 miles per second would occupy 4-1/3 years in crossing this interval. In the Northern hemisphere 61 Cygni is the nearest star, with a parallax of 0″·44 and a distance of about 470,000 times the Sun’s distance. Light would take more than seven years in reaching us from this star, α Lyræ has a parallax of 0″·15, equal to nearly 22 light-years. α Crucis shows a very small parallax (0″·03), and its distance is excessively remote—equal to about 108 light-years!
Proper Motion of Stars.—A very slight motion affects the places of many of the so-called fixed stars. This must, after the lapse of long intervals of time, materially alter the configuration of the constellations. But the change is a very gradual one, and must operate through many centuries before its effects will become appreciable in a general way. The greatest proper motion yet observed is that in regard to two small stars (one in Ursa Major and the other in Piscis Australis), which amounts to about 7″ annually. Another motion has been recognized, viz. in the line of sight. Dr. Huggins made the initiatory efforts in this research by measuring the displacement of the F line in the spectrum of Sirius. The work has been actively pursued at the observatories of Greenwich and Rugby, and with interesting results. While certain stars exhibit a motion of approach, others display a motion of recession. Thus Vega, Arcturus, and Pollux are approaching us at the rate of about 40 miles per second; while Rigel is receding at the rate of 17 miles per second, Castor at the rate of 19, Regulus 14, Betelgeuse 25, and Aldebaran 31. Sirius, in the years from 1875 to 1878, was receding from us at the rate of 22 miles per second; but this decreased in subsequent years, and in 1884-85 the star was approaching with a motion of about 22 miles per second. In 1886 and 1887 this rate was increased to about 30 miles per second, as observed both at Greenwich and Rugby. This confirms the idea that Sirius is moving in an elliptical orbit. Similar observations, in regard to the variable star Algol, have revealed that changes of velocity are connected with its changes of lustre. Before minimum the star recedes at the rate of 24½ miles per second, while after minimum the star approaches with a speed of 28½ miles per second (‘Monthly Notices,’ vol. 1. p. 241).
Double Stars and Binary Systems.—Telescopic power will often reveal two stars where but one is seen by the naked eye. Sometimes the juxtaposition of such stars is merely accidental; though they are placed nearly in the same line of sight the conjunction is an optical one only, and no connection apparently subsists between them. In other cases, however, pairs are found which have a physical relation, for one is revolving round the other; and these are termed binary stars. Sir W. Herschel was the first to announce them, from definite observations, in 1802. Of double stars more than 10,000 are now known; many of these are telescopic, but the list includes some fine examples of naked-eye stars.
Double Stars.
| β Orionis. | γ Leonis. | α Ursæ Minoris. | γ Virginis. |
| δ Cygni. | γ Arietis. | γ Andromedæ. | δ Serpentis. |
Double stars are excellent telescopic tests. A very close pair affords a good criterion as to the defining capacity of an instrument; while a pair more widely separated and of greatly unequal magnitude, like that of α Lyræ, offers a test of the light-grasping power. But in these delicate observations, as, indeed, in all others, the character of the seeing exercises an important and variable influence. A double star that is well shown on one night becomes utterly obliterated on another, owing to the unsteadiness and flaring of the image. On such occasions as the latter one is reminded of the “twitching, twirling, wrinkling, and horrible moulding” of which Sir John Herschel complained, and which unfortunately forms a too common experience of the astronomical observer. A close double, of nearly equal magnitudes, requires a steady night, such as is suitable for planetary details; but a wide double consisting of a bright and a minute star rather needs a very clear sky than the perfection of definition. Certain doubles, such as θ Aurigæ, δ Cygni, and ζ Herculis, are often more easily seen in twilight than on a dark sky; and some experienced observers, conscious of this advantage, have secured excellent measures in daylight. Mr. Gledhill says:—“Such stars as γ Leonis and γ Virginis are best measured before or very soon after sunset” (‘Observatory,’ vol. iii. p. 54).
List of Double Stars.