Variable Stars.—A proportion of the stars exhibit fluctuations in their visible brightness. In most cases, however, the variation is but slight, though there are instances in which the differences are considerable. The fluctuations are periodical in nature and capable of being exactly determined. But the character of the variation and the period are very dissimilar in different stars. Some are of short period, and their variations occupy a few days only; others, however, are more gradual, and twelve months or more may represent the complete cycle of their changes. These alterations of brightness generally escape the notice of casual observers of the heavens. To them the stars appear as constant in their relative magnitudes as they are in their relative positions. But a close observer of the firmament, who habitually watches and records the comparative lustre of the stars, must soon discover numerous evidences of change. He will remark certain stars which alternately grow bright and faint, and, in fact, display a regular oscillation of brilliancy. In the case of a pair of stars he may possibly notice that the superior lustre is emitted first by one and then by the other. The observation of these variables dates from a period anterior to the invention of the telescope. Nearly three centuries ago Fabricius remarked that ο Ceti (Mira) suffered a great diminution of light; for while it was of the 3rd mag. in Aug. 1596, it became invisible in the following autumn. It was re-observed by Holwarda in 1639, and as he appears to have been the first to estimate its period, some authors, including Argelander, have credited him with the discovery. The star has a period of about 331·3 days. Its variations are somewhat erratic, for at max. it is sometimes only 4th mag., while at others it is as bright as 2nd mag., and its min. are equally inconsistent.
β Persei (Algol) is another and perhaps the best known of all the variable stars. Its changes are very rapid, for it passes through its various gradations of brilliancy in less than three days. It was first noticed by Montanari in 1669, though it was left for Goodricke in 1782 to ascertain its period. The normal mag. of the star is 2·2, and it only shows distinct variation during the five hours which precede and follow a minimum, when it declines to 3·7 mag. Its period is shortening, for in 1782 it was 2d 20h 48m 59s·4, in 1842, 2d 40h 48m 55s·2, and at present Chandler finds it 2d 20h 48m 51s. As to the causes which contribute to these variations, they are invested in mystery. It has been conjectured that dark spots on the surfaces of the stars may, by the effects of rotation, introduce the observed alternations. Another surmise is that the temporary diminutions of lustre are to be ascribed to the interposition of dark satellites, and this theory seems tenable in regard to stars of the Algol type. It is satisfactory to note that a large amount of systematic work is being done in this important and delicate branch of research. Such stars as are subject to variation have been classed as follows:—1. Temporary or new stars; 2. Stars having long and pretty regular variation; 3. Stars irregularly variable; 4. Stars varying in short periods; 5. Stars of the type of Algol, which are liable to temporary diminutions of lustre. On the preceding page is a list of the most noteworthy variable stars.
List of Variable Stars.
| Name of Star. | Position, 1890. | Mags. | Period. | Observer. | |
|---|---|---|---|---|---|
| R.A. | Dec. | ||||
| h m | ° ′ | ||||
| μ Cephei | 0 52.5 | +81 17 | 7.2 9.4 | 2d 11h 50m | Ceraski, 1880. |
| ο Ceti | 2 13.8 | -3 29 | 2 0 | 331⅓ days | Fabricius, 1596. |
| β Persei | 3 1.0 | +40 32 | 2.2 3.7 | 2d 20h 49m | Montanari, 1669. |
| λ Tauri | 3 54·6 | +12 11 | 3.4 4.2 | 3d 22h 52m | Baxendell, 1848. |
| U Orionis | 5 49·3 | +20 9 | 6 12½ | Gore, 1885. | |
| ζ Geminorum | 6 8.2 | +22 32 | 3.2 4·2 | 135-151 days | Schmidt, 1865. |
| ζ Geminorum | 6 57.6 | +20 44 | 3.7 4·5 | 10d 3h 43m | Schmidt, 1847. |
| L2 Puppis | 7 10·2 | -44 28 | 3.5 6.3 | 136 days | Gould, 1872. |
| R Canis Maj. | 7 14.5 | -16 11 | 6.2 6.8 | 1d 3h 16m | Sawyer, 1887. |
| U Geminorum | 7 48.6 | +22 18 | 9 14 | 71-126 days | Hind, 1855. |
| S Cancri | 8 37.7 | +19 26 | 8.2 11·7 | 9d 11h 38m | Hind, 1848. |
| ζ Argûs | 10 40.8 | -59 6 | 1 6 | 46 or 67 yrs.? | Burchell, 1827. |
| R Hydræ | 13 23.7 | -22 43 | 4 10 | 436 days | Maraldi, 1704. |
| δ Libræ | 14 55.1 | -8 5 | 4.9 6·1 | 2d 7h 51m | Schmidt, 1859. |
| U Coronæ | 15 13.7 | +32 3 | 7.6 8·8 | 3d 10h 51m | Winnecke, 1869. |
| α Herculis | 17 9.6 | +14 31 | 3.1 3.9 | 88d 12h (irreg.) | W. Herschel, 1795. |
| U Ophiuchi | 17 11.0 | + 1 20 | 6 6.7 | 0d 20h 8m | Sawyer, 1881. |
| β Lyræ | 18 46.0 | +33 14 | 3.5 4.5 | 12d 21h 47m | Goodricke, 1784. |
| χ Cygni | 19 46·3 | +32 38 | 4-6.5 13 | 406 days | Kirch, 1686. |
| ζ Aquilæ | 19 46.9 | + 0 44 | 3.6 4.7 | 7d 4h 14m | Pigott, 1784. |
| Y Cygni | 20 47.7 | +34 15 | 7.1 7.9 | 1d 11h 57m | Chandler, 1886. |
| μ Cephei | 21 40.1 | +58 16 | 3.6 4·8 | 432 days? | Hind, 1848. |
| δ Cephei | 22 25.1 | +57 51 | 3.7 4.8 | 5d 8h 48m | Goodricke, 1784. |
New or Temporary Stars.—These stars (sometimes classed with variable stars) furnish us with rare instances of vast physical changes occurring among sidereal objects, usually so steadfast and endurable. The alternating lustre of certain variable stars represents phenomena of regular recurrence, and is probably to be explained by simple causes; but the sudden outbursts and rapid decline of temporary stars are facts of a more startling character, and need a more exceptional explanation. The first of these objects recorded in history appears to have been seen in Scorpio 134 years before the Christian era, and it suggested to Hipparchus of Rhodes the idea of forming a catalogue of stars, so that in future ages observers might have the means of recognizing new stars or any other changes in the configuration of the heavens. Hipparchus completed his catalogue in 128 B.C.; it contained 1025 stars, and forms one of the most valuable memorials we possess of the labours of the ancient astronomers. Another temporary star is said to have appeared in 130 A.D., but this and several other objects of presumably similar character noticed in later years may just possibly have been comets, and considerable doubt hangs over the descriptions. It will therefore be safest to confine our remarks to more modern and better attested instances of these phenomena[59]:—
1572, November 11.—The famous star of Tycho Brahe. He thus described his first view of it:—“One evening when I was considering, as usual, the celestial vault, the aspect of which is so familiar to me, I perceived with indescribable astonishment a bright star of extraordinary magnitude near the zenith in the constellation of Cassiopeia.” He adds:—“The new star was destitute of a tail, or of any appearance of nebulosity; it resembled the other stars in all respects, only that it twinkled even more than stars of the first magnitude. In brightness it surpassed Sirius, α Lyræ, or Jupiter. It could be compared in this respect only to Venus when she is nearest the earth (when a fourth part of her illuminated surface is turned towards us). Persons who were gifted with good sight could distinguish the star in the daytime, even at noon, when the sky was clear.” This brilliant NOVA began to fade early in Dec. 1572, and in April and May 1573 it resembled a star of the 2nd mag., in July and Aug. one of the 3rd mag., and in Oct. and Nov. one of the 4th mag. In March 1574 the star completely disappeared (to the naked eye), after a visibility of about 17 months. It exhibited some curious variations of colour. It was white when most brilliant; it then became yellow, and afterwards red, so that its hue in the early part of 1573 was similar to that of Mars. But in May it again became white, and continued so until it ceased to be visible. The position of this star (for 1890) is R.A. 0h 18m 41s, Dec. +63° 32′·2. It was supposed to be a reapparition of the brilliant stars which shone between Cepheus and Cassiopeia in 945 and 1264, and to have possibly been associated with the “Star of Bethlehem;” but there is no reliable evidence on which this view can be supported, as the alleged “stars” of 945 and 1264 were undoubtedly comets, misdescribed in old records. Cornelius Gemma is reputed to have seen the celebrated star of 1572 a few days before Tycho Brahe, viz., on November 8, 1572.
1604, October 10.—Discovered by Brunowski, who announced it to Kepler. It was brighter than a star of the 1st mag., also than Mars, Jupiter, or Saturn, which were not far distant at the time. It did not begin to fade immediately; for a month after its discovery it was still brighter than Jupiter, and of a white lustre. At the middle of November it surpassed Antares, but was inferior to Arcturus. In April 1605 it had fallen to the 3rd mag., and went on decreasing until in October it could scarcely be seen with the naked eye owing to the twilight resulting from its proximity to the Sun. In March 1606 it was invisible. The position of this object was about midway between ξ and 58 Ophiuchi, or at R.A. 17h 24m, Dec.-21° 207′ (1890).
1670, June 20.—Discovered by the Carthusian Monk Anthelme in R.A. 19h 43m 3s, Dec. +27° 3′ (1890), a few degrees east of β Cygni. It was of the 3rd mag., and continued in view, with constantly fluctuating brightness, for nearly two years. At the end of March 1672 it was 6th mag., and has never reappeared. Hind found a small, hazy, and ill-defined star in the same place, but this is probably not the same as Anthelme’s star of 1670.
1848, April 28.—During the long interval of 178 years separating 1670 from 1848 not a single new star appears to have revealed itself. Observers had multiplied, astronomical instruments had been much improved, star-catalogues were plentiful, and yet the sidereal heavens gave no intimation of a stellar outburst. No better proof than this could be afforded as to the great rarity of temporary stars. At length, in the spring of 1848, the spell was broken, and Mr. Hind announced that a new star of a reddish-yellow colour had appeared in Ophiuchus, R.A. 16h 53m 20s, Dec.-12° 43′ (1890). He expressed himself as certain that no star brighter than the 9th mag. had been there previous to April 5. After rising to the 4th mag. it soon faded, and in 1851 could only be observed in large instruments. In 1875 it was still visible as a very minute star.
1860, May 21.—M. Auwers, of Konigsberg, noticed a star of the 7th mag. near the centre of the bright resolvable nebula (M. 80), lying between α and β Scorpii, R.A. 16h 10m 29s, Dec.-22° 42′ (1890). On May 18 the star was not there, and it disappeared altogether in three weeks. It was independently seen by Pogson on May 28, and to him it seemed that “the nebula had been replaced by a star, so entirely were its dim rays overpowered by the concentrated blaze in their midst.”