Part 38

Aquarius is marked by no conspicuous stars. An irregular line of minute stars marks the course of the stream of water which flows from the Water-Bearer's Urn into the mouth of the Southern Fish. This mouth is marked by the first-magnitude star Fomalhaut. These constellations are shown in Fig. 397.

341. Pisces and Aries.--The remaining zodiacal constellations are Pisces, or the Fishes, Aries, or the Ram (Fig. 398), and Cancer, or the Crab.

The Fishes lie under Pegasus and Andromeda, but contain no bright stars. Aries (between Pisces and Taurus) is marked by a pair of stars on the head,--one of the second, and one of the third magnitude. Cancer (between Leo and Gemini) has no bright stars, but contains a remarkable cluster of small stars called Præsepe, or the Beehive.

Clusters.

342. The Hyades.--The Hyades are a very open cluster in the face of Taurus (334). The three brightest stars of this cluster form a letter V, the point of the V being on the nose, and the open ends at the eyes. This cluster is shown in Fig. 399. The name, according to the most probable etymology, means rainy; and they are said to have been so called because their rising was associated with wet weather. They were usually considered the daughters of Atlas, and sisters of the Pleiades, though sometimes referred to as the nurses of Bacchus.

343. The Pleiades.--The Pleiades constitute a celebrated group of stars, or a miniature constellation, on the shoulder of Taurus. Hesiod mentions them as "the seven virgins of Atlas born," and Milton calls them "the seven Atlantic sisters." They are referred to in the Book of Job. The Spaniards term them "the little nanny-goats;" and they are sometimes called "the hen and chickens."

Usually only six stars in this cluster can be seen with the naked eye, and this fact has given rise to the legend of the "lost Pleiad." On a clear, moonless night, however, a good eye can discern seven or eight stars, and some observers have distinguished as many as eleven. Fig. 400 shows the Pleiades as they appear to the naked eye under the most favorable circumstances. Fig. 401 shows this cluster as it appears in a powerful telescope. With such an instrument more than five hundred stars are visible.

344. Cluster in the Sword-handle of Perseus.--This is a somewhat dense double cluster. It is visible to the naked eye, appearing as a hazy star. A line drawn from Algenib, or Alpha of Perseus (338), to Delta of Cassiopeia (330), will pass through this cluster at about two-thirds the distance from the former. This double cluster is one of the most brilliant objects in the heavens, with a telescope of moderate power.

345. Cluster of Hercules.--The celebrated globular cluster of Hercules can be seen only with a telescope of considerable power, and to resolve it into distinct stars (as shown in Fig. 402) requires an instrument of the very highest class.

346. Other Clusters.--Fig. 403 shows a magnificent globular cluster in the constellation Aquarius. Herschel describes it as appearing like a heap of sand, being composed of thousands of stars of the fifteenth magnitude.

Fig. 404 shows a cluster in the constellation Toucan, which Sir John Herschel describes as a most glorious globular cluster, the stars of the fourteenth magnitude being immensely numerous. There is a marked condensation of light at the centre.

Fig. 405 shows a cluster in the Centaur, which, according to the same astronomer, is beyond comparison the richest and largest object of the kind in the heavens, the stars in it being literally innumerable. Fig. 406 shows a cluster in Scorpio, remarkable for the peculiar arrangement of its component stars.

Star clusters are especially abundant in the region of the Milky-Way, the law of their distribution being the reverse of that of the nebulæ.

Double and Multiple Stars.

347. Double Stars.--The telescope shows that many stars which appear single to the naked eye are really double, or composed of a pair of stars lying side by side. There are several pairs of stars in the heavens which lie so near together that they almost seem to touch when seen with the naked eye.

Pairs of stars are not considered double unless the components are so near together that they both appear in the field of view when examined with a telescope. In the majority of the pairs classed as double stars the distance between the components ranges from half a second to fifteen seconds.

Epsilon Lyræ is a good example of a pair of stars that can barely be separated with a good eye. Figs. 407 and 408 show this pair as it appears in telescopes magnifying respectively four and fifteen times; and Fig. 409 shows it as seen in a more powerful telescope, in which each of the two components of the pair is seen to be a truly double star.

348. Multiple Stars.--When a star is resolved into more than two components by a telescope, it is called a multiple star. Fig. 410 shows a triple star in Pegasus. Fig. 411 shows a quadruple star in Taurus. Fig. 412 shows a sextuple star, and Fig. 413 a septuple star. Fig. 414 shows the celebrated septuple star in Orion, called Theta Orionis, or the trapezium of Orion.

349. Optically Double and Multiple Stars.--Two or more stars which are really very distant from each other, and which have no physical connection whatever, may appear to be near together, because they happen to lie in the same direction, one behind the other. Such accidental combinations are called optically double or multiple stars.

350. Physically Double and Multiple Stars.--In the majority of cases the components of double and multiple stars are in reality comparatively near together, and are bound together by gravity into a physical system. Such combinations are called physically double and multiple stars. The components of these systems all revolve around their common centre of gravity. In many instances their orbits and periods of revolution have been ascertained by observation and calculation. Fig. 415 shows the orbit of one of the components of a double star in the constellation Hercules.

351. Colors of Double and Multiple Stars.--The components of double and multiple stars are often highly colored, and frequently the components of the same system are of different colors. Sometimes one star of a binary system is white, and the other red; and sometimes a white star is combined with a blue one. Other colors found in combination in these systems are red and blue, orange and green, blue and green, yellow and blue, yellow and red, etc.

If these double and multiple stars are accompanied by planets, these planets will sometimes have two or more suns in the sky at once. On alternate days they may have suns of different colors, and perhaps on the same day two suns of different colors. The effect of these changing colored lights on the landscape must be very remarkable.

New and Variable Stars.

352. Variable Stars.--There are many stars which undergo changes of brilliancy, sometimes slight, but occasionally very marked. These changes are in some cases apparently irregular, and in others periodic. All such stars are said to be variable, though the term is applied especially to those stars whose variability is periodic.

353. Algol.--Algol, a star of Perseus, whose position is shown in Fig. 416, is a remarkable variable star of a short period. Usually it shines as a faint second-magnitude star; but at intervals of a little less than three days it fades to the fourth magnitude for a few hours, and then regains its former brightness. These changes were first noticed some two centuries ago, but it was not till 1782 that they were accurately observed. The period is now known to be two days, twenty hours, forty-nine minutes. It takes about four hours and a half to fade away, and four hours more to recover its brilliancy. Near the beginning and end of the variations, the change is very slow, so that there are not more than five or six hours during which an ordinary observer would see that the star was less bright than usual.

This variation of light was at first explained by supposing that a large dark planet was revolving round Algol, and passed over its face at every revolution, thus cutting off a portion of its light; but there are small irregularities in the variation, which this theory does not account for.

354. Mira.--Another remarkable variable star is Omicron Ceti, or Mira (that is, the wonderful star). It is generally invisible to the naked eye; but at intervals of about eleven months it shines forth as a star of the second or third magnitude. It is about forty days from the time it becomes visible until it attains its greatest brightness, and is then about two months in fading to invisibility; so that its increase of brilliancy is more rapid than its waning. Its period is quite irregular, ranging from ten to twelve months; so that the times of its appearance cannot be predicted with certainty. Its maximum brightness is also variable, being sometimes of the second magnitude, and at others only of the third or fourth.

355. Eta Argus.--Perhaps the most extraordinary variable star in the heavens is Eta Argus, in the constellation Argo, or the Ship, in the southern hemisphere (Fig. 417). The first careful observations of its variability were made by Sir John Herschel while at the Cape of Good Hope. He says, "It was on the 16th of December, 1837, that, resuming the photometrical comparisons, my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude in a part of the heavens where, being perfectly familiar with it, I was certain that no such brilliant object had before been seen. After a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected, and referring to a map for its configuration with other conspicuous stars in the neighborhood, I became satisfied of its identity with my old acquaintance, Eta Argus. Its light was, however, nearly tripled. While yet low, it equalled Rigel, and, when it attained some altitude, was decidedly greater." It continued to increase until Jan. 2, 1838, then faded a little till April following, though it was still as bright as Aldebaran. In 1842 and 1843 it blazed up brighter than ever, and in March of the latter year was second only to Sirius. During the twenty-five years following it slowly but steadily diminished. In 1867 it was barely visible to the naked eye; and the next year it vanished entirely from the unassisted view, and has not yet begun to recover its brightness. The curve in Fig. 418 shows the change in brightness of this remarkable star. The numbers at the bottom show the years of the century, and those at the side the brightness of the star.