Part 18

2-1/2 stars of the 2d magnitude to make one of the 1st. 6 stars of the 3d magnitude to make one of the 1st. 16 stars of the 4th magnitude to make one of the 1st. 40 stars of the 5th magnitude to make one of the 1st. 100 stars of the 6th magnitude to make one of the 1st. 10,000 stars of the 11th magnitude to make one of the 1st. 1,000,000 stars of the 16th magnitude to make one of the 1st.

323. _The Number of the Stars._--The total number of stars in the celestial sphere visible to the average naked eye is estimated, in round numbers, at five thousand; but the number varies much with the perfection and the training of the eye and with the atmospheric conditions. For every star visible to the naked eye, there are thousands too minute to be seen without telescopic aid. Fig. 364 shows a portion of the constellation of the Twins as seen with the naked eye; and Fig. 365 shows the same region as seen in a powerful telescope.

Struve has estimated that the total number of stars visible with Herschel's twenty-foot telescope was about twenty million. The number that can be seen with the great telescopes of modern times has not been carefully estimated, but is probably somewhere between thirty million and fifty million.

The number of stars between the north pole and the circle thirty-five degrees south of the equator is about as follows:--

Of the 1st magnitude about 14 stars. Of the 2d magnitude about 48 stars. Of the 3d magnitude about 152 stars. Of the 4th magnitude about 313 stars. Of the 5th magnitude about 854 stars. Of the 6th magnitude about 2010 stars. ---- Total visible to naked eye 3391 stars.

The number of stars of the several magnitudes is approximately in inverse proportion to that of their brightness, the ratio being a little greater in the higher magnitudes, and probably a little less in the lower ones.

324. _The Division of the Stars into Constellations._--A glance at the heavens is sufficient to show that the stars are not distributed uniformly over the sky. The larger ones especially are collected into more or less irregular groups. The larger groups are called _constellations_. At a very early period a mythological figure was allotted to each constellation; and these figures were drawn in such a way as to include the principal stars of each constellation. The heavens thus became covered, as it were, with immense hieroglyphics.

There is no historic record of the time when these figures were formed, or of the principle in accordance with which they were constructed. It is probable that the imagination of the earlier peoples may, in many instances, have discovered some fanciful resemblance in the configuration of the stars to the forms depicted. The names are still retained, although the figures no longer serve any astronomical purpose. The constellation Hercules, for instance, no longer represents the figure of a man among the stars, but a certain portion of the heavens within which the ancients placed that figure. In star-maps intended for school and popular use it is still customary to give these figures; but they are not generally found on maps designed for astronomers.

325. _The Naming of the Stars._--The brighter stars have all proper names, as _Sirius_, _Procyon_, _Arcturus_, _Capella_, _Aldebaran_, etc. This method of designating the stars was adopted by the Arabs. Most of these names have dropped entirely out of astronomical use, though many are popularly retained. The brighter stars are now generally designated by the letters of the Greek alphabet,--_alpha_, _beta_, _gamma_, etc.,--to which is appended the genitive of the name of the constellation, the first letter of the alphabet being used for the brightest star, the second for the next brightest, and so on. Thus _Aldebaran_ would be designated as _Alpha Tauri_. In speaking of the stars of any one constellation, we simply designate them by the letters of the Greek alphabet, without the addition of the name of the constellation, which answers to a person's surname, while the Greek letter answers to his Christian name. The names of the seven stars of the "Dipper" are given in Fig. 366. When the letters of the Greek alphabet are exhausted, those of the Roman alphabet are employed. The fainter stars in a constellation are usually designated by some system of numbers.

326. _The Milky-Way, or Galaxy._--The Milky-Way is a faint luminous band, of irregular outline, which surrounds the heavens with a great circle, as shown in Fig. 367. Through a considerable portion of its course it is divided into two branches, and there are various vacant spaces at different points in this band; but at only one point in the southern hemisphere is it entirely interrupted.

The telescope shows that the Galaxy arises from the light of countless stars too minute to be separately visible with the naked eye. The telescopic stars, instead of being uniformly distributed over the celestial sphere, are mostly condensed in the region of the Galaxy. They are fewest in the regions most distant from this belt, and become thicker as we approach it. The greater the telescopic power, the more marked is the condensation. With the naked eye the condensation is hardly noticeable; but with the aid of a very small telescope, we see a decided thickening of the stars in and near the Galaxy, while the most powerful telescopes show that a large majority of the stars lie actually in the Galaxy. If all the stars visible with a twelve-inch telescope were blotted out, we should find that the greater part of those remaining were in the Galaxy.

The increase in the number of the stars of all magnitudes as we approach the plane of the Milky-Way is shown in Fig. 368. The curve _acb_ shows by its height the distribution of the stars above the ninth magnitude, and the curve _ACB_ those of all magnitudes.

327. _Star-Clusters._--Besides this gradual and regular condensation towards the Galaxy, occasional aggregations of stars into _clusters_ may be seen. Some of these are visible to the naked eye, sometimes as separate stars, like the "Seven Stars," or Pleiades, but more commonly as patches of diffused light, the stars being too small to be seen separately. The number visible in powerful telescopes is, however, much greater. Sometimes hundreds or even thousands of stars are visible in the field of view at once, and sometimes the number is so great that they cannot be counted.

328. _Nebulæ._--Another class of objects which are found in the celestial spaces are irregular masses of soft, cloudy light, known as _nebulæ_. Many objects which look like nebulæ in small telescopes are shown by more powerful instruments to be really star-clusters. But many of these objects are not composed of stars at all, being immense masses of gaseous matter.

The general distribution of nebulæ is the reverse of that of the stars. Nebulæ are thickest where stars are thinnest. While stars are most numerous in the region of the Milky-Way, nebulæ are most abundant about the poles of the Milky-Way. This condensation of nebulæ about the poles of the Milky-Way is shown in Figs. 367 and 369, in which the points represent, not stars, but nebulæ.

II. THE STARS.

The Constellations.

329. _The Great Bear._--The Great Bear, or _Ursa Major_, is one of the circumpolar constellations (4), and contains one of the most familiar _asterisms_, or groups of stars, in our sky; namely, the _Great Dipper_, or _Charles's Wain_. The positions and names of the seven prominent stars in it are shown in Fig. 370. The two stars Alpha and Beta are called the _Pointers_. This asterism is sometimes called the _Butcher's Cleaver_. The whole constellation is shown in Fig. 371. A rather faint star marks the nose of the bear, and three equidistant pairs of faint stars mark his feet.

330. _The Little Bear, Draco, and Cassiopeia._--These are all circumpolar constellations. The most important star of the Little Bear, or _Ursa Minor_, is _Polaris_, or the _Pole Star_. This star may be found by drawing a line from Beta to Alpha of the Dipper, and prolonging it as shown in Fig. 372. This explains why these stars are called the _Pointers_. The Pole Star, with the six other chief stars of the Little Bear, form an asterism called the _Little Dipper_. These six stars are joined with Polaris by a dotted line in Fig. 372.

The stars in a serpentine line between the two Dippers are the chief stars of _Draco_, or the _Dragon_; the trapezium marking its head. Fig. 373 shows the constellations of Ursa Minor and Draco as usually figured.

To find _Cassiopeia_, draw a line from Delta of the Dipper to Polaris, and prolong it about an equal distance beyond, as shown in Fig. 372. This line will pass near Alpha of Cassiopeia. The five principal stars of this constellation form an irregular _W_, opening towards the pole. Between Cassiopeia and Draco are five rather faint stars, which form an irregular _K_. These are the principal stars of the constellation _Cepheus_. These two constellations are shown in Fig. 374.

331. _The Lion, Berenice's Hair, and the Hunting-Dogs._--A line drawn from Alpha to Beta of the Dipper, and prolonged as shown in Fig. 375, will pass between the two stars _Denebola_ and _Regulus_ of _Leo_, or the _Lion_. Regulus forms a _sickle_ with several other faint stars, and marks the heart of the lion. Denebola is at the apex of a right-angled triangle, which it forms with two other stars, and marks the end of the lion's tail. This constellation is visible in the evening from February to July, and is figured in Fig. 376.

In a straight line between Denebola and Eta, at the end of the Great Bear's tail, are, at about equal distances, the two small constellations of _Coma Berenices_, or _Berenice's Hair_, and _Canes Venatici_, or the _Hunting-Dogs_. These are shown in Fig. 377. The dogs are represented as pursuing the bear, urged on by the huntsman _Boötes_.

332. _Boötes, Hercules, and the Northern Crown._--_Arcturus_, the principal star of _Boötes_, may be found by drawing a line from Zeta to Eta of the Dipper, and then prolonging it with a slight bend, as shown in Fig. 378. Arcturus and Polaris form a large isosceles triangle with a first-magnitude star called _Vega_. This triangle encloses at one corner the principal stars of Boötes, and the head of the Dragon near the opposite side. The side running from Arcturus to Vega passes through _Corona Borealis_, or the _Northern Crown_, and the body of _Hercules_, which is marked by a quadrilateral of four stars.

_Boötes_, who is often represented as a husbandman, _Corona Borealis_, and _Hercules_, are delineated in Fig. 379. These constellations are visible in the evening from May to September.

333. _The Lyre, the Swan, the Eagle, and the Dolphin._--_Altair_, the principal star of _Aquila_, or the _Eagle_, lies on the opposite side of the Milky-Way from Vega. Altair is a first-magnitude star, and has a faint star on each side of it, as shown in Fig. 380. Vega, also of the first magnitude, is the principal star of _Lyra_, or the _Lyre_. Between these two stars, and a little farther to the north, are several stars arranged in the form of an immense cross. The bright star at the head of this cross is called _Deneb_. The cross lies in the Milky-Way, and contains the chief stars of the constellation _Cygnus_, or the _Swan_. A little to the north of Altair are four stars in the form of a diamond. This asterism is popularly known as _Job's Coffin_. These four stars are the chief stars of _Delphinus_, or the _Dolphin_. These four constellations are shown together in Fig. 381. The _Swan_ is visible from June to December, in the evening.

334. _Virgo._--A line drawn from Alpha to Gamma of the Dipper, and prolonged with a slight bend at Gamma, will reach to a first-magnitude star called _Spica_ (Fig. 382). This is the chief star of the constellation _Virgo_, or the _Virgin_, and forms a large isosceles triangle with _Arcturus_ and _Denebola_.

_Virgo_ is represented in Fig. 383. To the right of this constellation, as shown in the figure, there are four stars which form a trapezium, and mark the constellation _Corvus_, or the _Crow_. This bird is represented as standing on the body of _Hydra_, or the _Water-Snake_. _Virgo_ is visible in the evening, from April to August.

335. _The Twins._--A line drawn from Delta to Beta of the Dipper, and prolonged as shown in Fig. 384, passes between two bright stars called _Castor_ and _Pollux_. The latter of these is usually reckoned as a first-magnitude star. These are the principal stars of the constellation _Gemini_, or the _Twins_, which is shown in Fig. 385. The constellation _Canis Minor_, or the _Little Dog_, is shown in the lower part of the figure. There are two conspicuous stars in this constellation, the brightest of which is of the first magnitude, and called _Procyon_.

The region to which we have now been brought is the richest of the northern sky, containing no less than seven first-magnitude stars. These are _Sirius_, _Procyon_, _Pollux_, _Capella_, _Aldebaran_, _Betelgeuse_, and _Rigel_. They are shown in Fig. 386.

_Betelgeuse_ and _Rigel_ are in the constellation _Orion_, being about equally distant to the north and south from the three stars forming the _belt_ of Orion. Betelgeuse is a red star. _Sirius_ is the brightest star in the heavens, and belongs to the constellation _Canis Major_, or the _Great Dog_. It lies to the east of the belt of Orion. _Aldebaran_ lies at about the same distance to the west of the belt. It is a red star, and belongs to the constellation _Taurus_, or the _Bull_. _Capella_ is in the constellation _Auriga_, or the _Wagoner_. These stars are visible in the evening, from about December to April.

336. _Orion and his Dogs, and Taurus._--_Orion_ and his _Dogs_ are shown in Fig. 387, and _Orion_ and _Taurus_ in Fig. 388. _Aldebaran_ marks one of the eyes of the bull, and is often called the _Bull's Eye_. The irregular _V_ in the face of the bull is called the _Hyades_, and the cluster on the shoulder the _Pleiades_.

337. _The Wagoner._--The constellation _Auriga_, or the _Wagoner_ (sometimes called the _Charioteer_), is shown in Fig. 389. _Capella_ marks the _Goat_, which he is represented as carrying on his back, and the little right-angled triangle of stars near it the _Kids_. The five chief stars of this constellation form a large, irregular pentagon. Gamma of _Auriga_ is also Beta of _Taurus_, and marks one of the horns of the _Bull_.

338. _Pegasus, Andromeda, and Perseus._--A line drawn from Polaris near to Beta of _Cassiopeia_ will lead to a bright second-magnitude star at one corner of a large square (Fig. 390). Alpha belongs both to the _Square of Pegasus_ and to _Andromeda_. Beta and Gamma, which are connected with Alpha in the figure by a dotted line, also belong to Andromeda. _Algol_, which forms, with the last-named stars and with the _Square of Pegasus_, an asterism similar in configuration to the _Great Dipper_, belongs to _Perseus_. _Algenib_, which is reached by bending the line at Gamma in the opposite direction, is the principal star of _Perseus_.

_Pegasus_ is shown in Fig. 391, and _Andromeda_ in Fig. 392. _Cetus_, the _Whale_, or the _Sea Monster_, shown in Fig. 393, belongs to the same mythological group of constellations.

339. _Scorpio, Sagittarius, and Ophiuchus._--During the summer months a brilliant constellation is visible, called _Scorpio_, or the _Scorpion_. The configuration of the chief stars of this constellation is shown in Fig. 394. They bear some resemblance to a boy's kite. The brightest star is of the first magnitude, and called _Antares_ (from _anti_, instead of, and _Ares_, the Greek name of Mars), because it rivals Mars in redness. The stars in the tail of the Scorpion are visible in our latitude only under very favorable circumstances. This constellation is shown in Fig. 395, together with _Sagittarius_ and _Ophiuchus_. _Sagittarius_, or the _Archer_, is to the east of _Scorpio_. It contains no bright stars, but is easily recognized from the fact that five of its principal stars form the outline of an inverted dipper, which, from the fact of its being partly in the Milky-Way, is often called the _Milk Dipper_.

_Ophiuchus_, or the _Serpent-Bearer_, is a large constellation, filling all the space between the head of _Hercules_ and _Scorpio_. It is difficult to trace, since it contains no very brilliant stars. This constellation and _Libra_, or the _Balances_, which is the zodiacal constellation to the west of Scorpio, are shown in Fig. 396.

340. _Capricornus, Aquarius, and the Southern Fish._--The two zodiacal constellations to the east of Sagittarius are _Capricornus_ and _Aquarius_. _Capricornus_ contains three pairs of small stars, which mark the head, the tail, and the knees of the animal.

_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.