Letters on Astronomy Part 20

By the aid of the telescope, new fields of stars present themselves, of boundless extent; the number continually augmenting, as the powers of the telescope are increased. Lalande, in his 'Histoire Celeste,' has registered the positions of no less than fifty thousand; and the whole number visible in the largest telescopes amounts to many millions.

When you look at the firmament on a clear Autumnal or Winter evening, it appears so thickly studded with stars, that you would perhaps imagine that the task of learning even the brightest of them would be almost hopeless. Let me a.s.sure you, this is all a mistake. On the contrary, it is a very easy task to become acquainted with the names and positions of the stars of the first magnitude, and of the leading constellations. If you will give a few evenings to the study, you will be surprised to find, both how rapidly you can form these new acquaintances, and how deeply you will become interested in them. I would advise you, at first, to obtain, for an evening or two, the a.s.sistance of some friend who is familiar with the stars, just to point out a few of the most conspicuous constellations. This will put you on the track, and you will afterwards experience no difficulty in finding all the constellations and stars that are particularly worth knowing; especially if you have before you a map of the stars, or, what is much better, a celestial globe. It is a pleasant evening recreation for a small company of young astronomers to go out together, and learn one or two constellations every favorable evening, until the whole are mastered. If you have a celestial globe, _rectify_ it for the evening; that is, place it in such a position, that the constellations shall be seen on it in the same position with respect to the horizon, that they have at that moment in the sky itself. To do this, I first elevate the north pole until the number of degrees on the bra.s.s meridian from the pole to the horizon corresponds to my lat.i.tude, (forty-one degrees and eighteen minutes.) I then find the sun's place in the ecliptic, by looking for the day of the month on the broad horizon, and against it noting the corresponding sign and degree. I now find the same sign and degree on the ecliptic itself, and bring that point to the bra.s.s meridian. As that will be the position of the sun at noon, I set the hour-index at twelve, and then turn the globe westward, until the index points to the given hour of the evening. If I now inspect the figures of the constellations, and then look upward at the firmament, I shall see that the latter are spread over the sky in the same manner as the pictures of them are painted on the globe. I will point out a few marks by which the leading constellations may be recognised; this will aid you in finding them, and you can afterwards learn the individual stars of a constellation, to any extent you please, by means of the globes or maps. Let us begin with the _Constellations of the Zodiac_, which, succeeding each other, as they do, in a known order, are most easily found.

_Aries_ (_the Ram_) is a small constellation, known by two bright stars which form his head, _Alpha_ and _Beta Arietis_. These two stars are about four degrees apart; and directly south of Beta, at the distance of one degree, is a smaller star, _Gamma Arietis_. It has been already intimated that the Vernal equinox probably was near the head of Aries, when the signs of the zodiac received their present names.

_Taurus_ (_the Bull_) will be readily found by the seven stars, or _Pleiades_, which lie in his neck. The largest star in Taurus is _Aldebaran_, in the Bull's eye, a star of the first magnitude, of a reddish color, somewhat resembling the planet Mars. Aldebaran and four other stars, close together in the face of Taurus, compose the _Hyades_.

_Gemini_ (_the Twins_) is known by two very bright stars, _Castor and Pollux_, five degrees asunder. Castor (the northern) is of the first, and Pollux of the second, magnitude.

_Cancer_ (_the Crab_.) There are no large stars in this constellation, and it is regarded as less remarkable than any other in the zodiac. It contains, however, an interesting group of small stars, called _Praesepe_, or the nebula of Cancer, which resembles a comet, and is often mistaken for one, by persons unacquainted with the stars. With a telescope of very moderate powers this nebula is converted into a beautiful a.s.semblage of exceedingly bright stars.

_Leo_ (_the Lion_) is a very large constellation, and has many interesting members. _Regulus_ (_Alpha Leonis_) is a star of the first magnitude, which lies directly in the ecliptic, and is much used in astronomical observations. North of Regulus, lies a semicircle of bright stars, forming a _sickle_, of which Regulus is the handle. _Denebola_, a star of the second magnitude, is in the Lion's tail, twenty-five degrees northeast of Regulus.

_Virgo_ (_the Virgin_) extends a considerable way from west to east, but contains only a few bright stars. _Spica_, however, is a star of the first magnitude, and lies a little east of the place of the Autumnal equinox. Eighteen degrees eastward of Denebola, and twenty degrees north of Spica, is _Vindemiatrix_, in the arm of Virgo, a star of the third magnitude.

_Libra_ (_the Balance_) is distinguished by three large stars, of which the two brightest const.i.tute the beam of the balance, and the smallest forms the top or handle.

_Scorpio_ (_the Scorpion_) is one of the finest of the constellations.

His head is formed of five bright stars, arranged in the arc of a circle, which is crossed in the centre by the ecliptic nearly at right angles, near the brightest of the five, _Beta Scorpionis_. Nine degrees southeast of this is a remarkable star of the first magnitude, of a reddish color, called _Cor Scorpionis_, or _Antares_. South of this, a succession of bright stars sweep round towards the east, terminating in several small stars, forming the tail of the Scorpion.

_Sagittarius_ (_the Archer_.) Northeast of the tail of the Scorpion are three stars in the arc of a circle, which const.i.tute the _bow_ of the Archer, the central star being the brightest, directly west of which is a bright star which forms the _arrow_.

_Capricornus_ (_the Goat_) lies northeast of Sagittarius, and is known by two bright stars, three degrees apart, which form the head.

_Aquarius_ (_the Water-Bearer_) is recognised by two stars in a line with _Alpha Capricorni_, forming the shoulders of the figure. These two stars are ten degrees apart; and three degrees southeast is a third star, which, together with the other two, make an acute triangle, of which the westernmost is the vertex.

_Pisces_ (_the Fishes_) lie between Aquarius and Aries. They are not distinguished by any large stars, but are connected by a series of small stars, that form a crooked line between them. _Piscis Australia_, the Southern Fish, lies directly below Aquarius, and is known by a single bright star far in the south, having a declination of thirty degrees.

The name of this star is _Fomalhaut_, and it is much used in astronomical measurements.

The constellations of the zodiac, being first well learned, so as to be readily recognised, will facilitate the learning of others that lie north and south of them. Let us, therefore, next review the princ.i.p.al _Northern Constellations_, beginning north of Aries, and proceeding from west to east.

_Andromeda_ is characterized by three stars of the second magnitude, situated in a straight line, extending from west to east. The middle star is about seventeen degrees north of Beta Arietis. It is in the girdle of Andromeda, and is named _Mirach_. The other two lie at about equal distances, fourteen degrees west and east of Mirach. The western star, in the head of Andromeda, lies in the equinoctial colure. The eastern star, _Alamak_, is situated in the foot.

_Perseus_ lies directly north of the Pleiades, and contains several bright stars. About eighteen degrees from the Pleiades is _Algol_, a star of the second magnitude, in the head of Medusa, which forms a part of the figure; and nine degrees northeast of Algol is _Algenib_, of the same magnitude, in the back of Perseus. Between Algenib and the Pleiades are three bright stars, at nearly equal intervals, which compose the right leg of Perseus.

_Auriga_ (_the Wagoner_) lies directly east of Perseus, and extends nearly parallel to that constellation, from north to south. _Capella_, a very white and beautiful star of the first magnitude, distinguishes this constellation. The feet of Auriga are near the Bull's horns.

The _Lynx_ comes next, but presents nothing particularly interesting, containing no stars above the fourth magnitude.

_Leo Minor_ consists of a collection of small stars north of the sickle in Leo, and south of the Great Bear. Its largest star is only of the third magnitude.

_Coma Berenices_ is a cl.u.s.ter of small stars, north of Denebola, in the tail of the Lion, and of the head of Virgo. About twelve degrees directly north of Berenice's hair, is a single bright star, called _Cor Caroli_, or Charles's Heart.

_Bootes_, which comes next, is easily found by means of _Arcturus_, a star of the first magnitude, of a reddish color, which is situated near the knee of the figure. Arcturus is accompanied by three small stars, forming a triangle a little to the southwest. Two bright stars, _Gamma_ and _Delta Bootis_, form the shoulders, and _Beta_, of the third magnitude, is in the head, of the figure.

_Corona Borealis_, (_the Crown_,) which is situated east of Bootes, is very easily recognised, composed as it is of a semicircle of bright stars. In the centre of the bright crown is a star of the second magnitude, called _Gemma_: the remaining stars are all much smaller.

_Hercules_, lying between the Crown on the west and the Lyre on the east, is very thickly set with stars, most of which are quite small.

This constellation covers a great extent of the sky, especially from north to south, the head terminating within fifteen degrees of the equator, and marked by a star of the third magnitude, called _Ras Algethi_, which is the largest in the constellation.

_Ophiucus_ is situated directly south of Hercules, extending some distance on both sides of the equator, the feet resting on the Scorpion.

The head terminates near the head of Hercules, and, like that, is marked by a bright star within five degrees of _Alpha Herculis_ Ophiucus is represented as holding in his hands the _Serpent_, the head of which, consisting of three bright stars, is situated a little south of the Crown. The folds of the serpent will be easily followed by a succession of bright stars, which extend a great way to the east.

_Aquila_ (_the Eagle_) is conspicuous for three bright stars in its neck, of which the central one, _Altair_, is a very brilliant white star of the first magnitude. _Antinous_ lies directly south of the Eagle, and north of the head of Capricornus.

_Delphinus_ (_the Dolphin_) is a small but beautiful constellation, a few degrees east of the Eagle, and is characterized by four bright stars near to one another, forming a small rhombic square. Another star of the same magnitude, five degrees south, makes the tail.

_Pegasus_ lies between Aquarius on the southwest and Andromeda on the northeast. It contains but few large stars. A very regular square of bright stars is composed of _Alpha Andromedae_ and the three largest stars in Pegasus; namely, _Scheat_, _Markab_, and _Algenib_. The sides composing this square are each about fifteen degrees. Algenib is situated in the equinoctial colure.

We may now review the _Constellations which surround the north pole_, within the circle of perpetual apparition.

_Ursa Minor_ (_the Little Bear_) lies nearest the pole. The pole-star, _Polaris_, is in the extremity of the tail, and is of the third magnitude. Three stars in a straight line, four degrees or five degrees apart, commencing with the pole-star, lead to a trapezium of four stars, and the whole seven form together a _dipper_,--the trapezium being the body and the three stars the handle.

_Ursa Major_ (_the Great Bear_) is situated between the pole and the Lesser Lion, and is usually recognised by the figure of a larger and more perfect dipper which const.i.tutes the hinder part of the animal.

This has also seven stars, four in the body of the Dipper and three in the handle. All these are stars of much celebrity. The two in the western side of the Dipper, Alpha and Beta, are called _Pointers_, on account of their always being in a right line with the pole-star, and therefore affording an easy mode of finding that. The first star in the tail, next the body, is named _Alioth_, and the second, _Mizar_. The head of the Great Bear lies far to the westward of the Pointers, and is composed of numerous small stars; and the feet are severally composed of two small stars very near to each other.

_Draco_ (_the Dragon_) winds round between the Great and the Little Bear; and, commencing with the tail, between the Pointers and the pole-star, it is easily traced by a succession of bright stars extending from west to east. Pa.s.sing under Ursa Minor, it returns westward, and terminates in a triangle which forms the head of Draco, near the feet of Hercules, northwest of Lyra. _Cepheus_ lies eastward of the breast of the Dragon, but has no stars above the third magnitude.

_Ca.s.siopeia_ is known by the figure of a _chair_, composed of four stars which form the legs, and two which form the back. This constellation lies between Perseus and Cepheus, in the Milky Way.

_Cygnus_ (_the Swan_) is situated also in the Milky Way, some distance southwest of Ca.s.siopeia, towards the Eagle. Three bright stars, which lie along the Milky Way, form the body and neck of the Swan, and two others, in a line with the middle one of the three, one above and one below, const.i.tute the wings. This constellation is among the few that exhibit some resemblance to the animals whose names they bear.

_Lyra_ (_the Lyre_) is directly west of the Swan, and is easily distinguished by a beautiful white star of the first magnitude, _Alpha Lyrae_.

The _Southern Constellations_ are comparatively few in number. I shall notice only the Whale, Orion, the Greater and Lesser Dog, Hydra, and the Crow.

_Cetus_ (_the Whale_) is distinguished rather for its extent than its brilliancy, reaching as it does through forty degrees of longitude, while none of its stars, except one, are above the third magnitude.

_Menkar_ (_Alpha Ceti_) in the mouth, is a star of the second magnitude; and several other bright stars, directly south of Aries, make the head and neck of the Whale. _Mira_, (_Omicron Ceti_,) in the neck of the Whale, is a variable star.

_Orion_ is one of the largest and most beautiful of the constellations, lying southeast of Taurus. A cl.u.s.ter of small stars forms the head; two large stars, _Betalgeus_ of the first and _Bellatrix_ of the second magnitude, make the shoulders; three more bright stars compose the buckler, and three the sword; and _Rigel_, another star of the first magnitude, makes one of the feet. In this constellation there are seventy stars plainly visible to the naked eye, including two of the first magnitude, four of the second, and three of the third.

_Canis Major_ lies southeast of Orion, and is distinguished chiefly by its containing the largest of the fixed stars, _Sirius_.

_Canis Minor_, a little north of the equator, between Canis Major and Gemini, is a small constellation, consisting chiefly of two stars, of which, _Procyon_ is of the first magnitude.

_Hydra_ has its head near Procyon, consisting of a number of stars of ordinary brightness. About fifteen degrees southeast of the head is a star of the second magnitude, forming the heart, (_Cor Hydrae_;) and eastward of this is a long succession of stars of the fourth and fifth magnitudes, composing the body and tail, and reaching a few degrees south of Spica Virginis.

_Corvus_ (_the Crow_) is represented as standing on the tail of Hydra.

It consists of small stars, only three of which are as large as the third magnitude.

In a.s.signing the places of individual stars, I have not aimed at great precision; but such a knowledge as you will acquire of the constellations and larger stars, by nothing more even than you can obtain from the foregoing sketch, will not only add greatly to the interest with which you will ever afterwards look at the starry heavens, but it will enable you to locate any phenomenon that may present itself in the nocturnal sky, and to understand the position of any object that may be described, by a.s.signing its true place among the stars; although I hope you will go much further than this mere outline, in cultivating an actual acquaintance with the stars. Leaving, now, these great divisions of the bodies of the firmament, let us ascend to the next order of stars, composing Cl.u.s.tERS.

In various parts of the nocturnal heavens are seen large groups which, either by the naked eye, or by the aid of the smallest telescope, are perceived to consist of a great number of small stars. Such are the Pleiades, Coma Berenices, and Praesepe, or the Bee-hive, in Cancer. The _Pleiades_, or Seven Stars, as they are called, in the neck of Taurus, is the most conspicuous cl.u.s.ter. When we look _directly_ at this group, we cannot distinguish more than six stars; but by turning the eye _sideways_ upon it, we discover that there are many more; for it is a remarkable fact that indirect vision is far more delicate than direct.

Thus we can see the zodiacal light or a comet's tail much more distinctly and better defined, if we fix one eye on a part of the heavens at some distance and turn the other eye obliquely upon the object, than we can by looking directly towards it. Telescopes show the Pleiades to contain fifty or sixty stars, crowded together, and apparently insulated from the other parts of the heavens. _Coma Berenices_ has fewer stars, but they are of a larger cla.s.s than those which compose the Pleiades. The _Bee-hive_, or Nebula of Cancer, as it is called, is one of the finest objects of this kind for a small telescope, being by its aid converted into a rich congeries of s.h.i.+ning points. The head of Orion affords an example of another cl.u.s.ter, though less remarkable than those already mentioned. These cl.u.s.ters are pleasing objects to the telescope; and since a common spygla.s.s will serve to give a distinct view of most of them, every one may have the power of taking the view. But we pa.s.s, now, to the third order of stars, which present themselves much more obscurely to the gaze of the astronomer, and require large instruments for the full developement of their wonderful organization. These are the NEBULae.

[Ill.u.s.tration Figures 70, 71, 72, 73. Cl.u.s.tERS OF STARS AND NEBULae.]

Nebulae are faint misty appearances which are dimly seen among the stars, resembling comets, or a speck of fog. They are usually resolved by the telescope into myriads of small stars; though in some instances, no powers of the telescope have been found sufficient thus to resolve them.

The _Galaxy_ or Milky Way, presents a continued succession of large nebulas. The telescope reveals to us innumerable objects of this kind.