Astronomy of To-day Part 20

The group of the Hyades occupies the "head" of the Bull, and is much more spread out than that of the Pleiades. It is composed besides of brighter stars, the brightest being one of the first magnitude, Aldebaran. This star is of a red colour, and is sometimes known as the "Eye of the Bull."

The constellation of _Orion_ is easily recognised as an irregular quadrilateral formed of four bright stars, two of which, Betelgeux (reddish) and Rigel (brilliant white), are of the first magnitude. In the middle of the quadrilateral is a row of three second magnitude stars, known as the "Belt" of Orion. Jutting off from this is another row of stars called the "Sword" of Orion.

The constellation of _Gemini_, or the Twins, contains two bright stars--Castor and Pollux--close to each other. Pollux, though marked with the Greek letter [b], is the brighter of the two, and nearly of the standard first magnitude.

Just further from the Pole than Gemini, is the constellation of _Canis Minor_, or the Lesser Dog. Its chief star is a white first magnitude one--Procyon.

Still further again from the Pole than Canis Minor is the constellation of _Canis Major_, or the Greater Dog. It contains the brightest star in the whole sky, the first magnitude star Sirius, bluish-white in colour, also known as the "Dog Star." This star is almost in line with the stars forming the Belt of Orion, and is not far from that constellation.

Taken in the following order, the stars Capella, [b] Aurigae, Castor, Pollux, Procyon, and Sirius, when they are all above the horizon at the same time, form a beautiful curve stretching across the heaven.

The groups of stars visible in the southern skies have by no means the same fascination for us as those in the northern. The ancients were in general unacquainted with the regions beyond the equator, and so their scheme of constellations did not include the sky around the South Pole of the heavens. In modern times, however, this part of the celestial expanse was also portioned out into constellations for the purpose of easy reference; but these groupings plainly lack that simplicity of conception and legendary interest which are so characteristic of the older ones.

The brightest star in the southern skies is found in the constellation of _Argo_, and is known as Canopus. In brightness it comes next to Sirius, and so is second in that respect in the entire heaven. It does not, however, rise above the English horizon.

Of the other southern constellations, two call for especial notice, and these adjoin each other. One is _Centaurus_ (or the Centaur), which contains the two first magnitude stars, [a] and [b] Centauri. The first of these, Alpha Centauri, comes next in brightness to Canopus, and is notable as being the nearest of all the stars to our earth. The other constellation is called _Crux_, and contains five stars set in the form of a rough cross, known as the "Southern Cross." The brightest of these, [a] Crucis, is of the first magnitude.

Owing to the Precession of the Equinoxes, which, as we have seen, gradually shifts the position of the Pole among the stars, certain constellations used to be visible in ancient times in more northerly lat.i.tudes than at present. For instance, some five thousand years ago the Southern Cross rose above the English horizon, and was just visible in the lat.i.tude of London. It has, however, long ago even ceased to be seen in the South of Europe. The constellation of Crux happens to be situated in that remarkable region of the southern skies, in which are found the stars Canopus and Alpha Centauri, and also the most brilliant portion of the Milky Way. It is believed to be to this grand celestial region that allusion is made in the Book of Job (ix. 9), under the t.i.tle of the "Chambers of the South." The "Cross" must have been still a notable feature in the sky of Palestine in the days when that ancient poem was written.

There is no star near enough to the southern pole of the heavens to earn the distinction of South Polar Star.

The Galaxy, or _Milky Way_ (see Plate XX., p. 296), is a broad band of diffused light which is seen to stretch right around the sky. The telescope, however, shows it to be actually composed of a great host of very faint stars--too faint, indeed, to be separately distinguished with the naked eye. Along a goodly stretch of its length it is cleft in two; while near the south pole of the heavens it is entirely cut across by a dark streak.

In this rapid survey of the face of the sky, we have not been able to do more than touch in the broadest manner upon some of the most noticeable star groups and a few of the most remarkable stars. To go any further is not a part of our purpose; our object being to deal with celestial bodies as they actually are, and not in those groupings under which they display themselves to us as a mere result of perspective.

[29] Attention must here be drawn to the fact that the name of the constellation is always put in the genitive case.

[30] The early peoples, as we have seen, appear to have been attracted by those groupings of the stars which reminded them in a way of the figures of men and animals. We moderns, on the other hand, seek almost instinctively for geometrical arrangements. This is, perhaps, symptomatic of the evolution of the race. In the growth of the individual we find, for example, something a.n.a.logous. A child, who has been given pencil and paper, is almost certain to produce grotesque drawings of men and animals; whereas the idle and half-conscious scribblings which a man may make upon his blotting-paper are usually of a geometrical character.

[31] Because the line joining them _points_ in the direction of the Pole Star.

CHAPTER XXIV

SYSTEMS OF STARS

Many stars are seen comparatively close together. This may plainly arise from two reasons. Firstly, the stars may happen to be almost in the same line of sight; that is to say, seen in nearly the same direction; and though one star may be ever so much nearer to us than the other, the result will give all the appearance of a related pair. A seeming arrangement of two stars in this way is known as a "double," or double star; or, indeed, to be very precise, an "optical double." Secondly, in a pair of stars, both bodies may be about the same distance from us, and actually connected as a system like, for instance, the moon and the earth. A pairing of stars in this way, though often casually alluded to as a double star, is properly termed a "binary," or binary system.

But collocations of stars are by no means limited to two. We find, indeed, all over the sky such arrangements in which there are three or more stars; and these are technically known as "triple" or "multiple"

stars respectively. Further, groups are found in which a great number of stars are closely ma.s.sed together, such a ma.s.sing together of stars being known as a "cl.u.s.ter."

The Pole Star (Polaris) is a double star, one of the components being of a little below the second magnitude, and the other a little below the ninth. They are so close together that they appear as one star to the naked eye, but they may be seen separate with a moderately sized telescope. The brighter star is yellowish, and the faint one white. This brighter star is found _by means of the spectroscope_ to be actually composed of three stars so very close together that they cannot be seen separately even with a telescope. It is thus a triple star, and the three bodies of which it is composed are in circulation about each other. Two of them are darker than the third.

The method of detecting binary stars by means of the spectroscope is an application of Doppler's principle. It will, no doubt, be remembered that, according to the principle in question, we are enabled, from certain shiftings of the lines in the spectrum of a luminous body, to ascertain whether that body is approaching us or receding from us. Now there are certain stars which always appear single even in the largest telescopes, but when the spectroscope is directed to them a spectrum _with two sets of lines_ is seen. Such stars must, therefore, be double.

Further, if the shiftings of the lines, in a spectrum like this, tell us that the component stars are making small movements to and from us which go on continuously, we are therefore justified in concluding that these are the orbital revolutions of a binary system greatly compressed by distance. Such connected pairs of stars, since they cannot be seen separately by means of any telescope, no matter how large, are known as "spectroscopic binaries."

In observations of spectroscopic binaries we do not always get a double spectrum. Indeed, if one of the components be below a certain magnitude, its spectrum will not appear at all; and so we are left in the strange uncertainty as to whether this component is merely faint or actually dark. It is, however, from the shiftings of the lines in the spectrum of the other component that we see that an orbital movement is going on, and are thus enabled to conclude that two bodies are here connected into a system, although one of these bodies resolutely refuses directly to reveal itself even to the all-conquering spectroscope.

Mizar, that star in the handle of the Plough to which we have already drawn attention, will be found with a small telescope to be a fine double, one of the components being white and the other greenish.

Actually, however, as the American astronomer, Professor F.R. Moulton, points out, these stars are so far from each other that if we could be transferred to one of them we should see the other merely as an ordinary bright star. The spectroscope shows that the brighter of these stars is again a binary system of two huge suns, the components revolving around each other in a period of about twenty days. This discovery made by Professor E.C. Pickering, the _first_ of the kind by means of the spectroscope, was announced in 1889 from the Harvard Observatory in the United States.

A star close to Vega, known as [e] (Epsilon) Lyrae (see Plate XIX., p.

292), is a double, the components of which may be seen separately with the naked eye by persons with very keen eyesight. If this star, however, be viewed with the telescope, the two companions will be seen far apart; and it will be noticed that each of them is again a double.

By means of the spectroscope Capella is shown to be really composed of two stars (one about twice as bright as the other) situated very close together and forming a binary system. Sirius is also a binary system; but it is what is called a "visual" one, for its component stars may be _seen_ separately in very large telescopes. Its double, or rather binary, nature, was discovered in 1862 by the celebrated optician Alvan G. Clark, while in the act of testing the 18-inch refracting telescope, then just constructed by his firm, and now at the Dearborn Observatory, Illinois, U.S.A. The companion is only of the tenth magnitude, and revolves around Sirius in a period of about fifty years, at a mean distance equal to about that of Ura.n.u.s from the sun. Seen from Sirius, it would shine only something like our full moon. It must be self-luminous and not a mere planet; for Mr. Gore has shown that if it shone only by the light reflected from Sirius, it would be quite invisible even in the Great Yerkes Telescope.

Procyon is also a binary, its companion having been discovered by Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of revolution in this system is about forty years. Observations by Mr. T.

Lewis of Greenwich seem, however, to point to the companion being a small nebula rather than a star.

The star [e] (Eta) Ca.s.siopeiae (see Plate XIX., p. 292), is easily seen as a fine double in telescopes of moderate size. It is a binary system, the component bodies revolving around their common centre of gravity in a period of about two hundred years. This system is comparatively near to us, _i.e._ about nine light years, or a little further off than Sirius.

In a small telescope the star Castor will be found double, the components, one of which is brighter than the other, forming a binary system. The fainter of these was found by Belopolsky, with the spectroscope, to be composed of a system of two stars, one bright and the other either dark or not so bright, revolving around each other in a period of about three days. The brighter component of Castor is also a spectroscopic binary, with a period of about nine days; so that the whole of what we see with the naked eye as Castor, is in reality a remarkable system of four stars in mutual orbital movement.

Alpha Centauri--the nearest star to the earth--is a visual binary, the component bodies revolving around each other in a period of about eighty-one years. The extent of this system is about the same as that of Sirius. Viewed from each other, the bodies would shine only like our sun as seen from Neptune.

Among the numerous binary stars the orbits of some fifty have been satisfactorily determined. Many double stars, for which this has not yet been done, are, however, believed to be, without doubt, binary. In some cases a parallax has been found; so that we are enabled to estimate in miles the actual extent of such systems, and the ma.s.ses of the bodies in terms of the sun's ma.s.s.

Most of the spectroscopic binaries appear to be upon a smaller scale than the telescopic ones. Some are, indeed, comparatively speaking, quite small. For instance, the component stars forming [b] Aurigae are about eight million miles apart, while in [z] Geminorum, the distance between the bodies is only a little more than a million miles.

Spectroscopic binaries are probably very numerous. Professor W.W.

Campbell, Director of the Lick Observatory, estimates, for instance, that, out of about every half-a-dozen stars, one is a spectroscopic binary.

It is only in the case of binary systems that we can discover the ma.s.ses of stars at all. These are ascertained from their movements with regard to each other under the influence of their mutual gravitative attractions. In the case of simple stars we have clearly nothing of the kind to judge by; though, if we can obtain a parallax, we may hazard a guess from their brightness.

Binary stars were incidentally discovered by Sir William Herschel. In his researches to get a stellar parallax he had selected a number of double stars for test purposes, on the a.s.sumption that, if one of such a pair were much nearer than the other, it might show a displacement with regard to its neighbour as a direct consequence of the earth's...o...b..tal movement around the sun. He, however, failed entirely to obtain any parallaxes, the triumph in this being, as we have seen, reserved for Bessel. But in some of the double stars which he had selected, he found certain alterations in the relative positions of the bodies, which plainly were not a consequence of the earth's motion, but showed rather that there was an actual circling movement of the bodies themselves under their mutual attractions. It is to be noted that the existence of such connected pairs had been foretold as probable by the Rev. John Mich.e.l.l, who lived a short time before Herschel.

The researches into binary systems--both those which can be seen with the eye and those which can be observed by means of the spectroscope, ought to impress upon us very forcibly the wide sway of the law of gravitation.

Of star cl.u.s.ters about 100 are known, and such systems often contain several thousand stars. They usually cover an area of sky somewhat smaller than the moon appears to fill. In most cl.u.s.ters the stars are very faint, and, as a rule, are between the twelfth and sixteenth magnitudes. It is difficult to say whether these are actually small bodies, or whether their faintness is due merely to their great distance from us, since they are much too far off to show any appreciable parallactic displacement. Mr. Gore, however, thinks there is good evidence to show that the stars in cl.u.s.ters are really close, and that the cl.u.s.ters themselves fill a comparatively small s.p.a.ce.

One of the finest examples of a cl.u.s.ter is the great globular one, in the constellation of Hercules, discovered by Halley in 1714. It contains over 5000 stars, and upon a clear, dark night is visible to the naked eye as a patch of light. In the telescope, however, it is a wonderful object. There are also fine cl.u.s.ters in the constellations of Auriga, Pegasus, and Canes Venatici. In the southern heavens there are some magnificent examples of globular cl.u.s.ters. This hemisphere seems, indeed, to be richer in such objects than the northern. For instance, there is a great one in the constellation of the Centaur, containing some 6000 stars (see Plate XXI., p. 306).

[Ill.u.s.tration: PLATE XXI. THE GREAT GLOBULAR Cl.u.s.tER IN THE SOUTHERN CONSTELLATION OF CENTAURUS

From a photograph taken at the Cape Observatory, on May 24th, 1903. Time of exposure, 1 hour.

(Page 306)]

Certain remarkable groups of stars, of a nature similar to cl.u.s.ters, though not containing such faint or densely packed stars as those we have just alluded to, call for a mention in this connection. The best example of such star groups are the Pleiades and the Hyades (see Plate XX., p. 296), Coma Berenices, and Praesepe (or the Beehive), the last-named being in the constellation of Cancer.

Stars which alter in their brightness are called _Variable Stars_, or "variables." The first star whose variability attracted attention is that known as Omicron Ceti, namely, the star marked with the Greek letter [o] (Omicron) in the constellation of Cetus, or the Whale, a constellation situated not far from Taurus. This star, the variability of which was discovered by Fabricius in 1596, is also known as Mira, or the "Wonderful," on account of the extraordinary manner in which its light varies from time to time. The star known by the name of Algol,[32]

popularly called the "Demon Star"--whose astronomical designation is [b]

(Beta) Persei, or the star second in brightness in the constellation of Perseus--was discovered by Goodricke, in the year 1783, to be a variable star. In the following year [b] Lyrae, the star in Lyra next in order of brightness after Vega, was also found by the same observer to be a variable. It may be of interest to the reader to know that Goodricke was deaf and dumb, and that he died in 1786 at the early age of twenty-one years!