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Astronomy of To-day By Cecil G. Dolmage Characters: 21752

Updated: 2017-12-06 00:02


THE STARS-continued

The stars appear to us to be scattered about the sky without any orderly arrangement. Further, they are of varying degrees of brightness; some being extremely brilliant, whilst others can but barely be seen. The brightness of a star may arise from either of two causes. On the one hand, the body may be really very bright in itself; on the other hand, it may be situated comparatively near to us. Sometimes, indeed, both these circumstances may come into play together.

Since variation in brightness is the most noticeable characteristic of the stars, men have agreed to class them in divisions called "magnitudes." This term, it must be distinctly understood, is employed in such classification without any reference whatever to actual size, being merely taken to designate roughly the amount of light which we receive from a star. The twenty brightest stars in the sky are usually classed in the first magnitude. In descending the scale, each magnitude will be noticed to contain, broadly speaking, three times as many stars as the one immediately above it. Thus the second magnitude contains 65, the third 190, the fourth 425, the fifth 1100, and the sixth 3200. The last of these magnitudes is about the limit of the stars which we are able to see with the naked eye. Adding, therefore, the above numbers together, we find that, without the aid of the telescope, we cannot see more than about 5000 stars in the entire sky-northern and southern hemispheres included. Quite a small telescope will, however, allow us to see down to the ninth magnitude, so that the total number of stars visible to us with such very moderate instrumental means will be well over 100,000.

It must not, however, be supposed that the stars included within each magnitude are all of exactly the same brightness. In fact, it would be difficult to say if there exist in the whole sky two stars which send us precisely the same amount of light. In arranging the magnitudes, all that was done was to make certain broad divisions, and to class within them such stars as were much on a par with regard to brightness. It may here be noted that a standard star of the first magnitude gives us about one hundred times as much light as a star of the sixth magnitude, and about one million times as much as one of the sixteenth magnitude-which is near the limit of what we can see with the very best telescope.

Though the first twenty stars in the sky are popularly considered as being of the first magnitude, yet several of them are much brighter than an average first magnitude star would be. For instance, Sirius-the brightest star in the whole sky-is equal to about eleven first magnitude stars, like, say, Aldebaran. In consequence of such differences, astronomers are agreed in classifying the brightest of them as brighter than the standard first magnitude star. On this principle Sirius would be about two and a half magnitudes above the first. This notation is usefully employed in making comparisons between the amount of light which we receive from the sun, and that which we get from an individual star. Thus the sun will be about twenty-seven and a half magnitudes above the first magnitude. The range, therefore, between the light which we receive from the sun (considered merely as a very bright star) and the first magnitude stars is very much greater than that between the latter and the faintest star which can be seen with the telescope, or even registered upon the photographic plate.

To classify stars merely by their magnitudes, without some definite note of their relative position in the sky, would be indeed of little avail. We must have some simple method of locating them in the memory, and the constellations of the ancients here happily come to our aid. A system combining magnitudes with constellations was introduced by Bayer in 1603, and is still adhered to. According to this the stars in each constellation, beginning with the brightest star, are designated by the letters of the Greek alphabet taken in their usual order. For example, in the constellation of Canis Major, or the Greater Dog, the brightest star is the well-known Sirius, called by the ancients the "Dog Star"; and this star, in accordance with Bayer's method, has received the Greek letter α (alpha), and is consequently known as Alpha Canis Majoris.[29] As soon as the Greek letters are used up in this way the Roman alphabet is brought into requisition, after which recourse is had to ordinary numbers.

Notwithstanding this convenient arrangement, some of the brightest stars are nearly always referred to by certain proper names given to them in old times. For instance, it is more usual to speak of Sirius, Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of α Canis Majoris, α Bo?tis, α Lyr?, α Aurig?, α Canis Minoris, α Tauri, α Leonis, &c. &c.

In order that future generations might be able to ascertain what changes were taking place in the face of the sky, astronomers have from time to time drawn up catalogues of stars. These lists have included stars of a certain degree of brightness, their positions in the sky being noted with the utmost accuracy possible at the period. The earliest known catalogue of this kind was made, as we have seen, by the celebrated Greek astronomer, Hipparchus, about the year 125 B.C. It contained 1080 stars. It was revised and brought up to date by Ptolemy in A.D. 150. Another celebrated list was that drawn up by the Persian astronomer, Al Sufi, about the year A.D. 964. In it 1022 stars were noted down. A catalogue of 1005 stars was made in 1580 by the famous Danish astronomer, Tycho Brahe. Among modern catalogues that of Argelander (1799–1875) contained as many as 324,198 stars. It was extended by Sch?nfeld so as to include a portion of the Southern Hemisphere, in which way 133,659 more stars were added.

In recent years a project was placed on foot of making a photographic survey of the sky, the work to be portioned out among various nations. A great part of this work has already been brought to a conclusion. About 15,000,000 stars will appear upon the plates; but, so far, it has been proposed to catalogue only about a million and a quarter of the brightest of them. This idea of surveying the face of the sky by photography sprang indirectly from the fine photographs which Sir David Gill took, when at the Cape of Good Hope, of the Comet of 1882. The immense number of star-images which had appeared upon his plates suggested the idea that photography could be very usefully employed to register the relative positions of the stars.

The arrangement of seven stars known as the "Plough" is perhaps the most familiar configuration in the sky (see Plate XIX., p. 292). In the United States it is called the "Dipper," on account of its likeness to the outline of a saucepan, or ladle. "Charles' Wain" was the old English name for it, and readers of C?sar will recollect it under Septentriones, or the "Seven Stars," a term which that writer uses as a synonym for the North. Though identified in most persons' minds with Ursa Major, or the Great Bear, the Plough is actually only a small portion of that famous constellation. Six out of the seven stars which go to make up the well-known figure are of the second magnitude, while the remaining one, which is the middle star of the group, is of the third.

The Greek letters, as borne by the individual stars of the Plough, are a plain transgression of Bayer's method as above described, for they have certainly not been allotted here in accordance with the proper order of brightness. For instance, the third magnitude star, just alluded to as being in the middle of the group, has been marked with the Greek letter δ (Delta); and so is made to take rank before the stars composing what is called the "handle" of the Plough, which are all of the second magnitude. Sir William Herschel long ago drew attention to the irregular manner in which Bayer's system had been applied. It is, indeed, a great pity that this notation was not originally worked out with greater care and correctness; for, were it only reliable, it would afford great assistance to astronomers in judging of what changes in relative brightness have taken place among the stars.

Though we may speak of using the constellations as a method of finding our way about the sky, it is, however, to certain marked groupings in them of the brighter stars that we look for our sign-posts.

Most of the constellations contain a group or so of noticeable stars, whose accidental arrangement dimly recalls the outline of some familiar geometrical figure and thus arrests the attention.[30] For instance, in an almost exact line with the two front stars of the Plough, or "pointers" as they are called,[31] and at a distance about five times as far away as the interval between them, there will be found a third star of the second magnitude. This is known as Polaris, or the Pole Star, for it very nearly occupies that point of the heaven towards which the north pole of the earth's axis is at present directed (see Plate XIX., p. 292). Thus during the apparently daily rotation of the heavens, this star looks always practically stationary. It will, no doubt, be remembered how Shakespeare has put into the mouth of Julius C?sar these memorable words:-

"But I am constant as the northern star,

Of whose true-fix'd and resting quality

There is no fellow in the firmament."

Plate XIX. The Sky around the North Pole

We see here the Plough, the Pole Star, Ursa Minor, Auriga, Cassiopeia's Chair, and Lyra. Also the Circle of Precession, along which the Pole makes a complete revolution in a period of 25,868 years, and the Temporary Star discovered by Tycho Brahe in the year 1572.

(Page 291)

On account of the curvature of the earth's surface, the height at which the Pole Star is seen above the horizon at any place depends regularly upon the latitude; that is to say, the distance of the place in question from the equator. For instance, at the north pole of the earth, where the latitude is greatest, namely, 90°, the Pole Star will appear directly overhead; whereas in England, where the latitude is about 50°, it will be seen a little more than half way up the northern sky. At the equator, where the latitude is nil, the Pole Star will be on the horizon due north.

In consequence of its unique position, the Pole Star is of very great service in the study of the constellations. It is a kind of centre around which to hang our celestial ideas-a starting point, so to speak, in our voyages about the sky.

According to the constellation figures, the Pole Star is in Ursa Minor, or the Little Bear, and is situated at the end of the tail of that imaginary figure (see Plate XIX., p. 292). The chief stars of this constellation form a group

not unlike the Plough, except that the "handle" is turned in the contrary direction. The Americans, in consequence, speak of it as the "Little Dipper."

Before leaving this region of the sky, it will be well to draw attention to the second magnitude star ζ in the Great Bear (Zeta Urs? Majoris), which is the middle star in the "handle" of the Plough. This star is usually known as Mizar, a name given to it by the Arabians. A person with good eyesight can see quite near to it a fifth magnitude star, known under the name of Alcor. We have here a very good example of that deception in the estimation of objects in the sky, which has been alluded to in an earlier chapter. Alcor is indeed distant from Mizar by about one-third the apparent diameter of the moon, yet no one would think so!

On the other side of Polaris from the Plough, and at about an equal apparent distance, will be found a figure in the form of an irregular "W", made up of second and third magnitude stars. This is the well-known "Cassiopeia's Chair"-portion of the constellation of Cassiopeia (see Plate XIX., p. 292).

On either side of the Pole Star, about midway between the Plough and Cassiopeia's Chair, but a little further off from it than these, are the constellations of Auriga and Lyra (see Plate XIX., p. 292). The former constellation will be easily recognised, because its chief features are a brilliant yellowish first magnitude star, with one of the second magnitude not far from it. The first magnitude star is Capella, the other is β Aurig?. Lyra contains only one first magnitude star-Vega, pale blue in colour. This star has a certain interest for us from the fact that, as a consequence of that slow shift of direction of the earth's axis known as Precession, it will be very near the north pole of the heavens in some 12,000 years, and so will then be considered the pole star (see Plate XIX., p. 292). The constellation of Lyra itself, it must also be borne in mind, occupies that region of the heavens towards which the solar system is travelling.

The handle of the Plough points roughly towards the constellation of Bo?tes, in which is the brilliant first magnitude star Arcturus. This star is of an orange tint.

Between Bo?tes and Lyra lie the constellations of Corona Borealis (or the Northern Crown) and Hercules. The chief feature of Corona Borealis, which is a small constellation, is a semicircle of six small stars, the brightest of which is of the second magnitude. The constellation of Hercules is very extensive, but contains no star brighter than the third magnitude.

Near to Lyra, on the side away from Hercules, are the constellations of Cygnus and Aquila. Of the two, the former is the nearer to the Pole Star, and will be recognised by an arrangement of stars widely set in the form of a cross, or perhaps indeed more like the framework of a boy's kite. The position of Aquila will be found through the fact that three of its brightest stars are almost in a line and close together. The middle of these is Altair, a yellowish star of the first magnitude.

At a little distance from Ursa Major, on the side away from the Pole Star, is the constellation of Leo, or the Lion. Its chief feature is a series of seven stars, supposed to form the head of that animal. The arrangement of these stars is, however, much more like a sickle, wherefore this portion of the constellation is usually known as the "Sickle of Leo." At the end of the handle of the sickle is a white first magnitude star-Regulus.

The reader will, no doubt, recollect that it is from a point in the Sickle of Leo that the Leonid meteors appear to radiate.

The star second in brightness in the constellation of Leo is known as Denebola. This star, now below the second magnitude, seems to have been very much brighter in the past. It is noted, indeed, as a brilliant first magnitude star by Al Sufi, that famous Persian astronomer who lived, as we have seen, in the tenth century. Ptolemy also notes it as of the first magnitude.

In the neighbourhood of Auriga, and further than it from the Pole Star, are several remarkable constellations-Taurus, Orion, Gemini, Canis Minor, and Canis Major (see Plate XX., p. 296).

The first of these, Taurus (or the Bull), contains two conspicuous star groups-the Pleiades and the Hyades. The Pleiades are six or seven small stars quite close together, the majority of which are of the fourth magnitude. This group is sometimes occulted by the moon. The way in which the stars composing it are arranged is somewhat similar to that in the Plough, though of course on a scale ever so much smaller. The impression which the group itself gives to the casual glance is thus admirably pictured in Tennyson's Locksley Hall:-

"Many a night I saw the Pleiads, rising through the mellow shade,

Glitter like a swarm of fire-flies tangled in a silver braid."

Plate XX. Orion and his Neighbours

We see here that magnificent region of the sky which contains the brightest star of all-Sirius. Note also especially the Milky Way, the Pleiades, the Hyades, and the "Belt" and "Sword" of Orion.

(Page 296)

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 β, 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, β Aurig?, 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, α and β 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, α 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 latitudes than at present. For instance, some five thousand years ago the Southern Cross rose above the English horizon, and was just visible in the latitude 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 title 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 analogous. 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.

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