Stars

Stars are in general distinguished from other celestial bodies by their fixity of position in the celestial sphere, by the scintillation of their light, and by the fact that they show no appreciable size even under the highest telescopic power. The first quality renders them of great use in that department of astronomy which relates to accurate time-keeping and to measures of latitude and longitude. They form fixed points of reference whose place from their minuteness can be very accurately determined; and as they are far removed from terrestrial and even solar influence we can refer the motions of the earth and other members of the solar system to them as to unvarying landmarks. Themselves immovable, they determine for us our own movement. It is true that many possess minute motions of their own, but these, as we shall see, are so small as in most cases to affect but little in any moderate time the accuracy of such observations. This practical fixity of the stars enables us to determine two fixed points in the sky called the north and south poles of the heavens. Our Pole Star (q.v.) is not far removed from the first of these. Stars near these points are but little affected by the diurnal motion of the heavens, moving with it in small circles, while stars in the Equator (q.v.) move with great rapidity, requiring to complete the entire circuit of the heavens in the same time (twenty-four hours) as the others take to traverse their shorter paths. Thus the changing aspect of the heavens is confusing to an observer at first; but on attentive watching it is seen that all the various speeds and paths of the stars result from one simple motion—viz. the apparent revolution of the whole sky once in every twenty-four hours, as if it were a vast hollow ball hung on opposite points at the two poles. The observer looks as from the centre of this ball, and were it not for the earth intercepting his view would see stars on all sides, below as well as above him. At sea or in a sufficiently extensive plain he will see almost exactly one-half of this sphere, the earth hiding the other. A celestial globe represents this sphere, but is necessarily examined from without, while the sky is seen from within, which must be remembered in using such globes.

Owing to the sun's yearly motion the stars present different aspects at successive times. In June we see nearly one-half of the sky at midnight. By December the sun, pursuing his annual track, has gained a position among the stars we saw at June midnight. They are above the horizon when the sun is in the meridian at noon. At midnight in December we therefore see the stars in the other half of the heavens. At intermediate seasons the condition of things is of course intermediate also. Speaking generally, the starry sky at midnight on the first of any month is the same as that visible at ten o'clock in the evening of the first of the following month, while the ten o'clock aspect in the first case has of course moved to eight o'clock, and so on.

On the inner surface of the sky sphere the stars are irregularly scattered in groups called Constellations (q.v.). From these groups the stars are named by adding a letter or number to the name of the group. The Greek alphabet is used for the brighter stars—a denoting the brightest, $\backslash beta$ the next brightest, and so on. Roman letters are used when the Greek ones are exhausted, and afterwards numerals. Sometimes, however, the stars are named by their number from some catalogue, with the initial letter of the observer or of the name of catalogue, and the hour of right ascension (see Vol. I. p. 475) in which the star stands. So great a variety and confusion exists in this nomenclature as to be a serious trouble to beginners, and better arrangements in regard to it are much to be desired. In 1891 arrangements were nearly completed for the charting of the stars by a uniform series of telescopic photographs of the entire heavens. In this great work all civilised nations are uniting with an energy promising success. It is expected to contain when finished all stars down to the 14th magnitude.

The Scintillation (q.v.) or twinkling of the stars, by which they are easily distinguished from the planets, is due to disturbances in our atmosphere, combined with the fact that the stars are to us mere luminous points. It is magnified by the telescope, and becomes often then sufficient to transform the star into something like a tangled mass of worsted. The various points on the surface of a planet, the sun, or the moon are similarly affected, causing an indistinctness of telescopic vision, which when the air is much disturbed renders accurate observation impossible. But the star, being one point of light, twinkles as a whole, while the planet, sun, or moon, being even in the smallest case many points, does not twinkle as a whole, and the average of its rays gives a steady impression to the eye. Scintillation is most observable on nights when the atmosphere is disturbed, and always greatest at the horizon, being least at the zenith and on quiet nights.

Stars are classed in magnitudes according to their brilliance. The 1st magnitude includes the brightest class, only some twenty-four in number, the 2d magnitude the next brightest class, and so on, descending in order of brightness to the 15th or even the 17th magnitude, where we encounter the present limit of telescopic vision. At first this classification was most arbitrary, astronomers differing as to the magnitude to which many stars should belong. Hence a decimal notation was introduced, and stars were classed as 1.5 magnitude, 2.3, 4.6, &c. This must not, however, be confounded with an obsolete notation found in some old books, in which 1.2 mag. meant a star between 1st mag. and 2d, but nearer 1st than 2d, and 2.1 meant the same, but nearer 2d than 1st. The regular instrumental measurement of star-magnitudes is an entirely modern work, proceeding rapidly still. Professor Pickering of Harvard has published (1891) two catalogues, together including more than 25,000 stars whose magnitudes are instrumentally determined, and other astronomers are at work in the same field. A rough guide to ordinary estimates is furnished by the fact that stars barely visible to a good eye are classed in mag. 6, while telescopic stars range from mag. 7 to mag. 15 or 16. Arcturus is nearly 1.0 mag. Pollux and Regulus are an average 2d mag. $\backslash beta$ Arietis is a shade above 3d mag.

Three explanations may be given for this great variety in brilliance among the stars: (1) that they are all at nearly the same distance from us, and are in themselves different in size or brightness; (2) that they are of nearly equal brightness, but ranged at very varied distances; (3) that they vary both in lustre and distance. The first view has long been obsolete, belonging to the infancy of astronomy; the second, though equally erroneous, is still to be found treated in many text-books as if it had some foundation, no doubt because it forms a theoretical basis for the star-gauging carried out by the Herschels (q.v.). The third view is unquestionably the correct one, as will more clearly appear when we examine the measured distances of the stars. There must also be considered the probability that space is not perfectly transparent, and may entirely absorb the light of a star, if its distance from the observer be great enough. If this were the case it would help to account for the observed variety in stellar light. It is at least remarkable that the number of faint stars is so much greater than of bright ones, the former exceeding the latter by millions.

Besides this variety between separate stars there are remarkable changes in the light of some stars from time to time. These are known as Variables. The number known is continually enlarged by fresh discoveries, and is now several hundreds. This variability is almost always periodic; but the length and form of the periods are strikingly different for different stars. $\backslash alpha$ Ceti, or Mira, as it is called, requires 331 days 8 hours to accomplish its changes. For about a fortnight it is nearly mag. 2, decreasing for three months it becomes invisible, remains so for five months, and then gradually increases for the rest of its period. Algol, or $\backslash beta$ Persei, has a period of 2 d. 20 h. 49 m.; but its actual change from mag. 2 to 4 is accomplished in 3½ h. $\backslash eta$ Argus, again, varies from mag. 1 to 6 in seventy years. The so-called 'new' stars appearing from time to time are possibly only extreme instances of this variability. A noted example is the 'Nova' (or new star) of 1572, which reached such brightness as to be visible at mid-day in November of that year, but immediately began to diminish, entirely disappearing by March 1574. It is probably not identical with the temporary stars of 945 and 1264 A.D. Several similar appearances are known. In August 1885 a 'Nova' appeared in the nucleus of the great Andromeda nebula. About the 6th mag. when first seen, it steadily faded, until by February 7, 1886, it was only of mag. 16. On February 1, 1892, another nova of mag. 6 was pointed out in Auriga by Mr T. D. Anderson of Edinburgh. Its place is R.A. 5 h. 25 m. 3 s., dec. + 30° 21'. Within two months it had grown very faint, decreasing since March 9 by half a mag. per day. Its spectrum shows many bright lines, including those of hydrogen and sodium. As to the cause of these outbursts and the other minor fluctuations of the variables there has been much speculation. The passage of attendant meteor-swarms in front of the star, the changes in its atmosphere, positive collision of stars or meteor-swarms, passage of the star behind the edge of a nebula owing to its parallax—these and other explanations have all been advanced; but as yet no satisfactory theory has been given. The spectroscopic has revealed in some 'Novas' outbursts of glowing gas; but the cause of such outbursts remains as yet unrevealed. The analogy of the solar spot period (see SUN) would seem to show that solar physics holds the key to this strange problem, our sun being really a slightly variable star.

Variety in colour is as great among the stars as variety in light. Comparing Sirius with Betelgeuse this is easily seen. The investigation into this subject is as yet in its infancy, and we may here only note that very red stars are never bright, and that the two constituents of a binary star usually are of different colours. Blue or green stars of marked colour are also never found alone, but always as members of a close double star.

The measurement of the distance of the stars, one of the most difficult problems presented to the human mind, has been also one of its greatest triumphs. It was one of the strongest objections to the Copernican system that if the earth moved as Copernicus said, then the stars would appear to move also, as trees and houses appear to move when seen from a train in motion, unless indeed the stars were at an inconceivable and impossible distance. The evidences for the motion of the earth were too strong to be long disputed, and the alternative of the inconceivable distance of the stars had to be admitted. Thus men became familiarised with the thought of immense distances, and soon began efforts to measure them. The movement of the earth transfers it every six months from one extremity to the other of a line 180,000,000 miles in length. So enormous is the distance of the stars that this change of place in our observatories has hardly any effect on the direction in which we view them. If a star is directly overhead on the meridian when we are at one end of this immense base-line it is still overhead when we have arrived at the other end. Some minute change, however, there must be, and to measure this astronomers set themselves. In 1805 Piazzi published some results, only to be disproved. Pond and Brinkley in England disputed concerning each other's results, coming to no definite conclusion. Their work, combined with that of Struve at Dorpat in 1820, showed that the change of place looked for—called Parallax (q.v.)—was even smaller than had been supposed, in fact inappreciable to the instruments of that day.

Finally Bessel of Königsberg, furnished with a fine heliometer by Fraunhofer, found in 1840 that the star 61 Cygni showed a parallax of 0".3483, since raised by modern observation to 0".475. This placed 61 Cygni at a distance = 600,000 times that of the sun from the earth. Struve found about the same time a parallax of 0".26 for the star Vega, a value since slightly diminished. Henderson also found for $\backslash alpha$ Centauri, a bright southern star, a parallax of about 1".0, since corrected to 0".75, a distance equal to $4\backslash frac\{1\}\{3\}$ years' journey of light. The year's journey of Light (q.v.) is now used as the unit in which to express star distance. The distance expressed thus is easily found from the parallax by the following equation: $\backslash frac\{3\; \backslash cdot\; 202\}\{p\}\; =\; d$, where $p$ is the parallax, as usually given, in seconds of arc, and $d$ = the distance required. The number 3.202 is a constant depending on the velocity of light. Succeeding these early observers, Auwers and Brunnow in Europe, and C. A. F. Peters in the United States, continued this great work, and in our own day Professor A. Hall, of Washington, and Drs Ball, Gill, and Elkin, in Britain and at the Cape of Good Hope, have wrought diligently in the same field. We give below a table of a few of the more interesting stars, with their parallaxes and distances:

Since 1889 Professor Pritchard of Oxford has employed photography in determining stellar parallax with considerable success. But very much yet remains to be done in this field of research.

From what is said above it is evident that the stars are distributed through all space, as far as we know it. To a cursory glance they appear to be capriciously scattered, but more minute observation shows traces of arrangement, though we are not yet able to state distinctly the law governing it. The Galaxy (q.v.) or Milky Way forms a ring in the heavens, entirely consisting of minute stars, and as we pass away from this ring the stars become much less abundant. The Herschels laid the foundation for this important discovery by their star-gauges. A telescope of 18 inches aperture, 20 feet focus, and a magnifying power of 180, giving a field of view 15' in diameter, was used in all these. The process consisted in directing this instrument to a part of the sky and counting the stars in its field. This repeated hundreds of times gave a fair idea of the average number of stars in a circle of 15' diameter at all parts of the sky. It was found that at 90° from the ring of the Galaxy this number was 4.15, at 45° from it 10.36, at 15° 30.30, and in the ring 122.00. Sir W. Herschel considered this to indicate that our sun was one of the stars of the Galaxy, which extended on all sides of us as a flat disc of stars. This view, however, presupposes that the stars in this disc-like cluster are somewhat uniformly distributed, a supposition which has little to support it. The Galaxy may better be considered as what it appears, a ring of small stars relatively near one another and vastly distant from us. The whole question remains still unsettled, awaiting more numerous measures of stellar absolute parallax than we yet possess.

In particular parts of the sky stars gather in clusters. The Pleiades, Hyades, and Præsepe are examples of loose clusters of comparatively bright stars. Telescopes reveal marvellous groups, such as that in Hercules, which contains in very small space thousands of stars, spreading at the edge of the mass into curious curved sprays, an arrangement seen also in less closely packed groups. These clusters are quite distinct from Nebulæ (q.v.), and form still a puzzle to the astronomer. The nebuculæ form a very curious instance of a clustering mixture of nebulæ and stars. In the case of all these clusters, and even of groups less closely compacted, it is probable that a physical connection binds the various members together. The five intermediate stars of the 'Plough' are known to be moving together, in the same direction and with sensibly equal speed. And this grouping is evident in many other cases. But the laws governing and producing it remain yet unknown.

We have alluded here to star motion, called by astronomers Proper Motion. This, although hardly affecting the places of the stars in the sky as marks of reference, is distinct and even measured in the case of very many stars. An annual motion of this kind amounting to 6" of a great circle of the heavens is accounted large, only some three or four stars exceeding this speed, which would require more than 300 years to transfer the star a distance in the sky equal to the moon's diameter. Although so minute in apparent amount, these motions are in reality of a speed inconceivable by us. The star Groombridge 1830 has a real velocity of more than 200 miles per second, although annually shifting its place in the sky by only 7".03; 61 Cygni has a proper motion of 5".12; a small star 1584, xxiii.^h Cordoba Catalogue, has recently been shown to have a proper motion of 6".205, while many others have motions of much less amount. Part of these motions is due to the solar movement in space (see SUN), which has been calculated from them.

Double and binary stars were first discovered by Sir William Herschel, and the known number is noted now in thousands, and is continually increasing. There are stars so close as to appear single to the eye, or even, in many cases, to considerable telescopic power, but which, when sufficiently magnified, are seen to consist of two or more separate stars. In some cases no known telescopic power suffices to show the double separate, but the periodical doubling of certain lines in its spectrum alone reveals its duplicity. In other cases a single star may show such irregularities in its proper motion as to lead to the suspicion that it is attended by a dark companion, whose presence is only indirectly revealed to us by the effect it has in disturbing its luminous neighbour. This is the case with Sirius (q.v.) and Procyon, the companion of the former having been actually discovered as an orb of feeble light, after Bessel had pointed out the probability of its existence. Other stars are also surmised to have companions of this kind. Some of the double stars appear to have no physical connection between their components. Others undoubtedly form systems, of two or more suns revolving in periods round their common centre of gravity. These periods vary from 1159 years in the case of 61 Cygni to 10.8 years in that of $\backslash delta$ Equulei. When the distance from us and period of such a 'double' are known, together with the dimensions of its orbit, its mass can be calculated, on the supposition that it is controlled by the force of gravitation.

In studying the physical nature of the stars, modern, unlike ancient and mediæval, science, assuming that the universe is continuous, has considered the laws governing terrestrial matter to obtain throughout the visible universe. It was therefore easy, once the question of stellar distance was partly answered, to reach the conclusion that the stars are really suns. For they have that in common with our sun, and even with our candles, that they emit light, and their light, when analysed, exhibits all the qualities of that emitted by luminous bodies in our laboratories. When their masses are calculated they are found to form bodies of the same order of magnitude as our sun. $\backslash alpha$ Centauri (a 'double') has a mass twice that of the sun. 61 Cygni only equals $\backslash frac\{1\}\{7\}$ the solar mass. Judging not by direct measurement of mass, but by distance and relative brightness, the star Arcturus is 10,000 times more luminous than our sun, and may have a volume equal to 1,000,000 suns. Other stars, either very brilliant or very distant, range in the same order of magnitude, though too much reliance should not be placed on these calculations, which are liable to large errors and discordances.

These investigations prepare us to expect that the stars are similar in materials and in history to our own sun. When the spectroscope (see SPECTRUM) is applied to examine their light this expectation is confirmed. The star Aldebaran shows the spectral lines of hydrogen, sodium, magnesium, iron, tellurium, antimony, calcium, bismuth, and mercury, indicating that these elements exist in its atmosphere in the vaporous state. The examination of numbers of stars confirms this, so that we know the stars to be of material similar to that composing our earth. The problem before astronomers is therefore now to determine the physical state of the matter composing the stars, and the history of their construction. Through Kant and Laplace the idea had become familiar that nebulae and stars represent different stages of star history, and that different stars correspond to different steps in the process of star life, if we may use the expression. Though certainly not yet proved, much of modern discovery confirms this view. There is no distinct line between stars and nebulae. From the cloudy mist of the Orion nebulae to the sparkling point of Vega or Sirius a gradation of forms may be traced with no sensible break.

Rutherford of New York, in 1863, first classified the stars according to their spectra into three groups: (1) Red and golden stars; (2) white stars with lines in their spectra; (3) white stars (as he supposed) without lines. In 1865 Secchi commenced also to classify, ultimately adopting a division into four types, which, though altered by Vogel in name, remains the standard classification of the stars. It is as follows:

• I. Continuous spectrum, with four heavy hydrogen lines. White stars, as Sirius and Vega.
• II. Closely resembling solar spectrum. Yellow stars. Aldebaran and our Sun.
• III. Red stars. Fluted bands, fainter towards the violet end.
• IV. Faint, deep red stars. Fluted bands wider than in III., and fading towards the red.

Two prominent explanations are given of these types. The first is that they represent the various stages of star history in order, Class I. containing stars newly formed and excessively hot and brilliant, while Class IV. contains stars cooled down nearly to extinction. The other explanation, due to Lockyer, is that Class III. represents an earlier stage in star history than either I. or II., showing, in fact, the fainter beginning of stellar luminosity; while Class IV. represents the closing period of the star's life as it approaches extinction. Lockyer has on these lines attempted a full classi- fication of the nebulae and stars, considering all to have their origin in swarms of meteorites (see METEORS), with which he supposes space to be everywhere peopled, and whose collisions, abrasions, and condensations give us ultimately all celestial bodies. This theory has grave objections, and is certainly not established as fact, but it has the merit of being an attempt to unite a vast variety of stellar phenomena under one intelligible principle. We therefore give Lockyer's development of Secchi's classes, with his explanations.

• I. Radiation lines and flutings chiefly. Nebulae and stars with bright line spectra.
• II. Mixed radiation and absorption chiefly. Red stars. Bright flutings—i.e. bands fading—towards the violet.
• III. Line absorption chiefly.
• IV. Few strong lines. Hottest stars. Central point of star life.
• V. Resembling group III.; but stars cooling.
• VI. Carbon absorption chiefly. Stars cooling to extinction.

New stars, on Lockyer's theory, are produced by the sudden collision of swarms of meteors, variable stars by the revolution of two or more swarms, which periodically ningle and collide; and in other ingenious ways various stellar features are accounted for. Time and future observations must be waited for before these explanations are either wholly accepted or rejected. Other eminent scientists strongly object to Lockyer's conclusions. In particular, Croll contended that all evidence goes to show that the physical universe had a beginning not as scattered meteorites, but as immense solid masses moving with a finite velocity, whose collisions formed the first step in the evolution of the various heavenly bodies. Caution in accepting any such theories is, however, very desirable.

For further information readers may consult G. F. Chambers's Descriptive Astronomy (4th ed. 1889-90, 3 vols.); Herschel's Outlines of Astronomy; Miss A. Clerke's History of Astronomy in the 19th Century, and The System of the Stars (1890); Proceedings Royal Society, vol. xlv., No. 266, being the Bakerian Lecture, 1885, by Lockyer; and Croll's Stellar Evolution. See also the articles ASTROLOGY, ASTRONOMY, ZODIAC, &c.