Many novae have recently been found in the spiral nebulae, especially in the great nebula of Andromeda.
CHAPTER LIII
THE DOUBLE STARS
Examining individual stars of the heavens more in detail, thousands of them are found to be double; not the stars that appear double to the naked eye, as Theta Tauri, Mizar, Epsilon Lyrae, and others; but pairs of stars much closer together, and requiring the power of the telescope to divide or separate them. Only a very few seconds apart they are or, in many cases, only the merest fraction of a second of arc. Some of them, called binaries, are found to be revolving around a common center, sometimes in only a few years, sometimes in stately periods of hundreds of years. Many such binary systems are now known, and the number is constantly increasing. Castor is one, Gamma Virginis another, Sirius also is one of these binaries, and a most interesting one, having a period of revolution of about 52 years.
Aitken, of the Lick Observatory, in his work on binary stars, directs special attention to the correlation between the elements of known binary orbits and the star"s spectral type, and presents a statistical study of the distribution of 54,000 visual double stars, of which the spectra of 3919 are known. That the ma.s.ses of binary systems average about twice that of the sun"s ma.s.s has long been known, and this fact can be employed with confidence in estimates of the probable parallax of these systems. Aitken applies the test to fourteen visual systems for which the necessary data are available, and deduces for them a mean ma.s.s of 1.76 times that of the sun. For the spectroscopic binaries the ma.s.ses are much greater.
Triple, quadruple and multiple stars are less frequent; but many exceedingly interesting objects of this cla.s.s exist. Epsilon Lyrae is one, a double-double, or four stars as seen with slender telescopic power, and six or seven stars with larger instruments. Sigma Orionis and 12 Lyncis, also Theta Cancri and Mu Bootis are good examples of triple stars.
CHAPTER LIV
THE STAR Cl.u.s.tERS
From multiple stars the transition is natural to star cl.u.s.ters although the gap between these types of stellar objects is very broad. The familiar group of the winter sky known as the Pleiades is a loose cl.u.s.ter, showing relatively very few stars even in telescopes or on photographic plates. The "Beehive," or cl.u.s.ter known as Praesepe in Cancer, and a double group in the sword-handle of Perseus, both just visible to the naked eye, are excellent examples of star cl.u.s.ters of the average type. When the moon is absent, they are easily recognized without a telescope as little patches of nebulous light; but every increase of optical power adds to their magnificence.
Then we come in regular succession to the truly marvelous globular cl.u.s.ters, that for instance in Hercules. Messier 13, a recent photograph of which, taken by Ritchey with the 60-inch reflector on Mount Wilson, reveals an aggregation of more than 50,000 stars. But the finest specimens are in the southern hemisphere. Sir John Herschel spent much time investigating them nearly a century ago at the Cape of Good Hope.
His description of the cl.u.s.ter in the constellation of Centaurus is as follows: "The n.o.ble globular cl.u.s.ter Omega Centauri is beyond all comparison the richest and largest object of the kind in the heavens.
The stars are literally innumerable, and as their total light when received by the naked eye affects it hardly more than a star of the fifth or fourth to fifth magnitude, the minuteness of each star may be imagined."
Others of these cl.u.s.ters are so remote that the separate stars are not distinguishable, especially at the center, and their distances are entirely beyond our present powers of direct measurement, although methods of estimating them are in process of development. If gravitation is regnant among the uncounted components of stellar cl.u.s.ters, as doubtless it is, these stars must be in rapid motion, although our photographs of measurements have been made too recently for us to detect even the slightest motion in any of the component stars of a cl.u.s.ter.
The only variations are changes of apparent magnitude, of a type first detected in a large number of stars in Omega Centauri, by Bailey of Harvard, who by comparison of photographs of the globular cl.u.s.ters was the first to find variable stars quite numerous in these objects. Their unexplained variations of magnitude take place with great rapidity, often within a few hours.
There are about a hundred of these globular cl.u.s.ters, and the radial velocities of ten of them have been measured by Slipher and found to range from a recession of 410 to an approach of 225 kilometers per second. These excessive velocities are comparable with those found for the spiral nebulae. Shapley has estimated the distances of many of these bodies, which contain a large number of variable stars of the Cepheid type. By a.s.suming their absolute magnitudes equal to those of similar Cepheids at known distances, he finds their distance represented by the inconceivably minute parallax of 0".00012, corresponding to 30,000 light-years. This research also places the globular cl.u.s.ters far outside and independent of our Galactic system of stars. The distribution of the globular cl.u.s.ters has also been investigated, and these interesting objects are found almost exclusively in but one hemisphere of the sky. Its center lies in the rich star clouds of Scorpio and Sagittarius. Success in finding the distances of these objects has made it possible to form a general idea of their distribution in three-dimensional s.p.a.ce.
The numerous variable stars in any one cl.u.s.ter are remarkable for their uniformity. Accepting variables of this type as a constant standard of absolute brightness, and a.s.suming that the differences of average magnitude of the variables in different cl.u.s.ters are entirely due to differences of distance, the relative distances of many cl.u.s.ters were ascertained with considerable accuracy. Then it was found that the average absolute magnitude of the twenty-five brightest stars in a cl.u.s.ter is also a uniform standard, or about 1.3 magnitudes brighter than the mean magnitude of the variables. This new standard was employed in ascertaining the distances of other cl.u.s.ters not containing many variables.
Shapley further shows that the linear dimensions of the cl.u.s.ters are nearly uniform, and the proper relative positions in s.p.a.ce are charted for sixty-nine of these objects. We can determine the scale of the charts, if we know the absolute brightness of our primary standard--the variable stars; and this is deduced from a knowledge of the distances of variables of the same type in our immediate stellar system.
The most striking of all the globular cl.u.s.ters, Omega Centauri, comes out the nearest; nevertheless it is distant 6.5 kilopa.r.s.ecs. A kilopa.r.s.ec is a thousand pa.r.s.ecs, and is the equivalent of 3,256 light-years. At the inconceivable distance of sixty-seven kilopa.r.s.ecs, or more than 200,000 light-years, is the most remote of the globular cl.u.s.ters, known to astronomers as N.G.C. 7006, from its number in the catalogue which records its position in the sky, the New General Catalogue of nebulae by Dreyer of Armagh.
The cl.u.s.ters are widely scattered, and their center of diffusion is about twenty kilopa.r.s.ecs on the Galactic plane toward the region of Scorpio-Sagittarius. Marked symmetry with reference to this plane makes it evident that the entire system of globular cl.u.s.ters is a.s.sociated with the Galaxy itself. But to conceive of this it is necessary to extend our ideas of the actual dimensions of the Galactic system. Almost on the circ.u.mference of the great system of globular cl.u.s.ters our local stellar system is found, and it contains probably all the naked-eye stars, with millions of fainter ones. Its size seems almost diminutive, only about one kilopa.r.s.ec in diameter. The relative location of our local stellar system shows why the globular cl.u.s.ters appear to be crowded into one hemisphere only.
Shapley suggests that globular cl.u.s.ters can exist only in empty s.p.a.ce, and that when they enter the regions of s.p.a.ce tenanted by stars, they dissolve into the well-known loose cl.u.s.ters and the star clouds of the Milky Way. Strangely the radial velocities of the cl.u.s.ters already observed show that most of them are traveling toward this region, and that some will enter the stellar regions within a period of the order of a hundred million years.
The actual dimensions of globular cl.u.s.ters are not easy to determine, because the outer stars are much scattered. To a typical cl.u.s.ter, Messier 3, Shapley a.s.signs a diameter of 150 pa.r.s.ecs, which makes it comparable with the size of the stellar cl.u.s.ter to which the sun belongs. Also on certain likely a.s.sumptions, he finds that the diameter of the great cl.u.s.ter in Hercules, the finest one in our northern sky, is about 350 pa.r.s.ecs, and its distance no less than 30,000 pa.r.s.ecs; in other words, the staggering distance that light would require 9,750,000 years to travel over. While these distances can never be verified by direct measurement, it lends great weight to the three methods of indirect measurement, or estimation, (1) from the diameter of the image of the cl.u.s.ters, (2) from the mean magnitude of the twenty-five brightest stars, and (3) from the mean magnitude of the short period variables, that they are in excellent agreement.
CHAPTER LV
MOVING Cl.u.s.tERS
Recent researches on the proper motions of stars have brought to light many groups of stars whose individual members have equal and parallel velocities. Eddington calls these moving cl.u.s.ters. The component stars are not exceptionally near to each other, and it often happens that other stars not belonging to the group are actually interspersed among them. They may be likened to double stars which are permanent neighbors, with some orbital motion, though exceedingly slow.
The connection is rather one of origin; occurring in the same region of s.p.a.ce, perhaps, from a single nebula. They set out with the same motion, and have "shared all the accidents of the journey together." Their equality of motion is intact because any possible deflections by the gravitative pull of the stellar system is the same for both. Mutual attraction may tend to keep the stars together, but their community of motion persists chiefly because no forces tend to interfere with it. In this way physically connected pairs may be separated by very great distances.
So with the moving cl.u.s.ters: their component stars may be widely separate on the celestial sphere, but equality of their motions affords a clue to their a.s.sociation in groups. The Hyades, a loose cl.u.s.ter in Taurus, is a group of thirty-nine stars, within an area of about 15 degrees square, which has been pretty fully investigated, especially by the late Professor Lewis Boss; and no doubt many fainter stars in the same region will ultimately be found to belong to the same group.
If we draw arrows on a chart representing the amount and direction of the proper motions of these stars, these arrows must all converge toward a point. This shows that their motions are parallel in s.p.a.ce. It is a relatively compact group, and the close convergence shows that their individual velocities must agree within a small fraction of a kilometer per second. Radial velocity measures of six of the component stars are in very satisfactory accord, giving 45.6 kilometers per second for the entire group.
We can get the transverse velocity, and therefrom the distances of the stars, which are among the best known in the heavens, because the proper motions are very accurately known. The mean parallax of the group by this indirect method comes out 0".025, agreeing almost exactly with the direct determination by photography, 0".023, by Kapteyn, De Sitter, and others.
Eddington concludes that this Taurus group is a globular cl.u.s.ter with a slight central condensation. Its entire diameter is about ten pa.r.s.ecs, and its known motion enables us to trace its past and future history. It was nearest the sun 800,000 years ago, when it was at about half its present distance. Boss calculated that in 65 million years, if the present motion is maintained, this group will have receded so far as to appear like an ordinary globular cl.u.s.ter 20" in diameter, its stars ranging from the ninth to the twelfth apparent magnitude. We may infer that the motion will likely continue undisturbed, because there are interspersed among the group many stars not belonging to it, and these have neither scattered its members nor sensibly interfered with the parallelism of their motion.
Another moving cl.u.s.ter, the similarity of proper motion of whose component stars was first pointed out by Proctor, is known as the Ursa Major system, which embraces primarily Beta, Gamma, Delta, Epsilon, and Zeta Ursae Majoris, or five of the seven stars that mark the familiar Dipper. But as many as eight other stars widely scattered are thought to belong to the same system, including Sirius and Alpha Coronae Borealis.
The absolute motion amounts to 28.8 kilometers per second, and is approximately parallel to the Galaxy. Turner has made a model of the cl.u.s.ter, which has the form of a flat disk.
Among stars of the Orion type of spectrum are several examples of moving cl.u.s.ters. The Pleiades together with many fainter stars form another moving cl.u.s.ter; as also do the brighter stars of Orion, together with the faint cloudlike extensions of the great nebula in Orion, whose radial velocity agrees with that of the stars in the constellation.
Still another very remarkable moving cl.u.s.ter is in Perseus, first detected by Eddington, and embracing eighteen stars, the brightest of which is Alpha Persei.
The further discovery of moving cl.u.s.ters is most important in the future development of stellar astronomy, because with their aid we can find out the relative distribution, luminosity, and distance of very remote stars. So far the stars found a.s.sociated in groups are of early types of spectrum; but the Taurus cl.u.s.ter embraces several members equally advanced in evolution with the sun, and in the more scattered system of Ursae Major there are three stars of Type F.
"Some of these systems," Eddington concludes, "would thus appear to have existed for a time comparable with the lifetime of an average star. They are wandering through a part of s.p.a.ce in which are scattered stars not belonging to their system--interlopers penetrating right among the cl.u.s.ter stars. Nevertheless, the equality of motion has not been seriously disturbed. It is scarcely possible to avoid the conclusion that the chance attractions of stars pa.s.sing in the vicinity have no appreciable effect on stellar motions; and that if the motions change in course of time (as it appears they must do) this change is due, not to the pa.s.sage of individual stars, but to the central attraction of the whole stellar universe, which is sensibly constant over the volume of s.p.a.ce occupied by a moving cl.u.s.ter."
CHAPTER LVI
THE TWO STAR STREAMS
Consider the ships on the Atlantic voyaging between Europe and America: at any one time there may be a hundred or more, all bound either east or west, some moving in interpenetrating groups, individuals frequently pa.s.sing each other, but rarely or never colliding. We might say, there are two great streams of ships, one moving east and the other west.
Now in place of each ship, imagine a hundred ships, and magnify their distances from each other to the vast distances that the stars are from each other, and all in motion in two great streams as before. This will convey some idea of the relatively recent discovery, called by astronomers "star-streaming."
Early in this century the investigation of moving cl.u.s.ters began to reveal the fact that the motions of the stars were not at random throughout the universe, and about 1904 Kapteyn was the first to show that the stellar motions considered in great groups are very far from being haphazard, but that the stars tend to travel in two great streams, or favored directions. This was ascertained by a.n.a.lyzing the proper motions of stars in the sky, many thousands of them, and correcting all for the effect which the known motion of the sun would have upon them.
The corrected motion, or part that is left over, is known as the star"s own motion, or _motus peculiaris_.
This important investigation was very greatly facilitated by the general catalogue of 6,188 stars well distributed over the entire sky, the work of the late Professor Boss. It was published by the Carnegie Inst.i.tution of Washington, and includes all stars down to the sixth magnitude. Boss was very critical in the matter of stellar positions and proper motions and his work is the most accurate at present available. Excluding stars of the Orion type and the known members of moving cl.u.s.ters, Kapteyn"s investigation was based on 5,322 stars, which he divided into seventeen regions of the sky, each northern region having an antipodal one in the southern hemisphere.
Mathematical a.n.a.lysis of these regions showed them all in substantial agreement, with one exception, and enabled Kapteyn to draw the conclusion that the stars of one stream, called Drift I, move with a speed of thirty-two kilometers per second, while those of the other, Drift II, travel with a speed of eighteen kilometers per second. Their directions are not, like those of east and west bound ships, 180 degrees from each other, but are inclined at an angle of 100 degrees. Drift I embraces about three-fifths of the stars, and Drift II the remaining two-fifths. Quite as remarkable as the drifts themselves is the fact that the relative motion of the two is very closely parallel to the plane of the Milky Way.
This epochal research has very great significance in all investigations of stellar motions, and it has been verified in various ways, particularly by the Astronomer Royal, Sir Frank Dyson, who limited the stars under consideration to 1,924 in number, but all having very large proper motions. In this way the two streams are even more characteristically marked. But radial velocity determinations afford the ultimate and most satisfactory test, and Campbell has this investigation in hand, cla.s.sifying the stars in their streaming according to the type.
Type A stars are so far found to be confirmatory. Turning to the question of physical differences between the stars of the two streams, Eddington inquires into the average magnitude of the stars in both drifts, and their spectral type. Also whether they are distributed at the same distance from the sun, and in the same proportion in all parts of the sky. His conclusion is that there is no important difference in the magnitudes of the stars const.i.tuting the two drifts. Regarding their spectra, stars of early and late types are found in both streams, with a somewhat higher proportion of late types among the stars of Drift II than those of Drift I. Campbell and Moore of the Lick Observatory have investigated seventy-three planetary nebulae which exhibit the phenomena of star-streaming, and have motions which are characteristic of the stars.