Popular Science Monthly/Volume 58/November 1900/Chapters on the Stars V
|CHAPTERS ON THE STARS.|
By PROFESSOR SIMON NEWCOMB, U. S. N.
BINARY AND MULTIPLE SYSTEMS.
SIR WILLIAM HERSCHEL was the first to notice that many stars which, to the unaided vision, seemed single, were really composed of two stars in close proximity to each other. The first question to arise in such a case would be whether the proximity is real or whether it is only apparent, arising from the two stars being in the same line from our system. This question was speedily settled by more than one consideration. If there were no real connection between any two stars, the chances would be very much against their lying so nearly in the same line from us as they are seen to do in the case of double stars. Out of 5,000 stars scattered at random over the celestial vault the chances would be against more than three or four being so close together that the naked eye could not separate them, and would be hundreds to one against any two being as close as the components of the closer double stars revealed by the telescope. The conclusion that the proximity is in nearly all cases real is also proved by the two stars generally moving together or revolving round each other.
Altogether there is no doubt that in the case of the brighter stars all that seem double in the telescope are really companions. But when we come to the thousands or millions of telescopic stars, there may be some cases in which the two stars of a pair have no real connection and are really at very different distances from us. The stars of such a pair are called 'optically double.' They have no especial interest for us and need not be further considered in the present work.
After Herschel, the first astronomer to search for double stars on a large scale was Wilhelm Struve, the celebrated astronomer of Dorpat. So thorough was his work in this field that he may fairly be regarded as the founder of a new branch of astronomy. Armed with what was, at that time (1815–35), a remarkable refracting telescope, he made a careful search of that part of the sky visible at Dorpat, with a view of discovering all the double stars within reach of his instrument. The angular distance apart of the components and the direction of the fainter from the brighter star were repeatedly measured with all attainable precision. The fine folio volume, 'Mensurse Micro-metricæ,' in which his results were published and discussed, must long hold its place as a standard work of reference on the subject.
Struve had a host of worthy successors, of whom we can name only a few. Sir John Herschel was rather a contemporary than a successor. His most notable enterprise was an expedition to the Cape of Good Hope for the purpose of exploring the southern heavens with greater telescopes that had then been taken to the southern hemisphere. Herschel,
Fig. 1. Position-angle and Distance of a Double Star.
South and Dawes, of England, were among the greatest English observers about the middle of the century. Otto Struve, son of Wilhelm, continued his father's work with zeal and success at Pulkowa. Later one of the most industrious observers was Dembowski, of Italy. During the last thirty years one of the most successful cultivators of double-star astronomy has been Burnham, of Chicago. He is to-day the leading authority on the subject. Enthusiasm, untiring industry and wonderful keenness of vision have combined to secure him this position.
The particulars which the careful observer of a double star should record are the position-angle and distance of the components and their respective magnitudes. To these Struve added their colors; but this has not generally been done.
Let P be the principal star and C the companion. Let N S be a north and south line through P, or an arc of the celestial meridian, the direction N being north and S south from the star P.
Then, the angle N P C is called the position-angle of the pair. It is counted round the circle from 0° to 360°. The angle drawn in the figure is nearly 120°. Were the companion C in the direction S the position angle would be 180°; to the right of P it would he 270°; to the right of N it would be between 270° and 360°.
The distance is the angle P C, which is expressed in seconds of arc.
We cannot set any well-defined limits to the range of distance. The general rule is that the greater the distance beyond a few seconds the less the interest that attaches to a double star, partly because the observation of distant pairs offers no difficulty, partly because of the increasing possibility that the components have no physical connection and so form only an optically double star. With every increase of telescopic power so many closer and closer pairs are found that we cannot set any limit to the number of stars that may have companions. It is therefore to the closer pairs that the attention of astronomers is more especially directed.
The difficulty of seeing a star as double, or, in the familiar language of observers, of 'separating' the components, arises from two sources, the proximity of the companion to the principal star and the difference in magnitude between the two. It was only in rare cases that Struve could separate a pair of distance half a second. Now Burnham finds pairs whose distance is one-quarter of a second or less; possibly the limit of a tenth of a second is being approached. It goes without saying that a very minute companion to a bright star may, when the distance is small, be lost in the rays of its brighter neighbor. For all these reasons no estimate can be made of the actual number of double stars in the heavens. With every increase of telescopic power and observing skill more difficult pairs are being found without a sign of a limit.
The great interest which attaches to double stars arises from the proof which they afford that the law of gravitation extends to the stars. Struve, by comparing his own observations with each other, or with those of Herschel, found that many of the pairs which he measured were in relative motion; the position angle progressively changing from year to year, and sometimes the distance also. The lesser star was therefore revolving round the greater, or, to speak with more precision, both were revolving round their common center of gravity. To such a pair the name binary system is now applied.
There can be no reasonable doubt that the two components of all physically connected double stars revolve round each other. If they did not their mutual gravitation would bring them together and fuse them into a single mass. We are therefore justified in considering all double stars as binary systems, except those which are merely optically double. For reasons already set forth, the pairs of the latter class which are near together must be very few in number; indeed, there are probably none among the close double stars whose brightest component can be seen by the naked eye.
The time of revolution of the binary systems is so long that there are only about fifty cases in which it has yet been determined with any certainty. Leaving out the 'spectroscopic binaries,' to be hereafter described, the shortest period yet found is eleven years. In only a small minority of cases is the period less than a century. In the large majority either no motion at all has yet been detected, or it is so slow as to indicate that the period must be several centuries, perhaps several thousand years.
There is a great difficulty in determining the period with precision until the stars have been observed through nearly a revolution, owing to the number of elements, seven in all, that fix the orbit, and the difficulty of making the measures of position angle and distance with precision. It thus happens that many of the orbits of binary systems which have been computed and published have no sound basis. Two cases in point may be mentioned.
The first magnitude star Castor, or α Geminorum, can be seen to be double with quite a small telescope. The components are in relative motion. Owing to the interesting character of the pair it has been well observed, and a number of orbits have been computed. The periodic times found by the components have a wide range. The fact is, nothing is known of the period except that it is to be measured by centuries, perhaps by thousands of years.
The history of 61 Cygni, a star ever memorable from being the first of which the parallax was determined, is quite similar. Although, since accurate observations have been made on it the components have moved through an apparent angle of 30°, the observations barely suffice to show a very slight curvature in the path which the two bodies are describing round each other. Whether the period is to be measured by centuries or by thousands of years cannot be determined for many years to come.
In his work on the 'Evolution of the Stellar Systems,' Prof. T. J. J. See has investigated the orbits of forty double stars having the shortest periods. There are twenty-eight periods of less than one hundred years
In considering the orbits of binary systems we must distinguish between the actual and the apparent orbit. The former is the orbit as it would appear to an observer looking at it from a direction perpendicular to its plane. This orbit, like that of a planet or comet moving round the sun, is an ellipse, having the principal star in its focus. The point nearest the latter is called the periastron, or pericenter, and corresponds to the perihelion of a planetary orbit. The point most distant from the principal star is the apocenter. It is opposite the pericenter and corresponds to the aphelion of a planetary orhit. The law of motion is here the same as in the case of a body of the solar system; the radius vector, joining the two bodies, sweeps over equal areas in equal times. The apparent orbit is the orbit as it appears to us. It differs from the actual orbit because we see it from a more or less oblique direction. In some cases the plane of the orbit passes near our system. Then to us the orbit will appear as a straight line and the small star will seem to swing from one side of the large one to the other like a pendulum, though the actual orbit may differ little from a circle. In some cases there may be two pericenters and two apocenters to the apparent orbit. This will be the case when a nearly circular orbit is seen at a considerable obliquity.
It is a remarkable and interesting fact that the law of areas holds good in the apparent as in the actual orbit. This is because all parts of the plane of the orbit are seen at the same angle, so that the obliquity of vision diminishes all the equal areas in the same proportion and thus leaves them equal.
The two most interesting binary systems are those of Sirius and Procyon. In the case of each the existence and orbit of the companion were inferred from the motions of the principal star before the companion had been seen. Before the middle of the century it was found that Sirius did not move with the uniform proper motion which characterizes the stars in general; and the inequality of its motion was attributed to the attraction of an unseen satellite. Later Auwers, from an exhaustive investigation of all the observations of the star, placed the inequality beyond doubt and determined the elements of the orbit of the otherwise unknown satellite. Before his final work was published the satellite was discovered by Alvan G. Clark, of Cambridgeport, Mass., son and successor of the first and greatest American maker of telescopes. Additional interest was imparted to the discovery by the fact that it was made in testing a newly constructed telescope, the largest refractor that had been made up to that time. The discoverer was, at the time, unaware of the work of Peters and Auwers demonstrating the existence of the satellite. The latter was, however, in the direction predicted by Auwers, and a few years of observation showed that it was moving in fairly close accordance with the prediction.
The orbit as seen from the earth is very eccentric, the greatest distance of the satellite from the star being about ten seconds, the least less than three seconds. Owing to the brilliant light of Sirius the satellite is quite invisible, even in the most powerful telescopes, when nearest its primary. This was the case in the years 1890-92 and will again be the case about 1940, when another revolution will be completed.
The history of Procyon is remarkably similar. An inequality of its motion was suspected, but not proved, by Peters. Auwers showed from observations that it described an orbit seemingly circular, having a radius of about 1". There could be no doubt that this motion must be due to the revolution of a satellite, but the latter long evaded discovery, though carefully searched for with the new telescopes which were from time to time brought into use. At length in 1895 Schaeberle found the long-looked-for object with the 36-inch telescope of the Lick Observatory. It was nearly in the direction predicted by Auwers, and a year's observation by Schaeberle, Barnard and others showed that it was revolving in accordance with the theory.
If the conclusion of Auwers that the apparent orbit of the principal star is circular were correct, the distance of the satellite should always be the same. It would then be equally easy to see at all times. The fact that neither Burnham nor Barnard ever succeeded in seeing the
Fig. 2. Apparent Orbit of α Centauri, by Professor See.
object with the Lick telescope would then be difficult to account for. The fact is, however, that the periodic motion of Procyon is so small that a considerable eccentricity might exist without being detected by observations. The probability is, therefore, that the apparent orbit is markedly eccentric and that the satellite was nearer the primary during the years 1878-92 than it was when discovered.
One very curious feature, common to both of these systems, is that the mass of each satellite, as compared with that of its primary, is out of all proportion to its brightness. The remarkable conclusions to be drawn from this fact will he discussed in a subsequent chapter.
The system of α Centauri is interesting from the shortness of the period, the brightness of the stars and the fact that it is the nearest star to ns so far as known. We reproduce a diagram of the apparent orbit from Dr. See's work. The period of revolution found by Dr. See is eighty-one years. The major axis of the apparent orbit is 32"; of the minor axis 6".
The pairs of which, so far as known, the period of revolution is the shortest, are these:
|κ||Pegasi;||R. A.||=||21h. 40m.;||Dec. =||+||25°11';||Period||= 11. 42.|
|δ||Equulei;||"||=||21h. 10m.;||"||+||9°37’;||"||=11. 45.|
|ε||Sagittarii;||"||=||18h. 56m.;||"||-||30° 1’;||"||=18. 85.|
|ρ||Argus;||"||=||7h. 47m.;||"||-||13°38’;||"||=22. 00.|
|85||Pegasi;||"||-||23h. 57m.;||"||+||26°34’;||"||=24. 00.|
TRIPLE AND MULTIPLE SYSTEMS.
Systems of three or more stars so close together that there must be a physical connection between them are quite numerous. There is every variety of such systems. Sometimes a small companion of a brighter star is found to be itself double. A curious case of this sort is that of γ Andromedæ. This object was observed and measured by Struve as an ordinary double star, of which the companion was much smaller than the principal star. Some years later Alvan Clark found that this companion was itself a close double star, of which the components, separated by about 1", were nearly equal. Moreover, it was soon found that these components revolved round each other in a period not yet accurately determined, but probably less than a century. Thus we have a binary system revolving round a central star, as the earth and moon revolve round the sun.
In most triple systems there is no such regularity as this. The magnitudes and relative positions of the components are so varied that no general description is possible. Stars of every degree of brightness are combined in every way. Observations on these systems extend over so short an interval that we have no data for determining the laws of motion that may prevail in any but one or two of the simplest cases. They are, in all probability, too complicated to admit of profitable mathematical investigation. There is, therefore, little more of interest to be said about them.
There is a very notable multiple system known as the Trapezium of Orion, from the fact that it is composed of four stars, one of which is plainly visible to the naked eye, while the others may he well seen in the smallest telescope. There are also two other very faint stars, each of which seems to be a companion of one of the bright ones. This system is situated in the great nebulæ of Orion, to be described in the next chapter, a circumstance which has made it one of the most interesting objects to observers. No motion has yet been certainly detected among the components.
SPECTROSCOPIC BINARY SYSTEMS.
Among the many striking results of recent astronomical research it would be difficult to name any more epoch-making than the discovery that great numbers of the stars have invisible dark bodies revolving round them of a mass comparable with their own. The existence of these revolving bodies is made known not only by their eclipsing the star, but by producing a periodic change in the radial motion of the star. How their motion is determined by means of the spectroscope has been briefly set forth in a former chapter. As a general rule the motion is uniform in the case of each star. We have described in a former chapter the periodic character of the radial motion of Algol, discovered by Vogel. This was followed by the discovery that α Virginis, though not variable, was affected by a similar inequality of the radial motion, having a period of four days and nineteen minutes. The velocity of the star in its apparent orbit is very great, about ninety-one kilometers, or fifty-six English miles, per second. It follows that the radius of the orbit is some three million miles. The mass of the invisible companion must, therefore, be very great.
Fig. 3. Radial Motion of a Binary System.
A new form of binary system was thus brought out which, from the method of discovery, was called the spectroscopic binary system. But there is really no line to be drawn between these and other binary systems. We have seen that as telescopic power is increased, closer and closer binary systems are constantly being formed. We naturally infer that there is no limit to the proximity of the pairs of stars of such systems and that innumerable stars may have satellites, planets or companion stars so close or so faint as to elude our powers of observation. Still, there is as yet a wide gap between the most rapidly moving visible binary system and the slowest spectroscopic one, which, however, will be filled by continued observation.
The actual orbit of such a system cannot be determined with the spectroscope, because only one component of the motion, that in the direction of the earth, can be observed. In the case of an orbit of which the plane was perpendicular to the line of sight from the earth to the star the spectroscope could give us no information as to the motion. The motion to or from the earth would be invariable. To show the result of the orbit being seen obliquely, let E be the earth and A S be the plane of the orbit seen edgewise. Drop the perpendicular A M upon the line of sight. Then, while the star is moving from S to A the spectroscope will measure the motion as if it took place from S to M. Since S M is less than A S, the measured velocity will always be less than the actual velocity, except in the rare case when the plane of the orbit is directed toward the earth. Since the spectroscope can give us no information as to the inclination under which we see the orbit, it follows that the actual orbital velocities of the spectroscopic binaries must remain unknown. We can only say that they cannot be less, but may be greater to any extent than that shown by our measures.
Fig. 4. The Mills Spectrograph of the Lick Observatory.
If the components of a binary system do not differ greatly in brightness, its character may be detected without actually measuring the radial velocities. Since the motion is shown by a displacement of the spectral lines and since, in any binary system, the two components must always move in opposite directions, it follows that the displacements of the spectral lines of the two stars will be in opposite directions. Hence, when one of the stars, say A, is moving toward us, and the other, say D, from us, all the spectral lines will appear double, the lines made by A being displaced toward the blue end of the spectrum and those by B toward the red end. After half a revolution the motion will be reversed and the lines will again be double; only the lines of star A will now be on the red side of the others. Between these two phases will be one in which the radial velocities of the two stars are the same; the lines will then appear single.
The first star of which the binary character was detected in this way is ε Ursæ Majoris. The discovery was made at the Harvard Observatory. Capella is supposed to be another of the same class.
About 1896 the Lick Observatory was supplied with the best spectrograph
Fig. 5. The New Photographic Refracting Telescope of the Astrophysical Observatory at Potsdam, near Berlin.
that Brashear could produce, the gift of Mr. D. O. Mills. In the hands of Campbell the measurements of radial motion with this instrument have reached an extraordinary degree of precision and brought to light the fact that systems of the kind in question are more numerous than would ever have been suspected. Campbell believes that the radial motion of about one star in every thirteen is affected by an observable inequality. Such an inequality can arise only through the action of a neighborhood of a mass at least comparable with that of our sun. A new field of astronomical research is thus opened, the exploration of which must occupy many years. The ultimate result may be to make as great an addition to our knowledge of the heavens as has been made during the last century by the telescope.
A star-cluster is a bunch or collection of stars separated from the great mass of stars which stud the heavens. The Pleiades, or 'seven stars,' as they are familiarly called, form a cluster, of which six of the components are easily seen by the naked eye, while five others may be distinguished by a good eye.
About 1780 Michell, of England, raised the question whether, supposingthe stars visible to the naked eye to be scattered over the sky at random, there would be a reasonable possibility that those of the Pleiades would all fall within so small a space as that filled by the constellation. His correct conclusion was in the negative. It follows that this cluster does not consist of disconnected stars at various distances, which happen to be nearly in a line from our system, but is really a collection of stars by itself. Besides the stars visible to the naked eye, the Pleiades comprise a great number of telescopic stars, of which about sixty have been catalogued and their relative positions determined. The principal star of the cluster is Alcyone or η Tauri, which is of the third magnitude. The five which come next in the order of brightness are not very unequal, being all between the fourth and fifth magnitudes. Six are near the sixth magnitude. The remainder, so far as catalogued, range from the seventh to the ninth.
In this case there is a fairly good method of distinguishing between a star which belongs to the cluster and one which probably lies beyond it. This test is afforded by the proper motion. All the stars of the group have a common proper motion in the same direction of about seven seconds per century. The first accurate measures made on the relative positions of the stars of the cluster were those of Bessel, about 1830. In recent years several observers have made yet more accurate determinations. The most thorough recent discussion is by Elkin. One result of his work is that there is as yet no certain evidence of any relative motion among the stars of the group. They all move on together with their common motion of seven seconds per century, as if they were a single mass.
A closer cluster, which is plainly visible to the naked eye and looks like a cloudy patch of light, is Præsepe in Cancer. It is very well seen in the early evenings of winter and spring. Although there is nothing in the naked-eye view to suggest a star, it is found on telescopic examination that the individual stars do not fall far below the limit of visibility, several being of about the seventh magnitude.
Another notable cluster of the same general nature is that in Perseus. This constellation is situated in the Milky Way, not far from its region of nearest approach to the pole. In the figure of the constellation the cluster forms the handle of the hero's sword. It may be seen
Fig. 6. The Great Cluster in Hercules, as Photographed with the Crossley Reflector of the Lick Observatory.
in the evening during almost any season except summer. To the naked eye it seems more diffused and star-like than Præepe; in fact, it has two distinct centers of condensation, so that it may be considered as a double cluster.
The two clusters last described may be resolved into stars with the smallest telescopes. But in the case of most of these objects the individual stars are so faint that the most powerful instruments scarcely suffice to bring them out. One of the most remarkable clusters in the northern heavens is that of Hercules. To the naked eye it is but a faint and insignificant patch which would be noticed only by a careful observer. But in a large telescope it is seen to be one of the most interesting objects in the heavens. Near the border the individual stars can be readily distinguished. But they grow continually thicker toward the center, where, even in a telescope of two feet aperture, the
Fig. 7. The Cluster ω Centauri, Photographed by Gill at the Cape Observatory.
observer can see only a patch of light, which is, however, as he scans it, suggestive of the countless stars that must there be collected. By the aid of photography, Professor Pickering has nearly succeeded in the complete resolution of this cluster.
In many cases the central portions of these objects are so condensed that they cannot be visually resolved into their separate stars, even with the most powerful telescopes. . A closer approach to complete resolution has been made by photography. We present copies of several photographs which have been made by Pickering, Gill and others.
The cluster which, according to Pickering, may he called the finest in the sky, is ω Centauri. It lies just within the border of the Milky Way, in right ascension, 13h. 20.8m., and declination—46° -47'. There are no bright stars near. To the naked eye it appears as a hazy star of the fourth magnitude. Its actual extreme diameter is about 40'. The brightest individual stars within this region are between the eighth and ninth magnitudes. Over six thousand have been counted on one of the photographs and the whole number is much greater.
The most remarkable and suggestive feature of the principal clusters is the number of variable stars which they contain. This feature has been brought out by the photographs taken at the Harvard Observatory and at its branch station in Arequipa. The count of stars and the detection of the variables was very largely made by Professor Bailey, who, for several years past, has been in charge of the Arequipa station. The proportion of variables is very different in different clusters. In the double cluster, 869-884, only one has been found among a thousand stars. The richest in variables is Messier, 3, in which one variable has been detected among every seven stars. It might be suspected that the closer and more condensed the cluster the greater the proportion of variables. This, however, does not hold universally true. In the great cluster of Hercules only two variables are found among a thousand stars.
Very remarkable, at least in the case of ω Centauri, is the shortness of the period of the variables. Out of one hundred and twenty-five found, ninety-eight have periods less than twenty-four hours. On the subject of the law of variation in these cases, Pickering says:
"The light curves of the ninety-eight stars whose periods are less than twenty-four hours may be divided into four classes. The first is well represented by No. 74. The period of this star is 12h. 4m. 3s. and the range in brightness two magnitudes. Probably the change in brightness is continuous. The increase of light is very rapid, occupying not more than one-fifth of the whole period. In some cases, possibly in this star, the light remains constant for a short time at minimum. In most cases, however, the change in brightness seems to be continuous. The simple type shown by No. 74 is more prevalent in this cluster than any other. There are, nevertheless, several stars, as No. 7, where there is a more or less well marked secondary maximum. The period of this star is 2d. llh. 51m. and the range in brightness one and a half magnitudes. The light curve is similar to that of well-known short-period variables, as δ Cephei and η Aquilæ. Another class may be represented by No. 126, in which the range is less than a magnitude and the times of increase and decrease are about equal. The period is 8h. 12m. 3s. No. 24 may perhaps be referred to as a fourth type. The range is about seven-tenths of a magnitude and the period is 11h. 5m. 7s. Apparently about 65 per cent, of the whole period is occupied by the increase of the light. This very slow rate of increase is especially striking from the fact that in many cases in this cluster the increase is extremely rapid, probably not more than ten per cent, of the whole period. In one case, No. 45, having a period of 14h. 8m., the rise from minimum to maximum, a change of two magnitudes takes place in about one hour, and in certain cases, chiefly owing to the necessary duration of a photographic exposure, there is no proof at present that the rise is not much more rapid.
"The marked regularity in the period of these stars is worthy of attention. Several have been studied during more than a thousand, and one during more than five thousand, periods without irregularities manifesting themselves."
It may be added that this regularity of the period, taken in connection with the case of η Aquilæ, already mentioned, affords a strong presumption that the variations in the light of these stars are in some way connected with the revolution of bodies around them, or of one star round another. Yet it is certain that the types are not of the Algol class and that the changes are not due merely to one star eclipsing another. That such condensed clusters should have a great number of close binary systems is natural, almost unavoidable, we might suppose. It will hereafter be shown to be probable that among the stars in general single stars are the exception rather than the rule. If such be the case, the rule should hold yet more strongly among the stars of a condensed cluster.
Perhaps the most important problem connected with clusters is the mutual gravitation of their component stars. Where thousands of stars are condensed into a space so small, what prevents them from all falling together into one confused mass? Are they really doing so, and will they ultimately form a single body? These are questions which can be satisfactorily answered only by centuries of observation; they must, therefore, be left to the astronomers of the future.
The first nebula, properly so-called, to be detected by an astronomical observer was that of Orion. Huyghens, in his 'Systema Saturnium,' gives a rude drawing of this object, with the following description:
"There is one phenomenon among the fixed stars worthy of mention which, so far as I know, has hitherto been noticed by no one, and, indeed, cannot be well observed except with large telescopes. In the sword of Orion are three stars quite close together. In 1656, as I chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). Three of these almost touched each other, and, with four others, shone through a nebula, so that the space around them seemed far brighter than the rest of the heavens, which was entirely clear, and appeared quite black, the effect being that of an opening in the sky, through which a brighter region was visible."
For a century after Huyghens made this observation it does not appear that these objects received special attention from astronomers. The first to observe them systematically on a large scale was Sir Wm. Herschel, whose vast researches naturally embraced them in their scope. His telescopes, large though they were, were not of good defining power and, in consequence, Herschel found it impossible to draw a certain line in all cases between nebulæ and clusters. At his time it was indeed a question whether all these bodies might not be clusters. This
Fig. 8. The Great Nebula of Orion, as Photographed by A. A. Common with a Four-foot Reflector.
question Herschel, with his usual sagacity, correctly answered in the negative. Up to the time of the spectroscope, all that astronomers could do with nebulæ was to discover, catalogue and describe them.
Several catalogues of these objects have been published. The one long established as a standard is the General Catalogue of Nebulæ and Clusters, by Sir John Herschel. With each object Herschel gave a condensed description. Recently Herschel's catalogue has been superseded by the general catalogue of Dreyer, based upon it.
Some of the more conspicuous of these objects are worthy of being individually mentioned. At the head of all must be placed the great nebula of Orion. This is plainly visible to the naked eye and can be seen without difficulty whenever the constellation is visible. Note the three bright stars lying nearly in an east and west line and forming the belt of the warrior. South of these will be seen three fainter ones, hanging below the belt, as it were, and forming the sword. To a keen eye, which sharply defines the stars, this middle star will appear hazy. It is the nebula in question. Its character will be strongly brought out by the smallest telescope, even by an opera-glass. Drawings of it have been made by numerous astronomers, the comparison of
Fig. 9. The Great Nebula of Andromeda Photographed by Roberts.
which has given rise to the question whether the object is variable. It cannot be said that this question is yet decided; but the best opinion would probably be in the negative. In recent times the improvements of the photographic process have led to the representation of the object by photography. A photograph made by Mr. A. A. Common, F.R.S., with a reflecting telescope, gives so excellent an impression of the object that by his consent we reproduce it.
The most remarkable feature connected with the nebula of Orion is the so-called Trapezium, already described. That these four stars form a system by themselves cannot be doubted. The darkness of the nebula immediately around them suggests that they were formed at the expense of the nebulous mass.
Great interest has recently been excited in the spiral form of certain nebulæ. The great spiral nebula M. 51 in Canes Venatici has long been known. We reproduce a photograph of this object and another. It is found by recent studies at the Lick Observatory that a spiral form can be detected in a great number of these objects by careful examination.
Fig. 10. The Great Spiral Nebula M. 51, as Photographed with the Crossley Reflector at the Lick Observatory.
Another striking feature of numerous nebulæ is their varied and fantastic forms, of which we give a number of examples. The ’Triphid nebula' is a noted one in this respect.
The great nebula of Andromeda is second only to that of Orion. It also is plainly visible to the naked eye and can be more readily recognized as a nebula than can the other. It has frequently been mistaken for a comet. Seen through a telescope of high power, its aspect is singular, as if a concealed light were seen shining through horn or semi-transparent glass. It is somewhat elliptical in form, as will be seen from a photograph by Sir William Roberts, F.R.S., which we reproduce (page 19).
Another nebula which, though not conspicuous to the naked eye, has attracted much attention from astronomers, is known, from the figure of one of its branches as the Omega nebula. Sir John Herschel, who first described this object in detail, says of it: "The figure is nearly that of the Greek capital Omega, somewhat distorted and very unequally bright." From one base of the letter extends out to the east a, long branch with a hook at the end, which, in most of the drawings, is more conspicuous than the portion included in the Omega. The
drawings, however, vary so much that the question has been raised whether changes have not taken place in the object. As in other cases, this question is one which it is not yet possible to decide. The appearance of such objects varies so much with the aperture of the telescope and the conditions of vision that it is not easy to decide whether the apparent change may not be due to these causes. It is curious that in a recent photograph the Omega element of it, if I may use the term, is far less conspicuous than in the older drawings, and is, in fact, scarcely recognizable.
Among the most curious of the nebulæ are the annular ones, which, as the term implies, have the form of a ring. It should be remarked that in such cases the interior of the ring is not generally entirely black, but is filled with nebulous light. We may, therefore, define these objects as nebulæ which are brighter round their circumference than in the center. The most striking of the annular nebulæ is that of Lyra. It may easily be found from being situated about half-way between
the stars Beta and Gamma. Although it is visible in a medium telescope, it requires a powerful one to bring out its peculiar features in a striking way. Recently it has been photographed by Keeler with the Crossley reflector of the Lick Observatory, who found that the best general impression was made with an exposure of only ten minutes.
The ring, as shown by Keeler's photographs, has a quite complicated structure. It seems to be made up of several narrower bright rings, interlacing somewhat irregularly, the spaces between them being filled with fainter nebulosity. One of these rings forms the outer boundary of the preceding end of the main ring. Sweeping around to the north end of the minor axis, it becomes very bright, perhaps by superposition on the broader main ring of the nebula at this place. It crosses this ring obliquely, forming the brightest part of the whole
nebula, and then forms the inner boundary of the main ellipse toward its following end. The remaining part of the ring is not so easily traced, as several other rings interlace on the south end of the ellipse.
The central star of this nebula has excited some interest. Its light seems to have a special actinic power, as the star is more conspicuous on the photographs than to the eye.
There are several other annular nebulæ which are fainter than than of Lyra. The one best visible in our latitudes is known as H IV. 13, or 4,565 of Dreyer's catalogue. It is situated in the constellation Cygnus which adjoins Lyra. Both Herschel and Lord Rosse have made drawings of it. It was photographed by Keeler with the
Crossley reflector on the nights of August 9 and 10, 1899, with exposures of one and two hours, respectively. Keeler states that the nebula, as shown by these photographs, is an elliptical, nearly circular ring, not quite regular in outline, pretty sharply defined at the outer edge." The outside dimensions are:
|Major axis||42". 5|
|Position angle of major axis||32°|
The nebula has a nucleus with a star exactly in the center. This is very conspicuous on a photograph, but barely if at all visible with a 36-inch reflector.
Another curious class of nebulæ are designated as planetary, on account of their form. These consist of minute, round disks of light, having somewhat the appearance of a planet. The appellation was suggested by this appearance. These objects are for the most part faint and difficult.
It is impossible to estimate the number of nebulæ in the heavens. New ones have been from time to time discovered, located and described by many observers during the last thirty years. Among these Lewis Swift is worthy of special mention. On photographing the sky near the galactic pole with the Crossley reflector, Keeler found no less than seven of these objects in a space of about one-half a square degree. He therefore estimates the whole number in the heavens capable of being photographed at several hundred thousand. It may be assumed that only a moderate fraction of these are visible to the eye, even aided by the largest telescopes.
Among the most singular of these objects are large diffused nebulæ, sometimes extending through a region of several degrees. A number of these were discovered by Herschel. Barnard, W. H. Pickering and others have photographed these for us. One of the most remarkable of them winds around in the constellation Orion in such a way that at first sight one might be disposed to inquire whether the impression on the photographic plate might not have been the result of some defect in the apparatus or some reflection of the light of the neighboring stars, which is so apt to occur in these delicate photographic operations. But its existence happens to be completely confirmed by independent testimony.It was first detected by W. H. Pickering and afterwards independently by Barnard.
A curious fact connected with the distribution of nebulæ over the sky is that it is in a certain sense the reverse of that of the stars. The latter are, as we shall hereafter show in detail, vastly more numerous in the regions near the Milky Way and fewer in number near the poles of that belt. But the reverse is the case with the nebulæ proper. They are least numerous in the Milky Way and increase in number as we go from it in either direction. Precisely what this signifies one would not at the present time be able to say. Perhaps the most obvious suggestion would be that in these two opposite nebulous regions the nebulæ have not yet condensed into stars. This, however, would be a purely speculative explanation.
On the other hand, star-clusters are more numerous in the galactic region. This, however, is little more than saying that in the regions where the stars are so much more numerous than elsewhere many of them naturally tend to collect in clusters. It is, however, a curious fact that, so far as yet been noticed, the large, diffused nebulæ which we have mentioned are more numerous in or near the Milky Way. If this tendency is established it will mark a curious distinction between them and the smaller nebulæ.
The most interesting question connected with these objects is that of their physical constitution. When, about 1866, the spectroscope was applied to astronomical investigation by Huggins and Secchi, these two observers found independently that the light of the great nebula of Orion formed a spectrum of bright lines, thus showing the object to be gaseous. This was soon found to be true of the nebulae generally. There is, however, a very curious exception in the case of the great nebula of Andromedæ. This object gives a more or less continuous spectrum. Why this is it is difficult to say.
Beyond the general fact that the light of a nebula does not come from solid matter, but from matter of a gaseous or other attenuated form, we have no certain knowledge of the physical constitution of these bodies. Certain features of their constitution can, however, be established with a fair approach to accuracy. Not only the spectroscopic evidence of bright lines, but the aspect of the objects themselves, show that they are transparent through and through. This is remarkable when taken in connection with their inconceivable size. Leaving out the large diffused nebulæ which we have mentioned, these objects are frequently several minutes in diameter. Of their distance we know nothing, except that they are probably situated in the distant stellar regions. Their parallax can be but a small fraction of a second. We shall probably err greatly in excess if we assume that it varies between one-hundredth and one-tenth of a second. To assign this parallax is the same thing as saying that at the distance of the nebulæ the dimensions of the earth's orbit would show a diameter which might range between one-fiftieth and one-fifth of a second, while that of Neptune would be more or less than one second. Great numbers of these objects are, therefore, thousands of times the dimensions of the earth's orbit, and probably most of them are thousands of times the dimensions of the whole solar system. That they should be completely transparent through such enormous dimensions shows their extreme tenuity. Were our solar system placed in the midst of one of them, it is probable that we should not be able to find any evidence of its existence.
A form of matter so different from any that can be found or produced on the surface of the earth can hardly be explained by our ordinary views of matter. A theory has, however, been propounded by Sir Norman Lockyer, so ingenious as to be worthy at least of mention. It is that these objects are vast collections of meteorites in rapid motion relatively to each other, which come into constant collision. Their velocity is such that at each collision heat and light are produced. In the language of our progenitors, who in the absence of matches used flint and steel, they 'strike fire' against each other. The idea of such a process originated with Prof. P. G. Tait, in an attempt to explain the tail of a comet, but it was elaborated and developed by Mr. Lockyer in his work on the 'Meteoritic Theory.'
The objections to this theory seem insuperable. A velocity so great, at such a distance from the center of the nebulæ, would be incompatible with the extreme tenuity of these objects. Every time that two meteors came into collision they would lose velocity, and, therefore, if the mass was sufficient to hold them from flying through space, would rapidly fall toward a common center. The amount of light produced by the collision of two such objects is only a minute fraction of the energy lost. The meteors which fall on the earth are mostly of iron, and, were the theory true, numerous lines of iron should be most conspicuous in the spectrum. But the fact is that in the great number of these objects there is but a single bright line, which does not seem to correspond to the line of any known substance. The supposed matter which produces it has, therefore, been called nebulum.