Popular Science Monthly/Volume 41/July 1892/New and Variable Stars
|"NEW" AND VARIABLE STARS.|
By J. ELLARD GORE.
TO ordinary observation the light of the stars seems to be constant. Although of various degrees of brilliancy, the brightness of each individual star appears to most people to be invariable. This is, of course, true with reference to the great majority of the stars which deck our midnight sky. There are, however, many objects the light of which is subject to considerable fluctuations. These are known as variable stars, and form one of the most interesting classes of objects visible in the stellar heavens. Over two hundred are now known to be certainly variable in light, and many others have been suspected of inconstancy. In some of these stars the changes of brightness can only be detected by careful watching, but in many the light is variable to a considerable extent. In the variable star Chi Cygni, for example, the star's light at maximum is about sixteen hundred times the light at minimum. At its brightest it is sometimes fairly visible to the naked eye, whereas in its faintest phase a pretty good telescope is required to see it at all.
These most interesting and mysterious objects have been divided into different classes, according to the character of the light-variation and the length of the period which completes the cycle of their curious changes. The classification now generally adopted is that proposed by Prof. Pickering, of the Harvard Observatory, U. S. A. This includes five classes, which are as follows: 1. Temporary or "new stars"; 2. Stars with regular periods of considerable length; 3. Irregular variables, having no definite period; 4. Variables of short period—say, under thirty days; and, 5. Variables of the type of Algol, or those which, at regular intervals, undergo a sudden (or comparatively sudden) diminution of brightness lasting for a few hours only, the star remaining constant in light (or nearly so) during the remainder of its period. A short account of these different classes may prove of interest to the general reader.
1. "Temporary" or "new stars" are perhaps the rarest phenomena visible in the heavens. Comets—at least, those visible to the naked eye are rare celestial visitors. Telescopic comets are, however, tolerably numerous, and scarcely a year passes without the discovery of several of these faint objects. Very few "new stars" have, however, been recorded in the annals of astronomical history. I refer, of course, to those which can properly be termed "new"—that is, stars the existence of which was previously unknown to astronomers, and which, blazing out suddenly, remained visible for a short time, and then faded away without again obtaining a maximum of light. Considered from this point of view, these novæ, as they are termed, can not correctly, perhaps, be classed among the variable stars at all. They appear once only, and then die out, never to return; at least, no return of a true nova has yet been recorded.
A remarkable peculiarity about these temporary stars is their usually sudden appearance. In all the well-authenticated cases the stars have blazed out with startling rapidity. Such were the brilliant stars of 1572 and 1604; and in later years, those of 1866 in Corona Borealis, and of 1876 in Cygnus. Tycho Brahe's star of 1572 made its appearance near the star Kappa Cassiopeiæ, the faintest of the four stars forming the well-known square in Cassiopeia's Chair. It appears to have been first noticed by Cornelius Gemma, on November 9th of that year, and it seems to have blazed out very suddenly, as he states that it was not visible on the preceding evening in a clear sky. The attention of Tycho Brahe, whose name is generally associated with the star, seems to have been first attracted to it on November 11th. When first seen, it surpassed Jupiter and rivaled Venus in brightness, and was visible at noonday! At this brilliancy, however, it did not long remain, but gradually diminished in luster, and in March, 1574, had completely disappeared, at least to the naked eye. Its curious changes are thus described: "As it decreased in size, so it varied in color; at first its light was white and extremely bright; it then became yellowish, afterward of a ruddy color, and finished with a pale livid color." Tycho Brahe has left an elaborate record of his observations of this wonderful object in a work of no less than 478 pages of printed matter.
"Kepler's nova" of 1604 appeared in the constellation Ophiuchus in October of that year. The planets Mars, Jupiter, and Saturn were near each other in this region of the heavens, near Eta Ophiuchi, and one evening Brunowski, a pupil of Kepler's, remarked that a new and very brilliant star had joined the planetary group. When first seen the new star was white, and exceeded in brilliancy Mars and Jupiter, and was even thought to rival Venus in splendor. It gradually diminished, and in six months was not equal in brightness to Saturn. In March, 1606, it had disappeared. It was also observed by the famous Galileo. Kepler wrote a work on the subject, which is still preserved. Only faint stars are now visible with the telescope near the positions assigned to these bright stars of 1572 and 1604.
In 1670 a star of the third magnitude was observed by Anthelm near Beta Cygni. It remained visible for about two years, and increased and diminished several times before its final disappearance.
A small temporary star was observed by Dr. Hind in Ophiochus on April 28, 1848. When first noticed it was about the fifth magnitude. It afterward rose to nearly the fourth magnitude, but gradually faded away. Hind was certain that up to April 3rd or 5th no object of even the ninth magnitude was visible in the position of the new star. This curious object is still visible, but has become very faint in recent years. In 1866 it was of the twelfth magnitude, and in 1875 not above the thirteenth magnitude.
On May 28, 1860, a new star was discovered by the late Mr. Pogson in the globular cluster known as 80 Messier in Scorpio. When first seen it was about the seventh magnitude, and nearly blotted out the nebula by its superior light. On June 10th the star had nearly vanished, and the cluster again shone out with its usual brilliancy, and with a condensed center. Pogson observed the cluster on May 9th, and noticed nothing remarkable; and, according to Schönfeld, it presented its usual appearance on May 15th in the heliometer of the Königsberg Observatory.
The star of 1866, known as the "Blaze Star," suddenly appeared in Corona Borealis in May of that year. Although it was subsequently found that the object had been previously observed and registered as a small star by the famous German astronomer, Argelander, it presented at the time of its discovery all the characteristics of a true nova. It seems to have blazed out very suddenly, for at about 9.30 p. m. on the evening of the 12th of May in that year Prof. Schmidt, observing the constellation Corona Borealis at Athens, saw nothing peculiar. Indeed, he afterward expressed his conviction that at that hour a star of even the fifth magnitude could not possibly have existed near the position without immediately attracting his attention. Within three hours afterward—about midnight—it was discovered by the late Mr. Birmingham, at Tuam, Ireland, shining as a star of the second magnitude, and rivaling in brilliancy Alphecca, "the gem of the coronet." Its light, however, rapidly faded. On May 14th it was of the third magnitude; on May 19th, only of the sixth. On May 24th it had become invisible to the naked eye, and by June 9th had faded to the ninth magnitude. When near its greatest brightness its light was examined by Dr. Huggins with the spectroscope, which showed the bright lines of hydrogen gas in addition to the ordinary stellar spectrum. During the ten years following this extraordinary outburst of light, Schmidt observed fluctuations in its brightness, which appeared to take place with a certain regularity. It would therefore seem that this object should be considered as an irregular variable rather than a "temporary star." Its rejection from the list of "new stars" would remove the only exception to the rule that all these wonderful objects have appeared in or near the Milky Way. Even the new star which was observed in August, 1885, in the great nebula in Andromeda forms no exception, for in Dr. Boeddicker's beautiful drawing of the galaxy, which has just been published, a faint extension of nebulous light is shown stretching from Cassiopeia's Chair to the nebula referred to.
A better example of a true temporary star is that which appeared in November, 1876, near Rho Cygni. It was first seen by Schmidt at Athens, soon after sunset, on the evening of November 24th, when it was about the third magnitude, and slightly brighter than Eta Pegasi. The appearance of this object was also probably sudden, for between November 1st and 20th Schmidt observed the vicinity, and was certain that no star of even the fifth magnitude could have escaped detection. Between November 20th and 24th the sky was, unfortunately, cloudy, so that the exact time of its appearance is unknown. This star was quite new, as it does not appear in any star-chart or catalogue. Like most of these curious objects, its light faded very rapidly. In the forty-eight hours following the night of November 27th it diminished to the extent of one and a half magnitude, and on November 30th it was reduced to the fifth magnitude. It afterward decreased with tolerable regularity, and in September, 1877, it was below the tenth magnitude. In subsequent years it became very faint. Ward found the star only sixteenth magnitude in October, 1881, and it was estimated of the fifteenth magnitude, at Mr. Wigglesworth's Observatory, in September, 1885. It was examined with the spectroscope a few days after its discovery, and its spectrum showed bright lines similar to the star in Corona Borealis. Subsequent observations seem to show that this extraordinary object changed into a small planetary nebula!
The star which appeared in August, 1885, in the great nebula in Andromeda (31 Messier) has been already referred to. It seems to have been independently noticed by several observers toward the end of August. It was, however, certainly seen by Mr. T. W. Ward, of Belfast, on August 19th, at 11 p. m., when he estimated it at nine and a half magnitude. On September 3rd the star was observed at seven and a half magnitude, at Dunecht, by Lord Crawford and Dr. Copeland, and its spectrum was found to be "fairly continuous." The star gradually faded away, and on February 7, 1886, was estimated only sixteenth magnitude with the twenty-six-inch refractor of the Naval Observatory at Washington. Dr. Auwers has pointed out the similarity between this outburst and the star of 1860 in the cluster 80 Messier, and thinks it very probable that both phenomena were due to physical changes in the nebulæ in which they occurred.
The most recent example of a new star is one discovered by Mr. T. D. Anderson, of Edinburgh, in the last week of January in the present year, and still visible with an opera-glass. It lies about two degrees south of the star Chi Aurigæ, in the Milky Way, and when first noticed was about magnitude four and a half. The star seems to have been visible for some time previously, as it has been found that its spectrum was photographed at Harvard Observatory, U.S.A., on December 1st, 10th, and 20th, before it was recognized as a new star. The actual time of its appearance therefore remains unknown, but that it is a new star there can be no doubt, as it does not appear in any star-chart or catalogue. The star is a very interesting object, and, according to observations by the present writer, is subject to sudden changes of brightness. It seems to be fading slowly, and on March 1st was still somewhat brighter than the sixth magnitude. Its spectrum is a very remarkable one, showing, it is thought, both bright and dark lines. The line C and other lines in the red are visible, the D line of sodium and the series of hydrogen lines being also present. Most of the lines are said to be double, each consisting of a bright and dark component. These double lines suggest the presence of two bodies, or systems of bodies, one approaching the eye and the other receding from it, with a relative velocity of between five hundred and six hundred miles per second. There is a suspicion that the bright lines characteristic of nebular spectra are also visible. These remarkable results suggest that the light of this star, and probably that of all "temporary" stars, is due either to the rush of a solid body through a gaseous nebula, or the clashing together of two meteoric swarms moving in opposite directions. The phenomenon might also be explained by two bodies forming a binary star passing through their perihelion, the great increase of light being due to a "violent grazing collision" at the point of nearest approach. Whether this new star is a veritable nova, or "temporary star," or merely represents the maximum of a hitherto unrecognized variable star of long period, like the so-called "Nova Orionis," discovered by the present writer in December, 1885, must be left to time to decide. In either case, it is a most interesting object, and its future career will be followed by astronomers with great interest.
Coming now to Class 2, we find regular variable stars with periods ranging from about 100 to 700 days, and with fluctuations in their light from about one magnitude to over eight magnitudes. Among the most remarkable of these are Mira Ceti, or the "wonderful star"; Chi Cygni, already referred to; R. Hydræ, R. Leonis, etc. Mira Ceti varies from about the second magnitude to a little below the ninth, with a mean period of about 331 days from maximum to maximum. Owing to its unusual brilliancy at maximum, and the great range of its light-fluctuations, this is perhaps the most interesting and remarkable of all the variable stars. The period of Chi Cygni is about 406 days, and its variation from about the fourth to nearly the thirteenth magnitude. R. Hydræ varies from the fourth to the eleventh mgnitude, with a period of about 437 days; and R. Leonis from about the fifth to the tenth magnitude, with a period of about 313 days. Most of the long-period variables are reddish in color, and show a banded spectrum, which seems to be a characteristic feature of this type of variable. Various theories have been proposed to account for the variation of light in long-period variables, but none of them are very satisfactory. The periodical outbreak of sun-spots on a large scale has been suggested, and also the clashing together of meteoric swarms revolving in an elongated orbit; but it must be confessed that the subject is still, to a great extent, a matter of mystery.
Class 3 includes the irregular variables—that is, stars which are undoubtedly variable, but have no regular periods. Sometimes these stars remain for long periods without any perceptible change, while at other times their fluctuations of light are very noticeable. Of these, perhaps the most remarkable are Mu Cephei (Sir William Herschel's "Garnet Star"), Alpha Herculis, Alpha Orionis (Betelgeuse), and Beta Pegasi. The variation is usually small, not exceeding one magnitude. Like the regular variables, these have also banded spectra.
In Class 4 are some very interesting objects—variable stars of short period. The greater number of these have periods of under eight days. The variation of light is generally small, but regular. In but few cases does it much exceed one magnitude, and in several it is less. In some, as in Beta Lyræ, Zeta Geminorum, and Eta Aquilæ, all the light-changes may be observed with the naked eye, while in others an opera-glass is necessary to follow the fluctuations.
In Class 5 are placed stars of the Algol type. These are the rarest of the regular variables, only ten having been hitherto detected. In these stars the light remains constant, or nearly so, for the greater portion of the period. A sudden diminution of brightness then commences, and all the light-changes are completed in the course of a few hours, after which the star returns to its normal brightness. The brightest of these remarkable stars are Algol (Beta Persei), Lambda Tauri, and Delta Libræ. The others are much fainter, only two being visible to the naked eye when at their normal brightness. A star of this class recently discovered in the Southern Constellation (Antlia) has the surprisingly short period of only seven and three quarters hours—the most rapid variation hitherto detected in any variable star. All the Algol variables are white, or only slightly colored.
It was long since suggested that the periodical diminution of light in the Algol variables might possibly be due to the interposition of a dark, eclipsing satellite. Some few years since Prof. Pickering undertook a mathematical investigation of the case of Algol, and showed that an eclipsing satellite revolving in a nearly circular orbit in a period indicated by the light-variations of the star would satisfactorily explain the observed phenomenon within the limits, of errors of observation, and he suggested that the orbit might be determined by spectroscopic observation of the star's light before and after the minimum. Observations of this kind made by Prof. Vogel at Potsdam, in 1888 and 1889, leave little doubt that the decrease of light is really due to an eclipsing satellite. He found that before the minimum the bright star is receding from the earth (and therefore the dark companion approaching), and after minimum it is approaching, thus proving the eclipse theory to be correct.
Herr J. Plassmann, of Warendorf, Germany, has lately announced his discovery of a secondary minimum in the light of Algol and Lambda Tauri. This, if confirmed, would seem to show that the eclipsing satellite is not absolutely dark, but possesses some inherent light of its own, this light being cut off when the satellite passes in its turn behind the disk of its primary.—The Gentleman's Magazine.
- ↑ Further observations on March 10th and 11th showed that the star had then faded to below the seventh magnitude; and on March 16th I could no longer see it through an opera-glass.