PUBLICATIONS OF THE Astronomical Society of the Pacific. VOL. XII. SAN FRANCISCO, CALIFORNIA, JUNE 1, 1900. No. 74. PHOTOGRAPH OF THE TRIFID NEBULA, IN BY JAMES E. Keeler. The "Trifid" Nebula, Messier 20, in Sagittarius, is one of the most beautiful and remarkable of the irregular nebulæ. Its name is derived from the appearance which it presents in visual telescopes, and more particularly from the description of Sir JOHN HERSCHEL: "One of them [several nebulæ in Sagittarius] is singularly trifid, consisting of three bright and irregularly formed nebulous masses, graduating away insensibly externally, but coming up to a great intensity of light at their interior edges, where they inclose and surround a sort of three-forked rift, or vacant area, abruptly and uncouthly crooked, and quite void of nebulous light. A beautiful triple star is situated precisely on the edge of one of these nebulous masses just where the interior vacancy forks out into two channels. A fourth nebulous mass spreads like a fan or downy plume from a star at a little distance from the triple nebula." * Numerous drawings of the Trifid Nebula have been published. Two by LASSELL in the Memoirs of the Royal Astronomical Society represent the brightest parts of nebula very well. There is a good drawing by TROUVELOT, showing the whole nebula, among the astronomical drawings in volume VIII of the Annals of the Harvard College Observatory. It has been copied into a number of popular books on astronomy. The photogravure of the Trifid Nebula which accompanies the present article was made from a photograph taken with the Crossley reflector on the night of July 6, 1899. The exposure given to the original negative was three hours. *"Outlines of Astronomy," p. 611. The enlarge ment of the photogravure, as compared with the negative, is 2.9 diameters, and Imm 13 seconds of arc. = The photograph agrees well with the drawings, but it shows very much more than they do. The three main rifts referred to in Sir JOHN HERSCHEL'S description are seen, ramifying in all directions, like the roots of a tree, and may be traced nearly to the limits of the plate. Besides these rifts, there are many others, extending through the fainter part of the nebula, which in TROUVELOT'S drawing is represented by a uniform patch of nebulosity. The triple star, referred to by HERSCHEL, and shown in the different drawings which have been mentioned, is, on the photograph, lost in the bright patch of nebulosity bordering on two of the main rifts. It is visible on the negative, but could not be brought out on the enlargement. According to Dr. HOLDEN, the observations and drawings of this nebula afford strong evidence that there has been a change in the relative position of the nebula and some of the involved stars. A drawing by HERSCHEL in the Philosophical Transactions (1833) shows the triple star in the middle of the dark space formed by the three principal lanes, and in several early observations the star is stated to have this position. Later observations and drawings place the star just within the outline of the nebula. But Dr. DREYER has pointed out that the drawing of 1833 was constructed from sketches "the rudest imaginable, aided by memory," while the observations were made under unfavorable circumstances. A drawing made by Sir JOHN HERSCHEL in 1835 shows the star in its present position. The change, if there was one, must have occurred suddenly, which is not, in general, the nature of cosmical changes. Since 1835 there has been no change in the relative positions of the star and the nebula. In all probability, therefore, the inferred proper motion of the nebula is illusory. It is certain that the more frequently one has to compare drawings and photographs of nebulæ, the less one is inclined to attach weight to evidence based on the drawings. NOTES ON THE PROGRESS OF DOUBLE-STAR ASTRONOMY.* BY WILLIAM J. HUSSEY. It is nearly a hundred years since Sir WILLIAM HERSCHEL, as a result of the comparison of measures which he had just obtained with others made by him some twenty years earlier, was able to announce half a dozen cases in which he had found one star revolving about another. At that time this was a wonderful discovery. Astronomers had not expected to find suns moving in this manner, and it is not surprising that they were amazed when HERSCHEL proved this to be the case by producing such an array of facts and arguments that no room was left to doubt the correctness of his conclusions. Previously but little attention had been given to the stars for their own sakes. With instruments such as were in use, perhaps it was thought that not much of interest could be ascertained concerning them. As reference-points in the sky they had been of use (and are so still) in facilitating the investigations of the motions of the bodies composing the solar system, and it does not appear to have been imagined that there is great variety among them. HERSCHEL'S discovery was one of the factors which have operated to change all this. He showed that the stars are not all after the same pattern, but that they have their several stories to tell to those who will interrogate them properly, and ever since his time astronomers have more and more been finding this to be the case. New departments of sidereal investigation have come into existence. Besides all that relates to the absolute positions and movements of the stars as determined by the meridian observations, and those relative changes which are ascertained from the micrometrical measures, there are the extensive fields of research in which are considered those strange variations of light exhibited by certain of the stars, and the multiplied diversity of phenomena which have been revealed by the study of stellar spectra. With larger telescopes, more efficient subsidiary apparatus, more exact theories of instruments and *This article does not pretend to cover the entire field of double-star astronomy. It gives no account of the numerous determinations of orbits which have appeared from time to time, and nothing concerning spectroscopic binaries of which quite a number have recently been discovered. methods, and a greater number of enthusiastic investigators, results of greater variety and ever-increasing interest and importance are being obtained. In this rapid movement forward which now characterizes sidereal astronomy, that part which relates to double stars, visually considered, follows methods which have been in use for many years, and it appears probable that this will continue to be the case for all of the more interesting sys tems. The number of stars which have been catalogued as double and multiple has reached the formidable aggregate of over thirteen thousand. The majority of these are the discoveries of long ago. The catalogues of the HERSCHELS and the STRUVES alone contain more than nine thousand entries. But many of these are wide and easy pairs, whose components sustain no known physical relation to each other, and which are of such a character that were they now found for the first time no one would think of cataloguing them as double stars. For nearly forty years, dating from 1782, Sir WILLIAM HERSCHEL was almost the only observer of double stars. Eight hundred and twelve of those which he observed have been identified. Many of these were his own discoveries, and all of the more important ones were included by STRUVE in the Dorpat Catalogue. Sir JOHN HERSCHEL began his long career in astronomy in 1816 by taking up, under his father's direction, the re-examination of all the double stars discovered by him. By a fortunate circumstance, he worked from 1821 to 1823 in conjunction with Sir JAMES SOUTH, at the latter's private observatory in London. This was provided with two telescopes of very modest dimensions according to our present standards. One was three and threequarters and the other five inches aperture. With these little instruments, not supplied with driving-clocks, these observers made a series of observations of 380 double stars, which is remarkable for the skill, patience, and accuracy with which it was executed, and for the confirmation of the phenomena first brought to light by Sir WILLIAM HERSCHEL. These observations of SOUTH and HERSCHEL were published in the Philosophical Transactions for 1824, and won for them the Lalande Prize of the French Academy in 1825, and gold medals from the Royal Astronomical Society in the following year. At the same time this society presented its gold medal to WILLIAM STRUVE for the double-star observations which he had made at Dorpat, and made honorable mention of the work of AMICI at Modena in the same direction. These four were the only doublestar observers then living. Previous to this time STRUVE had been using a very modest telescope in his double-star work. But in 1824 FRAUNHOFER completed for the Dorpat Observatory the celebrated refractor of nearly nine and a half inches aperture. This was equatorially mounted, and provided with a good driving-clock. For fifteen years it remained the largest refractor in the world, and during that time it was by far the most efficient telescope in existence for double-star work. On receiving this equatorial, STRUVE resolved to reobserve all the double stars then known from the north pole to 15° South Declination, and to examine carefully all the brighter stars within this region for the purpose of discovering new pairs. In the course of this survey he is said to have examined no fewer than 120,000 stars. As a result of the first two years' work upon this programme, he published in 1827 the Dorpat Catalogue of 3,065 double and multiple stars. In this were included not only his own discoveries, but also all other objects which had been found by other astronomers within 105° of the north pole, and which appeared to him worthy of being called double stars. The second part of STRUVE'S programme was more arduous and required a greater time. To obtain exact micrometrical observations of all the double stars of the Dorpat Catalogue which are worthy of attention is no small undertaking, and it is surprising that STRUVE accomplished it in the comparatively short interval of twelve years. In 1837 he published the "Mensuræ Micrometricæ," a ponderous folio, containing the observations made at Dorpat with the large equatorial from 1824 to 1836, together with the results which he had obtained with a smaller instrument from 1813 to 1824. It also contained many of the observations by the HERSCHELS and SOUTH, so that it be said that this volume gave all that was known at the time of its publication concerning the double stars of the Dorpat Catalogue. When the objects of this catalogue came to be critically examined, it was found that some had distances greater than 32", which was the maximum limit set by STRUVE; that some were not double; that some were identical with others, erroneous positions having been assigned to them. For these and various may |