Astronomical Curiosities: Facts and Fallacies

To add to the difficulty of solving this celestial problem, the spectroscope shows that the Andromeda nebula is not gaseous. The spectrum is, according to Scheiner, very similar to that of the sun, and “there is a surprising agreement of the two, even in respect to the relative intensities of the separate spectral regions.”365 He thinks that “the greater part of the stars comprising the nucleus of the nebula belong to the second spectral class” (solar), and that the nebula “is now in an advanced stage of development. No trace of bright nebular lines are present, so that the interstellar space in the Andromeda nebula, just as in our stellar system, is not appreciably occupied by gaseous matter.”[366] He suggests that the inner part of the nebula [the “nucleus”] “corresponds to the complex of those stars which do not belong to the Milky Way, while the latter corresponds to the spirals of the Andromeda nebula.”[366] On this view of the matter we may suppose that the component particles are small bodies widely separated, and in this way the mean density of the Andromeda nebula may be very small indeed. They cannot be large bodies, as the largest telescopes have failed to resolve the nebula into stars, and photographs show no sign of resolution.

It has often been suggested, and sometimes definitely stated, that the Andromeda nebula may possibly be an “external” universe, that is an universe entirely outside our sidereal system, and comparable with it in size. Let us examine the probability of such hypothesis. Assuming that the nebula has the same diameter as the Milky Way, or about 6000 “light years,” as estimated by Prof. Newcomb, I find that its distance from the earth would be about 150,000 “light years.” As this is about 8000 times the distance indicated by Bohlin’s parallax, its dimensions would be 8000 times as great, and hence its volume and mass would be 8000 cubed, or 512,000,000,000 times greater than that found above. That is, about 4 trillion (4 × 1018) times the sun’s mass! As this appears an incredibly large mass to be compressed into a volume even so large as that of our sidereal system, we seem compelled to reject the hypothesis that the nebula represents an external universe. The sun placed at the distance corresponding to 150,000 light years would, I find, shine as a star of less than the 23rd magnitude, a magnitude which would be invisible in the largest telescope that man could ever construct. But the combined light of 4 trillion of stars of even the 23rd magnitude would be equal to one of minus 23·5 magnitude, that is, 23½ magnitude brighter than the zero magnitude, or not very much inferior to the sun in brightness. As the Andromeda nebula shines only as a star of about the 5th magnitude the hypothesis of an external universe seems to be untenable.

It is evident, however, that the mass of the Andromeda nebula must be enormous; and if it belongs to our sidereal system, and if the other great nebulæ have similar masses, it seems quite possible that the mass of the visible universe may much exceed that of the visible stars, and may be equal to 1000 million times the sun’s mass – as supposed by the late Lord Kelvin – or even much more.

With reference to the small star which suddenly blazed out near the nucleus of the Andromeda nebula in August, 1885, Prof. Seeliger has investigated the decrease in the light of the star on the hypothesis that it was a cooling body which had suddenly been raised to an intense heat by the shock of a collision, and finds a fair agreement between theory and observation. Prof. Auwers points out the similarity between this outburst and that of the “temporary star” of 1860, which appeared in the cluster 80 Messier, and he thinks it very probable that both phenomena were due to physical changes in the nebulæ in which they appeared.

The appearance of this temporary star in the Andromeda nebula seems to afford further evidence against the hypothesis of the nebula being an external universe. For, as I have shown above, our sun, if placed at a distance of 150,000 light years, would shine only as a star of the 23rd magnitude, or over 15 magnitudes fainter than the temporary star. This would imply that the star shone with a brightness of over a million times that of the sun, and would therefore indicate a body of enormous size. But the rapid fading of its light would, on the contrary, imply a body of comparatively small dimensions. We must, therefore, conclude that the nebula, whatever it may be, is not an external universe, but forms a member of our own sidereal system.

In Sir John Herschel’s catalogue of Nebulæ and Clusters of Stars, published in 1833, in the Philosophical Transactions of the Royal Society, there are many curious objects mentioned. Of these I have selected the following: —

No. 496 is described as “a superb cluster which fills the whole field; stars 9, 10 … 13 magnitude and none below, but the whole ground of the sky on which it stands is singularly dotted over with infinitely minute points.” This is No. 22 of Sir William Herschel’s 6th class, and will be found about 3 degrees south and a little east of the triple star 29 Monocerotis.

No. 650. This object lies about 3 degrees north of the star μ Leonis, the most northern of the bright stars in the well-known “Sickle,” and is thus described by Sir John Herschel: “A star 12th magnitude with an extremely faint nebulous atmosphere about 10″ to 12″. It is between a star 8-9 magnitude north preceding, and one 10th magnitude south following, neither of which are so affected. A curious object.”

No. 1558. Messier 53. A little north-east of the star α Comæ Berenices. Described as “a most beautiful highly compressed cluster. Stars very small, 12th … 20th magnitude, with scattered stars to a considerable distance; irregularly round, but not globular. Comes up to a blaze in the centre; indicating a round mass of pretty equable density. Extremely compressed. A most beautiful object. A mass of close-wedged stars 5′ in diameter; a few 12th magnitude, the rest of the smallest size and innumerable.” Webb says, “Not very bright with 37⁄10 inches; beautiful with 9 inches.” This should be a magnificent object with a very large telescope, like the Lick or Yerkes.

No. 2018. “A more than usually condensed portion of the enormous cluster of the Milky Way. The field has 200 or 300 stars in it at once.” This lies about 2° south-west of the star 6 Aquilæ, which is near the northern edge of the bright spot of Milky Way light in “Sobieski’s Shield” – one of the brightest spots in the sky.

No. 2093. “A most wonderful phenomenon. A very large space 20′ or 30′ broad in Polar Distance, and 1m or 2m in Right Ascension, full of nebula and stars mixed. The nebula is decidedly attached to the stars, and is as decidedly not stellar. It forms irregular lace-work marked out by stars, but some parts are decidedly nebulous, wherein no star can be seen.” Sir John Herschel gives a figure of this curious spot, which he says represents its “general character, but not the minute details of this object, which would be extremely difficult to give with any degree of fidelity.” It lies about 3 degrees west of the bright star ζ Cygni.

Among the numerous curious objects observed by Sir John Herschel during his visit to the Cape of Good Hope, the following may be mentioned: —

h 2534 (H iv. 77). Near τ4 Eridani. Sir John Herschel says, “Attached cometically to a 9th magnitude star which forms its head. It is an exact resemblance to Halley’s comet as seen in a night glass.”… “A complete telescopic comet; a perfect miniature of Halley’s comet, only the tail is rather broader in proportion.”366

h 3075. Between γ Monocerotis and γ Canis Majoris. “A very singular nebula, and much like the profile of a bust (head, neck, and shoulders) or a silhouette portrait, very large, pretty well defined, light nearly uniform, about 12′ diameter. In a crowded field of Milky Way stars, many of which are projected on it.”367

h 3315 (Dunlop 323). In the Milky Way; about 3° east of the Eta Argûs nebula. Sir John Herschel says, “A glorious cluster of immense magnitude, being at least 2 fields in extent every way. The stars are 8, 9, 10, and 11th magnitudes, but chiefly 10th magnitude, of which there must be at least 200. It is the most brilliant object of the kind I have ever seen” … “has several elegant double stars, and many orange-coloured stars.”368 This should form a fine object in even a comparatively small telescope, and may be recommended to observers in the southern hemisphere. A telescope of 3-inches aperture should show it well.

Among astronomical curiosities may be counted “clusters within clusters.” A cluster in Gemini (N.G.C. 2331) has a small group of “six or seven stars close together and well isolated from the rest.”

Lord Rosse describes No. 4511 of Sir John Herschel’s General Catalogue of Nebulæ and Clusters (Phil. Trans., 1864) as “a most gorgeous cluster, stars 12-15 magnitude, full of holes.”369 His sketch of this cluster shows 3 rings of stars in a line, each ring touching the next on the outside. Sir John Herschel described it as “Cluster; very large; very rich; stars 11-15 magnitude (Harding, 1827),” but says nothing about the rings. This cluster lies about 5 degrees south of δ Cygni.

Dr. See, observing with the large telescope of the Lowell Observatory, found that when the sky is clear, the moon absent, and the seeing perfect, “the sky appeared in patches to be of a brownish colour,” and suggests that this colour owes its existence to immense cosmical clouds, which are shining by excessively feeble light! Dr. See found that these brown patches seem to cluster in certain regions of the Milky Way.370

From a comparison of Trouvelot’s drawing of the small elongated nebula near the great nebula in Andromeda with recent photographs, Mr. Easton infers that this small nebula has probably rotated through an angle of about 15° in 25 years. An examination I have made of photographs taken in different years seems to me to confirm this suspicion, which, if true, is evidently a most interesting phenomenon.

Dr. Max Wolf of Heidelberg finds, by spectrum photography, that the well-known “ring nebula” in Lyra consists of four rings composed of four different gases. Calling the inner ring A, the next B, the next C, and the outer D, he finds that A is the smallest ring, and is composed of an unknown gas; the next largest, B, is composed of hydrogen gas; the next, C, consists of helium gas; and the outer and largest ring, D, is composed – like A – of an unknown gas. As the molecular weight of hydrogen is 2·016, and that of helium is 3·96, Prof. Bohuslav Brauner suggests that the molecular weight of the gas composing the inner ring A is smaller than that of hydrogen, and the molecular weight of the gas forming the outer ring D is greater than that of helium. He also suggests that the gas of ring A may possibly be identical with the “coronium” of the solar corona, for which Mendelief found a hypothetical atomic and molecular weight of 0·4.371

With reference to the nebular hypothesis of Laplace, Dr. A. R. Wallace argues that “if there exists a sun in a state of expansion in which our sun was when it extended to the orbit of Neptune, it would, even with a parallax of 1⁄60th of a second, show a disc of half a second, which could be seen with the Lick telescope.” My reply to this objection is, that with such an expansion there would probably be very little “intrinsic brightness,” and if luminous enough to be visible the spectrum would be that of a gaseous nebula, and no known star gives such a spectrum. But some planetary nebulæ look like small stars, and with high powers on large telescopes would probably show a disc. On these considerations, Dr. Wallace’s objection does not seem to be valid.

It is usually stated in popular works on astronomy that the spectra of gaseous nebulæ show only three or four bright lines on a faint continuous background. But this is quite incorrect. No less than forty bright lines have been seen and measured in the spectra of gaseous nebulæ.372 This includes 2 lines of “nebulium,” 11 of hydrogen, 5 of helium, 1 of oxygen (?), 3 of nitrogen (?), 1 of silicon (?), and 17 of an unknown substance. In the great nebulæ in Orion 30 bright lines have been photographed.373

D’Arrest found that “gaseous nebulæ are rarely met with outside the Milky Way, and never at a considerable distance from it.”374

Mr. A. E. Fath thinks that “no spiral nebula investigated has a truly continuous spectrum.” He finds that so feeble is the intensity of the light of the spiral nebulæ that, while a spectrogram of Arcturus can be secured with the Mills spectrograph “in less than two minutes,” “an exposure of about 500 hours would be required for the great nebula in Andromeda, which is of the same spectral type.”375 Mr. Fath thinks that in the case of the Andromeda nebula, the “star cluster” theory “seems to be the only one that can at all adequately explain the spectrum obtained.”376

Prof. Barnard finds that the great cluster in Hercules (Messier 13) is “composed of stars of different spectral types.” This result was confirmed by Mr. Fath.377

From observations with the great 40-inch telescope of the Yerkes Observatory (U.S.A.), Prof. Barnard finds that the nucleus of the planetary nebula H. iv. 18 in Andromeda is variable to the extent of at least 3 magnitudes. At its brightest it is about the 12th magnitude; and the period seems to be about 28 days. Barnard says, “I think this is the first case in which the nucleus of a planetary or other nebula has been shown to be certainly variable.” “The normal condition seems to be faint – the nucleus remaining bright for a few days only. In an ordinary telescope it looks like a small round disc of a bluish green colour.” He estimated the brightness of the nebula as that of a star of 8·2 magnitude.378 Even in a telescope of 4 inches aperture, this would be a fairly bright object. It lies about 3½ degrees south-west of the star ι Andromedæ.

The so-called “globular clusters” usually include stars of different brightness; comparatively bright telescopic stars of the 10th to 13th magnitude with faint stars of the 15th to 17th magnitude. Prof. Perrine of the Lick Observatory finds that (a) “the division of the stars in globular clusters into groups, differing widely in brightness, is characteristic of these objects”; (b) “the globular clusters are devoid of true nebulosity”; and (c) “stars fainter than 15th magnitude predominate in the Milky Way and globular clusters, but elsewhere are relatively scarce.” He found that “exposures of one hour or thereabouts showed as many stars as exposures four to six times as long; the only effect of the longer exposures being in the matter of density.” This last result confirms the late Dr. Roberts’ conclusions. Perrine finds that for clusters in the Milky Way, the faint stars (15th to 17th magnitude) “are about as numerous in proportion to the bright stars (10th to 13th magnitude) as in the globular clusters themselves.” This is, however, not the case with globular clusters at a distance from the Milky Way. In these latter clusters he found that “in the regions outside the limits of the cluster there are usually very few faint stars, hardly more than one-fourth or one-tenth as many as there are bright stars”; and he thinks that “this paucity of faint stars” in the vicinity of these clusters “gives rise to the suspicion that all regions at a distance from the Galaxy may be almost devoid of these very faint stars.” The late Prof. Keeler’s series of nebular photographs “in or near the Milky Way” tend to confirm the above conclusions. Perrine finds the northernmost region of the Milky Way “to be almost, if not entirely, devoid of globular clusters.”379

According to Sir John Herschel, “the sublimity of the spectacle afforded” by Lord Rosse’s great telescope of 6 feet in diameter of some of the “larger globular and other clusters” “is declared by all who have witnessed it, to be such that no words can express.”380

In his address to the British Association at Leicester in 1907, Sir David Gill said —

“Evidence upon evidence has accumulated to show that nebulæ consist of the matter out of which stars have been and are being evolved… The fact of such an evolution with the evidence before us, can hardly be doubted. I most fully believe that, when the modifications of terrestrial spectra under sufficiently varied conditions of temperature, pressure, and environment, have been further studied, this connection will be greatly strengthened.”

CHAPTER XVIII

Historical

The grouping of the stars into constellations is of great antiquity. The exact date of their formation is not exactly known, but an approximate result may be arrived at from the following considerations. On the celestial spheres, or “globes,” used by the ancient astronomers, a portion of the southern heavens of a roughly circular form surrounding the South Pole was left blank. This space presumably contained the stars in the southern hemisphere which they could not see from their northern stations. Now, the centre of this circular blank space most probably coincided with the South Pole of the heavens at the time when the constellations were first formed. Owing to the “Precession of the Equinoxes” this centre has now moved away from the South Pole to a considerable distance. It can be easily computed at what period this centre coincided with the South Pole, and calculations show that this was the case about 2700 B.C. The position of this circle also indicates that the constellations were formed at a place between 36° and 40° north latitude, and therefore probably somewhere in Asia Minor north of Mesopotamia. Again, the most ancient observations refer to Taurus as the equinoxial constellation. Virgil says —

“Candidus auratis aperit cum cornibus annum Taurus.”381

This would indicate a date about 3000 B.C. There is no tradition, however, that the constellation Gemini was ever seen to occupy this position, so that 3000 B.C. seems to be the earliest date admissible.382

Prof. Sayce thinks that the “signs of the Zodiac” had their origin in the plains of Mesopotamia in the twentieth or twenty-third century B.C., and Brown gives the probable date as 2084 B.C.383

According to Seneca, the study of astronomy among the Greeks dates back to about 1400 B.C.; and the ancient constellations were already classical in the time of Eudoxus in the fourth century B.C. Eudoxus (408-355 B.C.) observed the positions of forty-seven stars visible in Greece, thus forming the most ancient star catalogue which has been preserved. He was a son of Eschinus, and a pupil of Archytas and probably Plato.

The work of Eudoxus was put into verse by the poet Aratus (third century B.C.). This poem describes all the old constellations now known, except Libra, the Balance, which was at that time included in the Claws of the Scorpion. About B.C. 50, the Romans changed the Claws, or Chelæ, into Libra. Curious to say, Aratus states that the constellation Lyra contained no bright star!384 Whereas its principal star, Vega, is now one of the brightest stars in the heavens!

With reference to the origin of the constellations, Aratus says —

“Some men of yoreA nomenclature thought of and devisedAnd forms sufficient found.”

This shows that even in the time of Aratus the constellations were of great antiquity.

Brown says —

“Writers have often told us, speaking only from the depths of their ignorance, how ‘Chaldean’ shepherds were wont to gaze at the brilliant nocturnal sky, and to imagine that such and such stars resemble this or that figure. But all this is merely the old effort to make capital out of nescience, and the stars are before our eyes to prove the contrary. Having already certain fixed ideas and figures in his mind, the constellation-former, when he came to his task, applied his figures to the stars and the stars to his figures as harmoniously as possible.”385 “Thus e. g. he arranged the stars of Andromeda into the representation of a chained lady, not because they naturally reminded him (or anybody else) of such a figure, but because he desired to express that idea.”

A coin of Manius Aquillus, B.C. 94, shows four stars in Aquila, and seems to be the oldest representation extant of a star group. On a coin of B.C. 43, Dr. Vencontre found five stars, one of which was much larger than the others, and concludes that it represents the Hyades (in Taurus). He attributes the coin to P. Clodius Turrinus, who probably used the constellation Taurus or Taurinus as a phonetic reference to his surname. A coin struck by L. Lucretius Trio in 74 B.C., shows the seven stars of the Plough, or as the ancients called them Septem Triones. Here we have an allusion to the name of the magistrate Trio.386

In a work published in Berne in 1760, Schmidt contends that the ancient Egyptians gave to the constellations of the Zodiac the names of their divinities, and expressed them by the signs which were used in their hieroglyphics.387

Hesiod mentions Orion, the Pleiades, Sirius, Aldebaran, and Arcturus; and Homer refers to Orion, Arcturus, the Pleiades, the Hyades, the Great Bear (under the name of Amaxa, the Chariot), and the tail of the Little Bear, or “Cynosura.”

Hipparchus called the constellations Asterisms (ἀστερίσμος), Aristotle and Hyginus Σομάτα (bodies), and Ptolemy Σχημάτα (figures). By some they were called Μορφώσεις (configurations), and by others Μετεώρε. Proclus called those near the ecliptic Ζωδία (animals). Hence our modern name Zodiac.

Hipparchus, Ptolemy, and Al-Sufi referred the positions of the stars to the ecliptic. They are now referred to the equator. Aboul Hassan in the thirteenth century (1282) was the first to use Right Ascensions and Declinations instead of Longitudes and Latitudes. The ancient writers described the stars by their positions in the ancient figures. Thus they spoke of “the star in the head of Hercules,” “the bright star in the left foot of Orion” (Rigel); but Bayer in 1603 introduced the Greek letters to designate the brighter stars, and these are now universally used by astronomers. These letters being sometimes insufficient, Hevelius added numbers, but the numbers in Flamsteed’s Catalogue are now generally used.

Ptolemy and all the ancient writers described the constellation figures as they are seen on globes, that is from the outside. Bayer in his Atlas, published in 1603, reversed the figures to show them as they would be seen from the interior of a hollow globe and as, of course, they are seen in the sky. Hevelius again reversed Bayer’s figures to make them correspond with those of Ptolemy. According to Bayer’s arrangement, Betelgeuse (α Orionis) would be on the left shoulder of Orion, instead of the right shoulder according to Ptolemy and Al-Sufi, and Rigel (β Orionis) on the right foot (Bayer) instead of the left foot (Ptolemy). This change of position has led to some confusion; but at present the positions of the stars are indicated by their Right Ascensions and Declinations, without any reference to their positions in the ancient figures.

The classical constellations of Hipparchus and Ptolemy number forty-eight, and this is the number described by Al-Sufi in his “Description of the Fixed Stars” written in the tenth century A.D.

Firminicus gives the names of several constellations not mentioned by Ptolemy. M. Fréret thought that these were derived from the Egyptian sphere of Petosiris. Of these a Fox was placed north of the Scorpion; a constellation called Cynocephalus near the southern constellation of the Altar (Ara); and to the north of Pisces was placed a Stag. But all these have long since been discarded. Curious to say neither the Dragon nor Cepheus appears on the old Egyptian sphere.388

Other small constellations have also been formed by various astronomers from time to time, but these have disappeared from our modern star maps. The total number of constellations now recognized in both hemispheres amounts to eighty-four.

The first catalogue formed was nominally that of Eudoxus in the fourth century B.C. (about 370 B.C.). But this can hardly be dignified by the name of catalogue, as it contained only forty-seven stars, and it omits several of the brighter stars, notably Sirius! The first complete (or nearly complete) catalogue of stars visible to the naked eye was that of Hipparchus about 129 B.C. Ptolemy informs us that it was the sudden appearance of a bright new or “temporary star” in the year 134 B.C. in the constellation Scorpio which led Hipparchus to form his catalogue, and there seems to be no reason to doubt the accuracy of this statement, as the appearance of this star is recorded in the Chinese Annals. The Catalogue of Hipparchus contains only 1080 stars; but as many more are visible to the naked eye, Hipparchus must have omitted those which are not immediately connected with the old constellation figures of men and animals.