Gore John Ellard
Astronomical Curiosities: Facts and Fallacies
The northern of the two “pointers” in the Plough (so called because they nearly point to the Pole Star) is about the 2nd magnitude, as Al-Sufi rated it. It was thought to be variable in colour by Klein, Konkoly, and Weber; and M. Lau has recently found a period of 50 days with a maximum of “jaune rougeâtre” on April 2, 1902.
The famous variable star η Argus did “not exceed the 8th magnitude” in February, 1907, according to Mr. Tebbutt.341 This is the faintest ever recorded for this wonderful star.
It is stated in Knowledge (vol. 5, p. 3, January 4, 1884) that the temporary star of 1876 (in the constellation of Cygnus) “had long been known and catalogued as a telescopic star of the 9th magnitude with nothing to distinguish it from the common herd.” But this is quite erroneous. The star was quite unknown before it was discovered by Schmidt at Athens on November 24 of that year. The remark apparently refers to the “Blaze Star” of 1866 in Corona Borealis, which was known previously as a star of about the 9th magnitude before its sudden outburst on May 12 of that year.
This “new star” of 1866 – T Coronæ, as it is now called – was, with the possible exception of Nova Persei (1901), the only example of a nova which was known to astronomers as a small star previous to the great outburst of light. It is the brightest of the novæ still visible. It was the first of these interesting objects to be examined with the spectroscope. It was observed by Burnham in the years 1904-1906 with the great 40-inch telescope of the Yerkes Observatory (U.S.A.). He found its colour white, or only slightly tinged with yellow. In August and September, 1906, he estimated its magnitude at about 9·3, and “it would seem therefore that the Nova is now essentially of the same brightness it was before the outburst in 1866.” It shows no indication of motion. Burnham found no peculiarity about its telescopic image. A small and very faint nebula was found by Burnham a little following (that is east of) the nova.342
The following details of the great new star of 1572 – the “Pilgrim Star” of Tycho Brahé – are given by Delambre.343 In November, 1572, it was brighter than Sirius, Vega, and Jupiter, and almost equal to Venus at its brightest. During December it resembled Jupiter in brightness. In January, 1573, it was fainter and only a little brighter than stars of the 1st magnitude. In February and March it was equal to 1st magnitude stars, and in April and May was reduced to the 2nd magnitude. In June and July it was 3rd magnitude; in September of the 4th, and at the end of 1573 it was reduced to the 5th magnitude. In February, 1574, it was 6th magnitude, and in March of the same year it became invisible to the naked eye.
From this account it will be seen that the decrease in light of this curious object was much slower than that of Nova Persei (1901) (“the new star of the new century”). This would suggest that it was a much larger body.
There were also changes in its colour. When it was of the brightness of Venus or Jupiter it shone with a white light. It then became golden, and afterwards reddish like Mars, Aldebaran, or Betelgeuse. It afterwards became of a livid white colour like Saturn, and this it retained as long as it was visible. Tycho Brahé thought that its apparent diameter might have been about 3½ minutes of arc, and that it was possibly 361 times smaller than the earth(!) But we now know that these estimates were probably quite erroneous.
Temporary stars were called by the ancient Chinese “Ke-sing,” or guest stars.344
A temporary star recorded by Ma-tuan-lin (Chinese Annals) in February, 1578, is described as “a star as large as the sun.” But its position is not given.345
About the middle of September, 1878, Mr. Greely, of Boston (U.S.A.), reported to Mr. E. F. Sawyer (the eminent observer of variable stars) that, about the middle of August of that year, he had seen the famous variable star Mira Ceti of about the 2nd magnitude, although the star did not attain its usual maximum until early in October, 1878. Mr. Greely stated that several nights after he first saw Mira it had faded to the 4th or 5th magnitude. If there was no mistake in this observation (and Sawyer could find none) it was quite an unique phenomenon, as nothing of the sort has been observed before or since in the history of this famous star. It looks as if Mr. Greely had observed a new or “temporary” star near the place of Mira Ceti; but as the spot is far from the Milky Way, which is the usual seat of such phenomena, this hypothesis seems improbable.
In the so-called Cepheid and Geminid variables of short period, the principal characteristics of the light variation are as follows: —
“1. The light varies without pause.
“2. The amount of their light variation is usually about 1 magnitude.
“3. Their periods are short – a few days only.
“4. They are of a spectral type approximately solar; no Orion, Sirian or Arcturian stars having been found among them.
“5. They seem to be found in greater numbers in certain parts of the sky, notably in the Milky Way, but exhibit no tendency to form clusters.
“6. All those stars whose radial velocities have been studied have been found to be binaries whose period of orbital revolution coincides with that of their light change.
“7. The orbits, so far as determined, are all small, a sin i being 2,000,000 kilometres or less.
“8. Their maximum light synchronizes with their maximum velocity of approach, and minimum light with maximum velocity of recession.
“9. No case has been found in which the spectrum of more than one component has been bright enough to be recorded in the spectrograms.”346
It is very difficult to find an hypothesis which will explain satisfactorily all these characteristics, and attempts in this direction have not proved very successful. Mr. J. C. Duncan suggests the action of an absorbing atmosphere surrounding the component stars.
On March 30, 1612, Scheiner saw a star near Jupiter. It was at first equal in brightness to Jupiter’s satellites. It gradually faded, and on April 8 of the same year it was only seen with much difficulty in a very clear sky. “After that date it was never seen again, although carefully looked for under favourable conditions.”
An attempted identification of Scheiner’s star was made in recent years by Winnecke. He found that its position, as indicated by Scheiner, agrees with that of the Bonn Durchmusterung star 15°, 2083 (8½ magnitude). This star is not a known variable. Winnecke watched it for 17 years, but found no variation of light. From Scheiner’s recorded observations his star seems to have reached the 6th magnitude, which is considerably brighter than the Durchmusterung star watched by Winnecke.347
With reference to the colours of the stars, the supposed change of colour in Sirius from red to white is well known, and will be considered in the chapter on the Constellations. The bright star Arcturus has also been suspected of variation in colour. About the middle of the nineteenth century Dr. Julius Schmidt, of Athens, the well-known observer of variable stars, thought it one of the reddest stars in the sky, especially in the year 1841, when he found its colour comparable with that of the planet Mars.348 In 1852, however, he was surprised to find it yellow and devoid of any reddish tinge; in colour it was lighter than that of Capella. In 1863, Mr. Jacob Ennis found it “decidedly orange.” Ptolemy and Al-Sufi called it red.
Mr. Ennis speaks of Capella as “blue” (classing it with Rigel), and comparing its colour with that of Vega!349 But the present writer has never seen it of this colour. To his eye it seems yellowish or orange. It was called red by Ptolemy, El Fergani, and Riccioli; but Al-Sufi says nothing about its colour.
Of β Ursæ Minoris, Heis, the eminent German astronomer said, “I have had frequent opportunities of convincing myself that the colour of this star is not always equally red; at times it is more or less yellow, at others most decidedly red.”350
Among double stars there are many cases in which variation of colour has been suspected. In some of these the difference in the recorded colour may possibly be due to “colour blindness” in some of the observers; but in others there seems to be good evidence in favour of a change. The following may be mentioned: —
η Cassiopeiæ. Magnitudes of the components about 4 and 7½. Recorded as red and green by Sir John Herschel and South; but yellow and orange by Sestini.
ι Trianguli. Magnitudes 5½ and 7. Secchi estimated them as white or yellow and blue; but Webb called them yellow and green (1862).
γ Leonis, 2 and 3½. Sir William Herschel noted them white and reddish white; but Webb, light orange and greenish yellow.
12 Canum Venaticorum, 2½ and 6½. White and red, Sir William Herschel; but Sir John Herschel says in 1830, “With all attention I could perceive no contrast of colours in the two stars.” Struve found them both white in 1830, thus agreeing with Sir John Herschel. Sestini saw them yellow and blue in 1844; Smyth, in 1855, pale reddish white and lilac; Dembowski, in 1856, white and pale olive blue; and Webb, in 1862, flushed white and pale lilac.
On October 13, 1907, Nova Persei, the great new star of 1901, was estimated to be only 11·44 magnitude, or about 11½. When at its brightest this famous star was about zero magnitude; so that it has in about 6 years faded about 11½ magnitudes in brightness; in other words, it has been reduced to 1⁄40000 of its greatest brilliancy!
CHAPTER XVII
Nebulæ and Clusters
In his interesting and valuable work on “The Stars,” the late Prof. Newcomb said —
“Great numbers of the nebulæ are therefore thousands of times the dimensions of the earth’s orbit, and 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”!
Prof. Perrine thinks that the total number of the nebulæ will ultimately be found to exceed a million.351
Dr. Max Wolf has discovered a number of small nebulæ in the regions near Algol and Nova Persei (the great “new star” of 1901). He says, “They mostly lie in two bands,” and are especially numerous where the two bands meet, a region of 12 minutes of arc square containing no less than 148 of them. They are usually “round with central condensation,” and form of Andromeda nebula.352
Some small nebulæ have been found in the vicinity of the globular clusters. They are described by Prof. Perrine as very small and like an “out of focus” image of a small star. “They appear to be most numerous about clusters which are farthest from the galaxy.” Prof. Perrine says, “Practically all the small nebulæ about the globular clusters are elliptical or circular. Those large enough to show structure are spirals. Doubtless the majority of these are spirals.”353 This seems further evidence in favour of the “spiral nebular hypothesis” of Chamberlin and Moulton.
A great photographic nebula in Orion was discovered by Prof. Barnard in 1894. In a drawing he gives of the nebula,354 it forms a long streak beginning a little south of γ Orionis (Bellatrix), passing through the star 38 Orionis north of 51 and south of 56 and 60 Orionis. Then turning south it sweeps round a little north of κ Orionis; then over 29 Orionis, and ends a little to the west of η Orionis. There is an outside patch west of Rigel. Barnard thinks that the whole forms a vast spiral structure; probably connected with the “great nebula” in the “sword of Orion,” which it surrounds.
From calculations of the brightness of surface (“intrinsic brightness”) of several “planetary” nebulæ made by the present writer in the year 1905, he finds that the luminosity is very small compared with that of the moon. The brightest of those examined (h 3365, in the southern hemisphere, near the Southern Cross) has a surface luminosity of only 1⁄400 of that of the moon.355 The great nebulæ in Orion and Andromeda seem to have “still smaller intrinsic brightness.”
Arago says —
“The spaces which precede or which follow simple nebulæ, and a fortiori groups of nebulæ, contain generally few stars. Herschel found this rule to be invariable. Thus every time that, during a short interval, no star appeared, in virtue of the diurnal motion, to place itself in the field of his motionless telescope, he was accustomed to say to the secretary who assisted him (Miss Caroline Herschel), ‘Prepare to write; nebulæ are about to arrive.’”356
Commenting on this remark of Arago, the late Herbert Spencer says —
“How does this fact consist with the hypothesis that nebulæ are remote galaxies? If there were but one nebula, it would be a curious coincidence were this one nebula so placed in the distant regions of space as to agree in direction with a starless spot in our sidereal system! If there were but two nebulæ, and both were so placed, the coincidence would be excessively strange. What shall we say on finding that they are habitually so placed? (the last five words replace some that are possibly a little too strong)… When to the fact that the general mass of nebulæ are antithetical in position to the general mass of the stars, we add the fact that local regions of nebulæ are regions where stars are scarce, and the further fact that single nebulæ are habitually found in comparatively starless spots, does not the proof of a physical connection become overwhelming?”357
With reference to the small elongated nebula discovered by Miss Caroline Herschel in 1783 near the great nebula in Andromeda, Admiral Smyth says, “It lies between two sets of stars, consisting of four each, and each disposed like the figure 7, the preceding group being the smallest.”358
Speaking of the “nebula” Messier 3 – a globular cluster in Canes Venatici – Admiral Smyth says, “This mass is one of those balls of compact and wedged stars whose laws of aggregation it is so impossible to assign; but the rotundity of the figure gives full indication of some general attractive bond of union.”359 The terms “compact and wedged” are, however, too strong, for we know that in the globular clusters the component stars must be separated from each other by millions of miles!
Prof. Chamberlin suggests that the secondary nebula (as it is called) in the great spiral in Canes Venatici (Messier 51) may possibly represent the body which collided with the other (the chief nucleus) in a grazing collision, and is now escaping. He considers this secondary body to have been “a dead sun” – that is, a dark body.360 This would be very interesting if it could be proved. But it seems to me more probable that the secondary nucleus is simply a larger portion of the ejected matter, which is now being gradually detached from the parent mass.
Scheiner says “the previous suspicion that the spiral nebulæ are star clusters is now raised to a certainty,” and that the spectrum of the Andromeda nebula is very similar to that of the sun. He says there is “a surprising agreement of the two, even in respect to the relative intensity of the separate spectral regions.”361
In the dynamical theory of spiral nebulæ, Dr. E. J. Wilczynski thinks that the age of a spiral nebula may be indicated by the number of its coils; those having the largest number of coils being the oldest, from the point of view of evolution.362 This seems to be very probable.
In the spectrum of the gaseous nebulæ, the F line of hydrogen (Hβ) is visible, but not the C line (Hα). The invisibility of the C line is explained by Scheiner as due to a physiological cause, “the eye being less sensitive to that part of the spectrum in which the line appears than to the part containing the F line.”363
An apparent paradox is found in the case of the gaseous nebulæ. The undefined outlines of these objects render any attempt at measuring their parallax very difficult, if not impossible. Their distance from the earth is therefore unknown, and perhaps likely to remain so for many years to come. It is possible that they may not be farther from us than some of the stars visible in their vicinity. On the other hand, they may lie far beyond them in space. But whatever their distance from the earth may be, it may be easily shown that their attraction on the sun is directly proportioned to their distance – that is, the greater their distance, the greater the attraction! This is evidently a paradox, and rather a startling one too. But it is nevertheless mathematically true, and can be easily proved. For, their distance being unknown, they may be of any dimensions. They might be comparatively small bodies relatively near the earth, or they may be immense masses at a vast distance from us. The latter is, of course, the more probable. In either case the apparent size would be the same. Take the case of any round gaseous nebula. Assuming it to be of a globular form, its real diameter will depend on its distance from the earth – the greater the distance, the greater the diameter. Now, as the volumes of spheres vary as the cubes of their diameters, it follows that the volume of the nebula will vary as the cube of its distance from the earth. As the mass of an attracting body depends on its volume and density, its real mass will depend on the cube of its distance, the density (although unknown) being a fixed quantity. If at a certain distance its mass is m, at double the distance (the apparent diameter being the same) it would have a mass of eight times m (8 being the cube of 2), and at treble the distance its mass would be 27 m, and so on, its apparent size being known, but not its real size. This is obvious. Now, the attractive power of a body varies directly as its mass – the greater the mass, the greater the attraction. Again, the attraction varies inversely as the square of the distance, according to the well-known law of Newton. Hence if d be the unknown distance of the nebula, we have its attractive power varying as d3 divided by d2, or directly as the distance d. We have then the curious paradox that for a nebula whose distance from the earth is unknown, its attractive power on the sun (or earth) will vary directly as the distance – the greater the distance the greater the attraction, and, of course, conversely, the smaller the distance the less the attractive power. This result seems at first sight absurd and incredible, but a little consideration will show that it is quite correct. Consider a small wisp of cloud in our atmosphere. Its mass is almost infinitesimal and its attractive power on the earth practically nil. But a gaseous nebula having the same apparent size would have an enormous volume, and, although probably formed of very tenuous gas, its mass would be very great, and its attractive power considerable. The large apparent size of the Orion nebula shows that its volume is probably enormous, and as its attraction on the sun is not appreciable, its density must be excessively small, less than the density of the air remaining in the receiver of the best air-pump after the air has been exhausted. How such a tenuous gas can shine as it does forms another paradox. Its light is possibly due to some phosphorescent or electrical action.
The apparent size of “the great nebula in Andromeda” shows that it must be an object of vast dimensions. The nearest star to the earth, Alpha Centauri, although probably equal to our sun in volume, certainly does not exceed one-hundredth of a second in diameter as seen from the earth. But in the case of the Andromeda nebula we have an object of considerable apparent size, not measured by seconds of arc, but showing an area about three times greater than that of the full moon. The nebula certainly lies in the region of the stars – much farther off than Alpha Centauri – and its great apparent size shows that it must be of stupendous dimensions. A moment’s consideration will show that whatever its distance may be, the farther it is from the earth the larger it must be in actual size. The sun is vastly larger than the moon, but its apparent size is about the same owing to its greater distance. Sir William Herschel thought the Andromeda nebula to be “undoubtedly the nearest of all the great nebulæ,” and he estimated its distance at 2000 times the distance of Sirius. This would not, however, indicate a relatively near object, as it would imply a “light journey” of over 17,000 years! (The distance of Sirius is about 88 “light years.”)
It has been generally supposed that this great nebula lies at a vast distance from the earth, possibly far beyond most of the stars seen in the same region of the sky; but perhaps not quite so far as Herschel’s estimate would imply. Recently, however, Prof. Bohlin of Stockholm has found from three series of measures made in recent years a parallax of 0″·17.364
This indicates a distance of 1,213,330 times the sun’s distance from the earth, and a “light journey” of about 19 years. This would make the distance of the nebula more than twice the distance of Sirius, about four times the distance of α Centauri, but less than that of Capella.
Prof. Bohlin’s result is rather unexpected, and will require confirmation before it can be accepted. But it will be interesting to inquire what this parallax implies as to the real dimensions and probable mass of this vast nebula. The extreme length of the nebula may be taken to represent its diameter considered as circular. For, although a circle seen obliquely is always foreshortened into an ellipse, still the longer axis of the ellipse will always represent the real diameter of the circle. This may be seen by holding a penny at various angles to the eye. Now, Dr. Roberts found that the apparent length of the Andromeda nebula is 2⅓ degrees, or 8400 seconds of arc. The diameter in seconds divided by the parallax will give the real diameter of the nebula in terms of the sun’s distance from the earth taken as unity. Now, 8400 divided by 0″·17 gives nearly 50,000, that is, the real diameter of the Andromeda nebula would be – on Bohlin’s parallax – nearly 50,000 times the sun’s distance from the earth. As light takes about 500 seconds to come from the sun to the earth, the above figures imply that light would take about 290 days, or over 9 months to cross the diameter of this vast nebula.
Elementary geometrical considerations will show that if the Andromeda nebula lies at a greater distance from the earth than that indicated by Bohlin’s parallax, its real diameter, and therefore its volume and mass, will be greater. If, therefore, we assume the parallax found by Bohlin, we shall probably find a minimum value for the size and mass of this marvellous object.
Among Dr. Roberts’ photographs of spiral nebulæ (and the Andromeda nebula is undoubtedly a spiral) there are some which are apparently seen nearly edgeways, and show that these nebulæ are very thin in proportion to their diameter. From a consideration of these photographs we may, I think, assume a thickness of about one-hundredth of the diameter. This would give a thickness for the Andromeda nebulæ of about 500 times the sun’s distance from the earth. This great thickness will give some idea of the vast proportions of the object we are dealing with. The size of the whole solar system – large as it is – is small in comparison. The diameter and thickness found above can easily be converted into miles, and from these dimensions the actual volume of the nebula can be compared with that of the sun. It is merely a question of simple mensuration, and no problem of “high mathematics” is involved. Making the necessary calculations, I find that the volume of the Andromeda nebula would be about 2·32 trillion times (2·32 × 1018) the sun’s volume! Now, assuming that the nebulous matter fills only one-half of the apparent volume of the nebula (allowing for spaces between the spiral branches), we have the volume = 1·16 × 1018. If the nebula had the same density as the sun, this would be its mass in terms of the sun’s mass taken as unity, a mass probably exceeding the combined mass of all the stars visible in the largest telescopes! But this assumption is, of course, inadmissible, as the sun is evidently quite opaque, whereas the nebula is, partially at least, more or less transparent. Let us suppose that the nebula has a mean density equal to that of atmospheric air. As water is about 773 times heavier than air, and the sun’s density is 1·4 (water = 1) we have the mass of the nebula equal to 1·16 × 1018 divided by 773 × 1·4, or about 1015 times the sun’s mass, which is still much greater than the probable combined mass of all the visible stars. As it seems unreasonable to suppose that the mass of an individual member of our sidereal system should exceed the combined mass of the remainder of the system, we seem compelled to further reduce the density of the Andromeda nebula. Let us assume a mean density of, say, a millionth of hydrogen gas (a sufficiently low estimate) which is about 14·44 times lighter than air, and we obtain a mass of about 8 × 107 or 80 million times the mass of the sun, which is still an enormous mass.
As possibly I may have assumed too great a thickness for the nebula, let us take a thickness of one-tenth of that used above, or one thousandth of the length of the nebula. This gives a mass of 8 million times the sun’s mass. This seems a more probable mass if the nebula is – as Bohlin’s parallax implies – a member of our sidereal system.
If we assume a parallax of say 0″·01 – or one-hundredth of a second of arc – which would still keep the nebula within the bounds of our sidereal system – we have the dimensions of the nebula increased 17 times, and hence its mass nearly 5000 times greater (173) than that found above. The mass would then be 40,000 million times the sun’s mass! This result seems highly improbable, for even this small parallax would imply a light journey of only 326 years, whereas the distance of the Milky Way has been estimated by Prof. Newcomb at about 3000 years’ journey for light.
In Dr. Roberts’ photograph many small stars are seen scattered over the surface of the nebula; but these do not seem to be quite so numerous as in the surrounding sky. If the nebula lies nearer to us than the fainter stars visible on the photograph, some of them may be obscured by the denser portions of the nebula; some may be visible through the openings between the spiral branches; while others may be nearer to us and simply projected on the nebula.
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.