Gore John Ellard
Astronomical Curiosities: Facts and Fallacies
CHAPTER XIV
The Stars
Pliny says that Hipparchus “ventured to count the stars, a work arduous even for the Deity.” But this was quite a mistaken idea. Those visible to the naked eye are comparatively few in number, and the enumeration of those visible in an opera-glass – which of course far exceed those which can be seen by unaided vision – is a matter of no great difficulty. Those visible in a small telescope of 2¾ inches aperture have all been observed and catalogued; and even those shown on photographs taken with large telescopes can be easily counted. The present writer has made an attempt in this direction, and taking an average of a large number of counts in various parts of the sky, as shown on stellar photographs, he finds a total of about 64 millions for the whole sky in both hemispheres.269 Probably the total number will not exceed 100 millions. But this is a comparatively small number, even when compared with the human population of our little globe.
With reference to the charts made by photography in the International scheme commenced some years ago, it has now been estimated that the charts will probably contain a total of about 9,854,000 stars down to about the 14th magnitude (13·7). The “catalogue plates” (taken with a shorter exposure) will, it is expected, include about 2,676,500 stars down to 11½ magnitude. These numbers may, however, be somewhat increased when the work has been completed.270 If this estimate proves to be correct, the number of stars visible down to the 14th magnitude will be considerably less than former estimates have made it.
Prof. E. C. Pickering estimates that the total number of stars visible on photographs down to the 16th magnitude (about the faintest visible in the great Lick telescope) will be about 50 millions.271 In the present writer’s enumeration, above referred to, many stars fainter than the 16th magnitude were included.
Admiral Smyth says, with reference to Sir William Herschel – perhaps the greatest observer that ever lived – “As to Sir William himself, he could unhesitatingly call every star down to the 6th magnitude, by its name, letter, or number.”272 This shows great powers of observation, and a wonderful memory.
On a photographic plate of the Pleiades taken with the Bruce telescope and an exposure of 6 hours, Prof. Bailey of Harvard has counted “3972 stars within an area 2° square, having Alcyone at its centre.”273 This would give a total of about 41 millions for the whole sky, if of the same richness.
With an exposure of 16 hours, Prof. H. C. Wilson finds on an area of less that 110′ square a total of 4621 stars. He thinks, “That all of these stars belong to the Pleiades group is not at all probable. The great majority of them probably lie at immense distances beyond the group, and simply appear in it by projection.”[274] He adds, “It has been found, however, by very careful measurements made during the last 75 years at the Königsbergh and Yale Observatories, that of the sixty-nine brighter stars, including those down to the 9th magnitude, only eight show any certain movement with reference to Alcyone. Since Alcyone has a proper motion or drift of 6″ per century, this means that all the brightest stars except the eight mentioned are drifting with Alcyone and so form a true cluster, at approximately the same distance from the earth. Six of the eight stars which show relative drift are moving in the opposite direction to the movement of Alcyone, and at nearly the same rate, so that their motion is only apparent. They are really stationary, while Alcyone and the rest of the cluster are moving past them.”274 This tends to show that the faint stars are really behind the cluster, and are unconnected with it.
It is a popular idea with some people that the Pole Star is the nearest of all the stars to the celestial pole. But photographs show that there are many faint stars nearer to the pole than the Pole Star. The Pole Star is at present at a distance of 1° 13′ from the real pole of the heavens, but it is slowly approaching it. The minimum distance will be reached in the year 2104. From photographs taken by M. Flammarion at the Juvisy Observatory, he finds that there are at least 128 stars nearer to the pole than the Pole Star! The nearest star to the pole was, in the year 1902, a small star of about 12½ magnitude, which was distant about 4 minutes of arc from the pole.275 The estimated magnitude shows that the Pole Star is nearly 10,000 times brighter than this faint star!
It has been found that Sirius is bright enough to cast a shadow under favourable conditions. On March 22, 1903, the distinguished French astronomer Touchet succeeded in photographing the shadow of a brooch cast by this brilliant star. The exposure was 1h 5m.
Martinus Hortensius seems to have been the first to see stars in daylight, perhaps early in the seventeenth century. He mentions the fact in a letter to Gassendi dated October 12, 1636, but does not give the date of his observation. Schickard saw Arcturus in broad daylight early in 1632. Morin saw the same bright star half an hour after sunset in March, 1635.
Some interesting observations were made by Professors Payne and H. C. Wilson, in the summer of 1904, at Midvale, Montana (U.S.A.), at a height of 4790 feet above sea-level. At this height they found the air very clear and transparent. “Many more stars were visible at a glance, and the familiar stars appeared more brilliant… In the great bright cloud of the Milky Way, between β and γ Cygni, one could count easily sixteen or seventeen stars, besides the bright ones η and χ,276 while at Northfield it is difficult to distinctly see eight or nine with the naked eye.” Some nebulæ and star fields were photographed with good results by the aid of a 2½-inch Darlot lens and 3 hours’ exposure.277
Prof. Barnard has taken some good stellar photographs with a lens of only 1½ inches in diameter, and 4 or 5 inches focus belonging to an ordinary “magic lantern”! He says that these “photographs with the small lens show us in the most striking manner how the most valuable and important information may be obtained with the simplest means.”278
With reference to the rising and setting of the stars due to the earth’s rotation on its axis, the late Sir George B. Airy, Astronomer Royal of England, once said to a schoolmaster, “I should like to know how far your pupils go into the first practical points for which reading is scarcely necessary. Do they know that the stars rise and set? Very few people in England know it. I once had a correspondence with a literary man of the highest rank on a point of Greek astronomy, and found that he did not know it!”279
Admiral Smyth says, “I have been struck with the beautiful blue tint of the smallest stars visible in my telescope. This, however, may be attributed to some optical peculiarity.” This bluish colour of small stars agrees with the conclusion arrived at by Prof. Pickering in recent years, that the majority of faint stars in the Milky Way have spectra of the Sirian type and, like that brilliant star, are of a bluish white colour. Sir William Herschel saw many stars of a redder tinge than other observers have noticed. Admiral Smyth says, “This may be owing to the effect of his metallic mirror or to some peculiarity of vision, or perhaps both.”280
The ancient astronomers do not mention any coloured stars except white and red. Among the latter they only speak of Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse as of a striking red colour. To these Al-Sufi adds Alphard (α Hydræ).
Sir William Herschel remarked that no decidedly green or blue star “has ever been noticed unassociated with a companion brighter than itself.” An exception to Herschel’s rule seems to be found in the case of the star β Libræ, which Admiral Smyth called “pale emerald.” Mr. George Knott observed it on May 19, 1852, as “beautiful pale green” (3·7 inches achromatic, power 80), and on May 9, 1872, as “fine pale green” (5·5 inches achromatic, power 65).
The motion of stars in the line of sight, as shown by the spectroscope – should theoretically alter their brightness in the course of time; those approaching the earth becoming gradually brighter, while those receding should become fainter. But the distance of the stars is so enormous that even with very high velocities the change would not become perceptible for ages. Prof. Oudemans found that to change the brightness of a star by only one-tenth of a magnitude – a quantity barely perceptible to the eye-a number of years would be necessary, which is represented by the formula
5916 years
parallax × motion
for a star approaching the earth, and for a receding star
6195 years
p × m
This is in geographical miles, 1 geographical mile being equal to 4·61 English miles.
Reducing the above to English miles, and taking an average for both approaching and receding stars, we have
27,660 years
p × m
where p = parallax in seconds of arc, and m = radial velocity in English miles per second.
Prof. Oudemans found that the only star which could have changed in brightness by one-tenth of a magnitude since the time of Hipparchus is Aldebaran. This is taking its parallax as 0″·52. But assuming the more reliable parallax 0″·12 found by Dr. Elkin, this period is 4⅓ times longer. For Procyon, the period would be 5500 years.281 The above calculation shows how absurd it is to suppose that any star could have gained or lost in brightness by motion in the line of sight during historical times. The “secular variation” of stars is quite another thing. This is due to physical changes in the stars themselves.
The famous astronomer Halley, the second Astronomer Royal at Greenwich, says (Phil. Trans., 1796), “Supposing the number of 1st magnitude stars to be 13, at twice the distance from the sun there may be placed four times as many, or 52; which with the same allowance would nearly represent the star we find to be of the 2nd magnitude. So 9 × 13, or 117, for those at three times the distance; and at ten times the distance 100 × 13, or 1300 stars; of which distance may probably diminish the light of any of the stars of the 1st magnitude to that of the 6th, it being but the hundredth part of what, at their present distance, they appear with.” This agrees with the now generally accepted “light ratio” of 2·512 for each magnitude, which makes a first magnitude star 100 times the light of a 6th magnitude.
On the 4th of March, 1796,282 the famous French astronomer Lalande observed on the meridian a star of small 6th magnitude, the exact position of which he determined. On the 15th of the same month he again observed the star, and the places found for 1800 refer to numbers 16292-3 of the reduced catalogue. In the observation of March 4 he attached the curious remark, “Étoile singulière” (the observation of March 15 is without note). This remark of Lalande has puzzled observers who failed to find any peculiarity about the star. Indeed, “the remark is a strange one for the observer of so many thousands of stars to attach unless there was really something singular in the star’s aspect at the time.” On the evening of April 18, 1887, the star was examined by the present writer, and the following is the record in his observing book, “Lalande’s étoile singulière (16292-3) about half a magnitude less than η Cancri. With the binocular I see two streams of small stars branching out from it, north preceding like the tails of comet.” This may perhaps have something to do with Lalande’s curious remark.
The star numbered 1647 in Baily’s Flamsteed Catalogue is now known to have been an observation of the planet Uranus.283
Prof. Pickering states that the fainter stars photographed with the 8-inch telescope at Cambridge (U.S.A.) are invisible to the eye in the 15-inch telescope.284
Sir Norman Lockyer finds that the lines of sulphur are present in the spectrum of the bright star Rigel (β Orionis).285
About 8½° south of the bright star Regulus (α Leonis) is a faint nebula (H I, 4 Sextantis). On or near this spot the Capuchin monk De Rheita fancied he saw, in the year 1643, a group of stars representing the napkin of S. Veronica – “sudarium Veronicæ sive faciem Domini maxima similitudina in astris expressum.” And he gave a picture of the napkin and star group. But all subsequent observers have failed to find any trace of the star group referred to by De Rheita!286
The Bible story of the star of the Magi is also told in connection with the birth of the sun-gods Osiris, Horus, Mithra, Serapis, etc.287 The present writer has also heard it suggested that the phenomenon may have been an apparition of Halley’s comet! But as this famous comet is known to have appeared in the year B.C. 11, and as the date of the Nativity was probably not earlier than B.C. 5, the hypothesis seems for this (and other reasons) to be inadmissible. It has also been suggested that the phenomenon might have been an appearance of Tycho Brahé’s temporary star of 1572, known as the “Pilgrim star”; but there seems to be no real foundation for such an hypothesis. There is no reason to think that “temporary” or new stars ever appear a second time.
Admiral Smyth has well said, “It checks one’s pride to recollect that if our sun with the whole system of planets, asteroids, and moons, and comets were to be removed from the spectator to the distance of the nearest fixed star, not one of them would be visible, except the sun, which would then appear but as a star of perhaps the 2nd magnitude. Nay, more, were the whole system of which our globe forms an insignificant member, with its central luminary, suddenly annihilated, no effect would be produced on those unconnected and remote bodies; and the only annunciation of such a catastrophe in the Sidereal “Times” would be that a small star once seen in a distant quarter of the sky had ceased to shine.”288
Prof. George C. Comstock finds that the average parallax of 67 selected stars ranging in brightness between the 9th and the 12th magnitude, is of the value of 0″·0051.289 This gives a distance representing a journey for light of about 639 years!
Mr. Henry Norris Russell thinks that nearly all the bright stars in the constellation of Orion are practically at the same distance from the earth. His reasons for this opinion are: (1) the stars are similar in their spectra and proper motions, (2) their proper motions are small, which suggests a small parallax, and therefore a great distance from the earth. Mr. Russell thinks that the average parallax of these stars may perhaps be 0″·005, which gives a distance of about 650 “light years.”290
According to Sir Norman Lockyer’s classification of the stars, the order of increasing temperature is represented by the following, beginning with those in the earliest stage of stellar evolution: – Nebulæ, Antares, Aldebaran, Polaris, α Cygni, Rigel, ε Tauri, β Crucis. Then we have the hottest stars represented by ε Puppis, γ Argus, and Alnitam (ε Orionis). Decreasing temperature is represented by (in order), Achernar, Algol, Markab, Sirius, Procyon, Arcturus, 19 Piscium, and the “Dark Stars.”291 But other astronomers do not agree with this classification. Antares and Aldebaran are by some authorities considered to be cooling suns.
According to Ritter’s views of the Constitution of the Celestial Bodies, if we “divide the stars into three classes according to age corresponding to these three stages of development, we shall assign to the first class, A, those stars still in the nebular phase of development; to the second class, B, those in the transient stage of greatest brilliancy; and to the class C, those stars which have already entered into the long period of slow extinction. It should be noted in this classification that we refer to relative and not absolute age, since a star of slight mass passes through the successive phases of its development more rapidly than the star of greater mass.”292 Ritter comes to the conclusion that “the duration of the period in which the sun as a star had a greater brightness than at present was very short in comparison with the period in which it had and will continue to have a brightness differing only slightly from its present value.”293
In a valuable and interesting paper on “The Evolution of Solar Stars,”294 Prof. Schuster says that “measurements by E. F. Nichols on the heat of Vega and Arcturus indicated a lower temperature for Arcturus, and confirms the conclusion arrived at on other grounds, that the hydrogen stars have a higher temperature than the solar stars.” “An inspection of the ultraviolet region of the spectrum gives the same result. These different lines of argument, all leading to the same result, justify us in saying that the surface temperature of the hydrogen stars is higher than that of the solar stars. An extension of the same reasoning leads to the belief that the helium stars have a temperature which is higher still.” Hence we have Schuster, Hale, and Sir William Huggins in agreement that the Sirian stars are hotter than the solar stars; and personally I agree with these high authorities. The late Dr. W. E. Wilson, however, held the opinion that the sun is hotter that Sirius!
Schuster thinks that Lane’s law does not apply to the temperature of the photosphere and the absorbing layers of the sun and stars, but only to the portions between the photosphere and the centre, which probably act like a perfect gas. On this view he says the interior might become “hotter and hotter until the condensation had reached a point at which the laws of gaseous condensation no longer hold.”
With reference to the stars having spectra of the 3rd and 4th type (usually orange and red in colour), Schuster says —
“The remaining types of spectra belong to lower temperature still, as in place of metallic lines, or in addition to them, certain bands appear which experiments show us invariably belong to lower temperature than the lines of the same element.
“If an evolutionary process has been going on, which is similar for all stars, there is little doubt that from the bright-line stars down to the solar stars the order has been (1) helium or Orion stars, (2) hydrogen or Sirian stars, (3) calcium or Procyon stars, (4) solar or Capellan stars.”
My investigations on “The Secular Variation of Starlight” (Studies in Astronomy, chap. 17, and Astronomical Essays, chap. 12) based on a comparison of Al-Sufi’s star magnitudes (tenth century) with modern estimates and measures, tend strongly to confirm the above views.
With regard to the 3rd-type stars, such as Betelgeuse and Mira Ceti, Schuster says, “It has been already mentioned that observers differ as to whether their position is anterior to the hydrogen or posterior to the solar stars, and there are valid arguments on both sides.”
Scheiner, however, shows, from the behaviour of the lines of magnesium, that stars of type I. (Sirian) are the hottest, and type III. the coolest, and he says, we have “for the first time a direct proof of the correctness of the physical interpretation of Vogel’s spectral classes, according to which class II. is developed by cooling from I., and III. by a further process of cooling from II.”295
Prof. Hale says that “the resemblance between the spectra of sun-spots and of 3rd-type stars is so close as to indicate that the same cause is controlling the relative intensities of many lines in both instances. This cause, as the laboratory work indicates, is to be regarded as reduced temperature.”296
According to Prof. Schuster, “a spectrum of bright lines may be given by a mass of luminous gas, even if the gas is of great thickness. There is, therefore, no difficulty in explaining the existence of stars giving bright lines.” He thinks that the difference between “bright line” stars and those showing dark lines depends upon the rate of increase of the temperature from the surface towards the centre. If this rate is slow, bright lines will be seen. If the rate of increase is rapid, the dark-line spectrum shown by the majority of the stars will appear. This rate, he thinks, is regulated by the gravitational force. So that in the early stages of condensation bright lines are more likely to occur. “If the light is not fully absorbed,” both bright and dark lines of the same element may be visible in the same star. Schuster considers it quite possible that if we could remove the outer layers of the Sun’s atmosphere, we should obtain a spectrum of bright lines.297
M. Stratonoff finds that stars having spectra of the Orion and Sirian types – supposed to represent an early stage in stellar evolution – tend to congregate in or near the Milky Way. Star clusters in general show a similar tendency, “but to this law the globular clusters form an exception.”298 We may add that the spiral nebulæ – which seem to be scattered indifferently over all parts of the sky – also seem to form an exception; for the spectra of these wonderful objects seem to show that they are really star clusters, in which the components are probably relatively small; that is, small in comparison with our sun.
If we accept the hypothesis that suns and systems were evolved from nebulæ, and if we consider the comparatively small number of nebulæ hitherto discovered in the largest telescopes – about half a million; and if we further consider the very small number of red stars, or those having spectra of the third and fourth types – usually considered to be dying-out suns – we seem led to the conclusion that our sidereal system is now at about the zenith of its life-history; comparatively few nebulæ being left to consolidate into stars, and comparatively few stars having gone far on the road to the final extinction of their light.
Prof. Boss of Albany (U.S.A.) finds that about forty stars of magnitudes from 3½ to 7 in the constellation Taurus are apparently drifting together towards one point. These stars are included between about R.A. 3h 47m to 5h 4m, and Declination + 5° to + 23° (that is, in the region surrounding the Hyades). These motions apparently converge to a point near R.A. 6h, Declination + 7° (near Betelgeuse). Prof. Boss has computed the velocity of the stars in this group to be 45·6 kilometres (about 28 miles) a second towards the “vanishing point,” and he estimated the average parallax of the group to be 0″·025 – about 130 years’ journey for light. Although the motions are apparently converging to a point, it does not follow that the stars in question will, in the course of ages, meet at the “vanishing point.” On the contrary, the observed motions show that the stars are moving in parallel lines through space. About 15 kilometres of the observed speed is due to the sun’s motion through space in the opposite direction. Prof. Campbell finds from spectroscopic measures that of these forty stars, nine are receding from the earth with velocities varying from 12 to 60 kilometres a second, and twenty-three others with less velocities than 38 kilometres.299 It will be obvious that, as there is a “vanishing point,” the motion in the line of sight must be one of recession from the earth.
It has been found that on an average the parallax of a star is about one-seventh of its “proper motion.”300
Adopting Prof. Newcomb’s parallax of 0″·14 for the famous star 1830 Groombridge, the velocity perpendicular to the line of sight is about 150 miles a second. The velocity in the line of sight – as shown by the spectroscope – is 59 miles a second approaching the earth. Compounding these two velocities we find a velocity through space of about 161 miles a second!
An eminent American writer puts into the mouth of one of his characters, a young astronomer, the following: —
“I read the page
Where every letter is a glittering sun.”
From an examination of the heat radiated by some bright stars, made by Dr. E. F. Nicholls in America with a very sensitive radiometer of his own construction, he finds that “we do not receive from Arcturus more heat than we should from a candle at a distance of 5 or 6 miles.”
With reference to the progressive motion of light, and the different times taken by light to reach the earth from different stars, Humboldt says, “The aspect of the starry heavens presents to us objects of unequal date. Much has long ceased to exist before the knowledge of its presence reaches us; much has been otherwise arranged.”301
The photographic method of charting the stars, although a great improvement on the old system, seems to have its disadvantages. One of these is that the star images are liable to disappear from the plates in the course of time. The reduction of stellar photograph plates should, therefore, be carried out as soon as possible after they are taken. The late Dr. Roberts found that on a plate originally containing 364 stars, no less than 130 had completely disappeared in 9¼ years!
It has been assumed by some writers on astronomy that the faint stars visible on photographs of the Pleiades are at practically the same distance from the earth as the brighter stars of the cluster, and that consequently there must be an enormous difference in actual size between the brighter and fainter stars. But there is really no warrant for any such assumption. Photographs of the vicinity show that the sky all round the Pleiades is equally rich in faint stars. It seems, therefore, more reasonable to suppose that most of the faint stars visible in the Pleiades are really far behind the cluster in space. For if all the faint stars visible on photographs belonged to the cluster, then if we imagine the cluster removed, a “hole” would be left in the sky, which is of course utterly improbable, and indeed absurd. An examination of the proper motions tends to confirm this view of the matter, and indicates that the Pleiades cluster is a comparatively small one and simply projected on a background of fainter stars.
It has long been suspected that the famous star 61 Cygni, which is a double star, forms a binary system – that is, that the two stars composing it revolve round their common centre of gravity and move together through space. But measures of parallax made by Herman S. Davis and Wilsing seem to show a difference of parallax between the two components of about 0·08 of a second of arc. This difference of parallax implies a distance of about 2¼ “light years” between the two stars, and “if this is correct, the stars are too remote to form a binary system. The proper motions of 5″·21 and 5″·15 seem to show that they are moving in nearly parallel directions; but are probably slowly separating.” Mr. Lewis, however, thinks that a physical connection probably exists.302
Dante speaks of the four bright stars of the Southern Cross as emblematical of the four cardinal virtues, Justice, Temperance, Fortitude, and Prudence; and he seems to refer to the stars Canopus, Achernar, and Foomalhaut under the symbols of Faith, Hope, and Charity. The so-called “False Cross” is said to be formed by the stars κ, δ, ε, and ι of the constellation Argo Navis. But it seems to me that a better (although larger) cross is formed by the stars α Centauri and α, β, and γ of Triangulum Australis.
Mr. Monck has pointed out that the names of the brightest stars seem to be arranged alphabetically in order of colour, beginning with red and ending with blue. Thus we have Aldebaran, Arcturus, Betelgeuse, Capella, Procyon, Regulus, Rigel, Sirius, Spica and Vega. But as the origin of these names is different, this must be merely a curious coincidence.303 And, to my eye at least, Betelgeuse is redder than Arcturus.
The poet Longfellow speaks of the —
“Stars, the thoughts of God in the heavens,”304
and Drayton says —
“The stars to me an everlasting book
In that eternal register, the sky.”305
Observing at a height of 12,540 feet on the Andes, the late Dr. Copeland saw Sirius with the naked eye less than 10 minutes before sunset.306 He also saw Jupiter 3m 47s before sunset; and the following bright stars – Canopus, 0m 52s before sunset; Rigel (β Orionis) 16m 32s after sunset; and Procyon 11m 28s after sunset. From a height of 12,050 feet at La Paz, Bolivia, he saw with the naked eye in February, 1883, ten stars in the Pleiades in full moonlight, and seventeen stars in the Hyades. He also saw σ Tauri double.307
Humboldt says, “In whatever point the vault of heaven has been pierced by powerful and far-penetrating telescopic instruments, stars or luminous nebulæ are everywhere discoverable, the former in some cases not exceeding the 20th or 24th degree of telescopic magnitude.”308 But this is a mistake. No star of even the 20th magnitude has ever been seen by any telescope. Even on the best photographic plates it is doubtful that any stars much below the 18th magnitude are visible. To show a star of the 20th magnitude – if such stars exist – would require a telescope of 144 inches or 12 feet in aperture. To show a star of the 24th magnitude – if such there be – an aperture of 33 feet would be necessary!309
It is a popular idea that stars may be seen in the daytime from the bottom of a deep pit or high chimney. But this has often been denied. Humboldt says, “While practically engaged in mining operations, I was in the habit, during many years, of passing a great portion of the day in mines where I could see the sky through deep shafts, yet I never was able to observe a star.”310
Stars may, however, be seen in the daytime with even small telescopes. It is said that a telescope of 1 inch aperture will show stars of the 2nd magnitude; 2 inches, stars of the 3rd magnitude; and 4 inches, stars of the 4th magnitude. But I cannot confirm this from personal observation. It may be so, but I have not tried the experiment.
Sir George Darwin says —
“Human life is too short to permit us to watch the leisurely procedure of cosmical evolution, but the celestial museum contains so many exhibits that it may become possible, by the aid of theory, to piece together, bit by bit, the processes through which stars pass in the course of their evolutions.”311
The so-called “telluric lines” seen in the solar spectrum, are due to water vapour in the earth’s atmosphere. As the light of the stars also passes through the atmosphere, it is evident that these lines should also be visible in the spectra of the stars. This is found to be the case by Prof. Campbell, Director of the Lick Observatory, who has observed all the principal bands in the spectrum of every star he has examined.312