Astronomical Curiosities: Facts and Fallacies

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 bookIn 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

The largest “proper motion” now known is that of a star of the 8½ magnitude in the southern hemisphere, known as Cordoba Zone V. No. 243. Its proper motion is 8·07 seconds of arc per annum, thus exceeding that of the famous “runaway star,” 1830 Groombridge, which has a proper motion of 7·05 seconds per annum. This greater motion is, however, only apparent. Measures of parallax show that the southern “runaway” is much nearer to us than its northern rival, its parallax being 0″·32, while that of Groombridge 1830 is only 0″·14. With these data the actual velocity across the line of sight can be easily computed. That of the southern star comes out 80 miles a second, while that of Groombridge 1830 is 148 miles a second. The actual velocity of Arcturus is probably still greater.

The poet Barton has well said —

“The stars! the stars! go forth at night,Lift up thine eyes on high,And view the countless orbs of light,Which gem the midnight sky.Go forth in silence and alone,This glorious sight to scan,And bid the humbled spirit ownThe littleness of man.”

CHAPTER XV

Double and Binary Stars

Prof. R. G. Aitken, the eminent American observer of double stars, finds that of all the stars down to the 9th magnitude – about the faintest visible in a powerful binocular field-glass – 1 in 18, or 1 in 20, on the average, are double, with the component stars less than 5 seconds of arc apart. This proportion of double stars is not, however, the same for all parts of the sky; while in some regions double stars are very scarce, in other places the proportion rises to 1 in 8.

For the well-known binary star Castor (α Geminorum), several orbits have been computed with periods ranging from 232 years (Mädler) to 1001 years (Doberck). But Burnham finds that “the orbit is absolutely indeterminate at this time, and likely to remain so for another century or longer.”313 Both components are spectroscopic binaries, and the system is a most interesting one.

The well-known companion of Sirius became invisible in all telescopes in the year 1890, owing to its near approach to its brilliant primary. It remained invisible until August 20, 1896, when it was again seen by Dr. See at the Lowell Observatory.314 Since then its distance has been increasing, and it has been regularly measured. The maximum distance will be attained about the year 1922.

The star β Cephei has recently been discovered to be a spectroscopic binary with the wonderfully short period of 4h 34m 11s. The orbital velocity is about 10½ miles a second, and as this velocity is not very great, the distance between the components must be very small, and possibly the two component bodies are revolving in actual contact. The spectrum is of the “Orion type.”315

According to Slipher the spectroscopic binary γ Geminorum has the comparatively long period (for a spectroscopic binary) of about 3½ years. This period is comparable with that of the telescopic binary system, δ Equulei (period about 5·7 years). The orbit is quite eccentric. I have shown elsewhere316 that γ Geminorum has probably increased in brightness since the time of Al-Sufi (tenth century). Possibly its spectroscopic duplicity may have something to do with the variation in its light.

With reference to the spectra of double stars, Mr. Maunder suggests that the fact of the companion of a binary star showing a Sirian spectrum while the brighter star has a solar spectrum may be explained by supposing that, on the theory of fission, “the smaller body when thrown off consisted of the lighter elements, the heavier remaining in the principal star. In other words, in these cases spectral type depends upon original chemical constitution, and not upon the stage of stellar development attained.”317

A curious paradox with reference to binary stars has recently come to light. For many years it was almost taken for granted that the brighter star of a pair had a larger mass than the fainter component. This was a natural conclusion, as both stars are practically at the same distance from the earth. But it has been recently found that in some binary stars the fainter component has actually the larger mass! Thus, in the binary star ε Hydræ, the “magnitude” of the component stars are 3 and 6, indicating that the brighter star is about 16 times brighter than the fainter component. Yet calculations by Lewis show that the fainter star has 6 times the mass of the brighter, that is, contains 6 times the quantity of matter! In the well-known binary 70 Ophiuchi, Prey finds that the fainter star has about 4 times the mass of the brighter! In 85 Pegasi, the brighter star is about 40 times brighter than its companion, while Furner finds that the mass of the fainter star is about 4 times that of the brighter! And there are other similar cases. In fact, in these remarkable combinations of suns the fainter star is really the “primary,” and is, so far as mass is concerned, “the predominant partner.” This is a curious anomaly, and cannot be well explained in the present state of our knowledge of stellar systems. In the case of α Centauri the masses of the components are about equal, while the primary star is about 3 times brighter than the other. But here the discrepancy is satisfactorily explained by the difference in character of the spectra, the brighter component having a spectrum of the solar type, while the fainter seems further advanced on the downward road of evolution, that is, more consolidated and having, perhaps, less intrinsic brightness of surface.

In the case of Sirius and its faint attendant, the mass of the bright star is about twice the mass of the satellite, while its light is about 40,000 times greater! Here the satellite is either a cooled-down sun or perhaps a gaseous nebula. There seems to be no other explanation of this curious paradox. The same remark applies to Procyon, where the bright star is about 100,000 times brighter than its faint companion, although its mass is only 5 times greater.

The bright star Capella forms a curious anomaly or paradox. Spectroscopic observations show that it is a very close binary pair. It has been seen “elongated” at the Greenwich Observatory with the great 28-inch refractor – the work of Sir Howard Grubb – and the spectroscopic and visual measurements agree in indicating that its mass is about 18 times the mass of the sun. But its parallax (about 0″·08) shows that it is about 128 times brighter than the sun! This great brilliancy is inconsistent with the star’s computed mass, which would indicate a much smaller brightness. The sun placed at the distance of Capella would, I find, shine as a star of about 5½ magnitude, while Capella is one of the brightest stars in the sky. As the spectrum of Capella’s light closely resembles the solar spectrum, we seem justified in assuming that the two bodies have pretty much the same physical composition. The discrepancy between the computed and actual brightness of the star cannot be explained satisfactorily, and the star remains an astronomical enigma.

Three remarkable double-star systems have been discovered by Dr. See in the southern hemisphere. The first of these is the bright star α Phœnicis, of which the magnitude is 2·4, or only very slightly fainter than the Pole Star. It is attended by a faint star of the 13th magnitude at a distance of less than 10 seconds (1897). The bright star is of a deep orange or reddish colour, and the great difference in brightness between the component stars “renders the system both striking and difficult.” The second is μ Velorum, a star of the 3rd magnitude, which has a companion of the 11th magnitude, and only 2½″ from its bright primary (1897). Dr. See describes this pair as “one of the most extraordinary in the heavens.” The third is η Centauri, of 2½ magnitude, with a companion of 13½ magnitude at a distance of 5″·65 (1897); colours yellow and purple. This pair is “extremely difficult, requiring a powerful telescope to see it.” Dr. See thinks that these three objects “may be regarded as amongst the most splendid in the heavens.”