Astronomical Curiosities Part 11

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 9 years!

It has been a.s.sumed 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 cl.u.s.ter, 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 a.s.sumption. 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 cl.u.s.ter in s.p.a.ce. For if _all_ the faint stars visible on photographs belonged to the cl.u.s.ter, then if we imagine the cl.u.s.ter 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 cl.u.s.ter 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 s.p.a.ce. But measures of parallax made by Herman S. Davis and Wilsing seem to show a difference of parallax between the two components of about 008 of a second of arc. This difference of parallax implies a distance of about 2 "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.[303]

Dante speaks of the four bright stars of the Southern Cross as emblematical of the four cardinal virtues, Justice, Temperance, Fort.i.tude, 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 ?, d, e, and ? of the constellation Argo Navis. But it seems to me that a better (although larger) cross is formed by the stars a Centauri and a, , and ? 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.[304] And, to my eye at least, Betelgeuse is redder than Arcturus.

The poet Longfellow speaks of the--

"Stars, the thoughts of G.o.d in the heavens,"[305]

and Drayton says--

"The stars to me an everlasting book In that eternal register, the sky."[306]

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.[307] He also saw Jupiter 3{m} 47{s} before sunset; and the following bright stars--Canopus, 0{m} 52{s} before sunset; Rigel ( Orionis) 16{m} 32{s} after sunset; and Procyon 11{m} 28{s} 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 s Tauri double.[308]

Humboldt says, "In whatever point the vault of heaven has been pierced by powerful and far-penetrating telescopic instruments, stars or luminous nebulae are everywhere discoverable, the former in some cases not exceeding the 20th or 24th degree of telescopic magnitude."[309] 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![310]

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 pa.s.sing 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."[311]

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 pa.s.s in the course of their evolutions."[312]

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 pa.s.ses 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 princ.i.p.al bands in the spectrum of every star he has examined.[313]

The largest "proper motion" now known is that of a star of the 8 magnitude in the southern hemisphere, known as Cordoba Zone V. No. 243.

Its proper motion is 807 seconds of arc per annum, thus exceeding that of the famous "runaway star," 1830 Groombridge, which has a proper motion of 705 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...o...b.. of light, Which gem the midnight sky.

Go forth in silence and alone, This glorious sight to scan, And bid the humbled spirit own The 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-gla.s.s--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 (a Geminorum), several orbits have been computed with periods ranging from 232 years (Madler) 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."[314]

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.[315] 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 Cephei has recently been discovered to be a spectroscopic binary with the wonderfully short period of 4{h} 34{m} 11{s}. The orbital velocity is about 10 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."[316]

According to Slipher the spectroscopic binary ? Geminorum has the comparatively long period (for a spectroscopic binary) of about 3 years. This period is comparable with that of the telescopic binary system, d Equulei (period about 57 years). The orbit is quite eccentric.

I have shown elsewhere[317] that ? 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 princ.i.p.al star. In other words, in these cases spectral type depends upon original chemical const.i.tution, and not upon the stage of stellar development attained."[318]

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 ma.s.s 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 ma.s.s! Thus, in the binary star e Hydrae, 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 ma.s.s of the brighter, that is, contains 6 times the quant.i.ty of matter! In the well-known binary 70 Ophiuchi, Prey finds that the fainter star has about 4 times the ma.s.s of the brighter! In 85 Pegasi, the brighter star is about 40 times brighter than its companion, while Furner finds that the ma.s.s 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 ma.s.s 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 a Centauri the ma.s.ses 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 ma.s.s of the bright star is about twice the ma.s.s 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 ma.s.s 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 ma.s.s is about 18 times the ma.s.s 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 ma.s.s, which would indicate a much smaller brightness.

The sun placed at the distance of Capella would, I find, s.h.i.+ne as a star of about 5 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 a.s.suming 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 a Phnicis, of which the magnitude is 24, 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 Velorum, a star of the 3rd magnitude, which has a companion of the 11th magnitude, and only 2" from its bright primary (1897). Dr.

See describes this pair as "one of the most extraordinary in the heavens."

The third is ? Centauri, of 2 magnitude, with a companion of 13 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."

The following notes are from Burnham's recently published _General Catalogue of Double Stars_.

The Pole Star has a well-known companion of about the 9th magnitude, which is a favourite object for small telescopes. Burnham finds that the bright star and its faint companion are "relatively fixed," and are probably only an "optical pair." Some other companions have been suspected by amateur observers, but Burnham finds that "there is nothing nearer" than the known companion within the reach of the great 36-inch telescope of the Lick Observatory (_Cat._, p. 299).

The well-known companion to the bright star Rigel ( Orionis) has been suspected for many years to be a close double star. Burnham concludes that it is really a binary star, and its "period may be shorter than that of any known pair" (_Cat._, p. 411).

Burnham finds that the four brighter stars in the trapezium in the great Orion nebula (in the "sword") are relatively fixed (_Cat._, p. 426).

? Leonis. This double star was for many years considered to be a binary, but Burnham has shown that all the measures may be satisfactorily represented by a straight line, and that consequently the pair merely forms an "optical double."

42 Comae Berenices. This is a binary star of which the orbit plane pa.s.ses nearly through the earth. The period is about 25 years, and Burnham says the orbit "is as accurately known as that of any known binary."

s Coronae Borealis. Burnham says that the orbits. .h.i.therto computed--with periods ranging from 195 years (Jacob) to 846 years (Doberck) are "mere guess work," and it will require the measures of at least another century, and perhaps a much longer time, to give an approximate period (_Cat._, p.

209). So here is some work left for posterity to do in this field.

70 Ophiuchi. With reference to this well-known binary star, Burnham says, "the elements of the orbit are very accurately known." The periods computed range from 8666 years (Doolittle) to 9815 years (Powell). The present writer found a period of 8784 years, which cannot be far from the truth. Burnham found 8775 years (_Cat._, p. 774). In this case there is not much left for posterity to accomplish.

61 Cygni. With reference to this famous star Burnham says, "So far the relative motion is practically rectilinear. If the companion is moving in a curved path, it will require the measures of at least another half-century to make this certain. The deviation of the measured positions during the last 70 years from a right line are less than the average errors of the observations."

Burnham once saw a faint companion to Sirius of the 16th magnitude, and measured its position with reference to the bright star (2806: 40"25: 189986). But he afterwards found that it was "not a real object but a reflection from Sirius" (in the eye-piece). Such false images are called "ghosts."

With reference to the well-known double (or rather quadruple) star e Lyrae, near Vega, and supposed faint stars near it, Burnham says, "From time to time various small stars in the vicinity have been mapped, and much time wasted in looking for and speculating about objects which only exist in the imagination of the observer." He believes that many of these faint stars, supposed to have been seen by various observers, are merely "ghosts produced by reflection."