“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 orOrionstars, (2) hydrogen or Sirian stars, (3) calcium or Procyon stars, (4) solar or Capellan stars.”
“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 orOrionstars, (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, andAstronomical 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 asto 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.”[296]
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.”[297]
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 increaseis 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.[298]
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.”[299]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 ofnebulæ 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. 3h47mto 5h4m, 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.[300]It will be obvious that, as there is a “vanishing point,” the motion in the line of sight must be one ofrecessionfrom the earth.
It has been found that on an average the parallax of a star is about one-seventh of its “proper motion.”[301]
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 velocityinthe 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 pageWhere every letter is a glittering sun.”
From an examination of the heat radiated bysome 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 ofunequal date. Much has long ceased to exist before the knowledge of its presence reaches us; much has been otherwise arranged.”[302]
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 thebrighter 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 ifallthe 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 areprobably 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, 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.[304]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,”[305]
and Drayton says—
“The stars to me an everlasting bookIn 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 3m47sbefore sunset; and the following bright stars—Canopus, 0m52sbefore sunset; Rigel (β Orionis) 16m32safter sunset; and Procyon 11m28safter 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.[308]
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.”[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 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.”[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 pass in the course of their evolutions.”[312]
“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.”[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 passes through the atmosphere, it is evident that these lines should also be visible in the spectraof 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.[313]
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.”
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.”[314]Both components are spectroscopic binaries, and the system is a most interesting one.
The well-known companion of Sirius becameinvisible 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 β Cephei has recently been discovered to be a spectroscopic binary with the wonderfully short period of 4h34m11s. 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.”[316]
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 elsewhere[317]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.”[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 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 themass 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). Thebright 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.”
The following notes are from Burnham’s recently publishedGeneral 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 starRigel (β 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).
γ 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 Comæ Berenices. This is a binary star of which the orbit plane passes nearly through the earth. The period is about 25½ years, and Burnham says the orbit “is as accurately known as that of any known binary.”
σ Coronæ Borealis. Burnham says that the orbits hitherto 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.” Theperiods computed range from 86·66 years (Doolittle) to 98·15 years (Powell). The present writer found a period of 87·84 years, which cannot be far from the truth. Burnham found 87·75 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 (280°·6: 40″·25: 1899·86). 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 ε Lyræ, 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 faintstars, supposed to have been seen by various observers, are merely “ghosts produced by reflection.”
The binary star ζ Boötis, which has long been suspected of small and irregular variation of light, showed remarkable spectral changes in the year 1905,[319]somewhat similar to those of anova, or temporary star. It is curious that such changes should occur in a star having an ordinary Sirian type of spectrum!
A curious quadruple system has been discovered by Mr. R. T. A. Innes in the southern hemisphere. The star κ Toucani is a binary star with components of magnitudes 5 and 7·7, and a period of revolution of perhaps about 1000 years. Within 6′ of this pair is another star (Lacaille 353), which is also a binary, with a period of perhaps 72 years. Both pairs have the same proper motion through space, and evidently form a vast quadruple system; for which Mr. Innes finds a possible period of 300,000 years.[320]
It is a curious fact that the performance of a really good refracting telescope actually exceeds what theory would indicate! at least so far as double stars are concerned. For example, the famous double-star observer Dawes found that the distance between the components of a doublestar which can just be divided, is found by dividing 4″·56 by the aperture of the object-glass in inches. Now theory gives 5″·52 divided by the aperture. “The actual telescope—if a really good one—thus exceeds its theoretical requirements. The difference between theory and practice in this case seems to be due to the fact that in the ‘spurious’ star disc shown by good telescopes, the illumination at the edges of the star disc is very feeble, so that its full size is not seen except in the case of a very bright star.”[321]
Variable Stars
Inthat interesting workA Cycle of Celestial Objects, Admiral Smyth says (p. 275), “Geminiano Montanari, as far back as 1670, was so struck with the celestial changes, that he projected a work to be intituled theInstabilities of the Firmament, hoping to show such alterations as would be sufficient to make even Aristotle—were he alive—reverse his opinion on the incorruptibility of the spangled sky: ‘There are now wanting in the heavens,’ said he, ‘two stars of the 2nd magnitude in the stem and yard of the ship Argo. I and others observed them in the year 1664, upon occasion of the comet that appeared in that year. When they first disappeared I know not; only I am sure that on April 10, 1668, there was not the least glimpse of them to be seen.’” Smyth adds, “Startling as this account is—and I am even disposed to question the fact—it must be recollected that Montanari was a man of integrity, and well versed in the theory and practice of astronomy;and his account of the wonder will be found—in good set Latin—in page 2202 of thePhilosophical Transactionsfor 1671.”
There must be, I think—as Smyth suggests—some mistake in Montanari’s observations, for it is quite certain that of the stars mentioned by Ptolemy (second centuryA.D.) there is no star of the 2nd magnitude now missing. It is true that Al-Sufi (tenth century) mentions a star of thethirdmagnitude mentioned by Ptolemy in the constellation of the Centaur (about 2° east of the star ε Centauri) which he could not find. But this has nothing to do with Montanari’s stars. Montanari’s words are very clear. He says, “Desunt in Cœlo duæ stellæSecundæ MagnitudinisinPuppi Navisejusve TranstrisBayero β et γ,propeCanem Majoris,à me et aliis, occasione præsertim CometæA. 1664observatæ et recognitæ. Earum Disparitionemcui Anno debeam, non novi;hoc indubium, quod à die10 April, 1668,nevestigium quidemillarum adesse amplius observe; cæteris circa eas etium quartæ et quintæ magnitudinis, immotis.” So the puzzle remains unsolved.
Sir William Herschel thought that “of all stars which are singly visible, about one in thirty are undergoing an observable change.”[322]Now taking the number of stars visible to the naked eye at 6000, this would give about 200 variable starsvisible at maximum to the unaided vision. But this estimate seems too high. Taking all the stars visible in the largest telescopes—possibly about 100 millions—the proportion of variable stars will probably be much smaller still.
The theory that the variation of light in the variable stars of the Algol type is due to a partial eclipse by a companion star (not necessarily a dark body) is now well established by the spectroscope, and is accepted by all astronomers. The late Miss Clarke has well said “to argue this point would beenforcer une porte ouverte.”
According to Dr. A. W. Roberts, the components of the following “Algol variables” “revolve in contact”: V Puppis, X Carinæ, β Lyræ, and υ Pegasi. Of those V Puppis and β Lyræ are known spectroscopic binaries. The others are beyond the reach of the spectroscope, owing to their faintness.
A very curious variable star of the Algol type is that known as R R Draconis. Its normal magnitude is 10, but at minimum it becomes invisible in a 7½-inch refracting telescope. The variation must, therefore, be over 3 magnitudes, that is, at minimum its light must be reduced to about one-sixteenth of its normal brightness. The period of variation from maximum to minimum is about 2·83 days. The variation of light near minimum is extraordinarily rapid, thelight decreasing by about 1 magnitude in half an hour.[323]
A very remarkable variable star has been recently discovered in the constellation Auriga. Prof. Hartwig found it of the 9th magnitude on March 6, 1908, the star “having increased four magnitudes in one day, whilst within eight days it was less than the 14th magnitude.”[324]In other words its light increased at least one-hundredfold in eight days!
The period of the well-known variable star β Lyræ seems to be slowly increasing. This Dr. Roberts (of South Africa) considers to be due to the component stars slowly receding from each other. He finds that “a very slight increase of one-thousandth part of the radius of the orbit would account for the augmentation in time, 30min a century.” According to the theory of stellar evolution the lengthening of the period of revolution of a binary star would be due to the “drag” caused by the tides formed by each component on the other.[325]
M. Sebastian Albrecht finds that in the short-period variable star known as T Vulpeculæ (and other variables of this class, such as Y Ophiuchi), there can be no eclipse to explain the variation of light (as in the case of Algol). The star is a spectroscopic binary, it is true, but the maximumof light coincides with the greatest velocity ofapproachin the line of sight, and the minimum with the greatest velocity ofrecession. Thus the light curve and the spectroscopic velocity curve are very similar in shape, but one is like the other turned upside down. “That is, the two curves have a very close correspondence in phase in addition to correspondence of shape and period.”[326]
The star now known as W Ursæ Majoris (the variability of which was discovered by Müller and Kempf in 1902), and which lies between the stars θ and υ of that constellation, has the marvellously short period of 4 hours (from maximum to maximum). Messrs. Jordan and Parkhurst (U.S.A.), find from photographic plates that the star varies from 7·24 to 8·17 magnitude.[327]The light at maximum is, therefore, more than double the light at minimum. A sun which loses more than half its light and recovers it again in the short period of 4 hours is certainly a curious and wonderful object.
In contrast with the above, the same astronomers have discovered a star in Perseus which seems to vary from about the 6th to the 7th magnitude in the very long period of 7½ years! It is now known as X Persei, and its position for 1900 is R.A. 3h49m8s, Dec. N. 30° 46′, or about one degree south-east of the star ζ Persei.It seems to be a variable of the Algol type, as the star remained constant in light at about the 6th magnitude from 1887 to 1891. It then began to fade, and on December 1, 1897, it was reduced to about the 7th magnitude.
On the night of August 20, 1886, Prof. Colbert, of Chicago, noticed that the star ζ Cassiopeiæ increased in brightness “by quite half a magnitude, and about half an hour afterwards began to return to its normal magnitude.”[328]This curious outburst of light in a star usually constant in brightness is (if true) a very unusual phenomenon. But a somewhat similar fluctuation of light is recorded by the famous German astronomer Heis. On September 26, 1850, he noted that the star “ζ Lyræ became, for a moment,very bright, and then again faint.” (The words in his original observing book are: “ζ Lyræ wurde einenMoment sehr hellund hierauf wieder dunkel.”) As Heis was a remarkably accurate observer of star brightness, the above remark deserves the highest confidence.[329]
The variable star known as the V Delphini was found to be invisible in the great 40-inch telescope of the Yerkes Observatory on July 20, 1900. Its magnitude was, therefore, below the 17th. At its maximum brightness it is about 7½, or easily visible in an ordinary opera-glass, so that itsrange of variation is nearly, or quite, ten magnitudes. That is, its light at maximum is about 10,000 times its light at minimum. That a sun should vary in light to this enormous extent is certainly a wonderful fact. A variable discovered by Ceraski (and numbered 7579 in Chandlers’ Catalogue) “had passed below the limit of the 40-inch in June, 1900, and was, therefore, not brighter than 17 mag.”[330]
The late Sir C. E. Peck and his assistant, Mr. Grover, made many valuable observations of variable stars at the Rousden Observatory during many years past. Among other interesting things noted, Peck sometimes saw faint stars in the field of view of his telescope which were at other times invisible for many months, and he suggested that these are faint variable stars with a range of brightness from the 13th to the 20th magnitude. He adds, “Here there is a practically unemployed field for the largest telescopes.” Considering the enormous number of faint stars visible on stellar photographs the number of undiscovered variable stars must be very large.
Admiral Smyth describes a small star near β Leonis, about 5′ distant, of about 8th magnitude, and dull red. In 1864 Mr. Knott measured a faint star close to Smyth’s position, but estimated it only 11·6 magnitude. The Admiral’s star would thereupon seem to be variable.[331]
The famous variable star η Argus, which Sir John Herschel, when at the Cape of Good Hope in 1838, saw involved in dense nebulosity, was in April, 1869, “seen on the bare sky,” with the great Melbourne telescope, “the nebula having disappeared for some distance round it.” Other changes were noticed in this remarkable nebula. The Melbourne observers saw “three times as many stars as were seen by Herschel.” But of course their telescope is much larger—48 inches aperture, compared with Herschel’s 20 inches.
Prof. E. C. Pickering thinks that the fluctuations of light of the well-known variable star R Coronæ (in the Northern Crown), “are unlike those of any known variable.” This very curious object—one of the most curious in the heavens—sometimes remains for many months almost constant in brightness (just visible to the naked eye), and then rapidly fades in light by several magnitudes! Thus its changes of light in April and May, 1905, were as follows:—
Thus between April 1 and May 1, its light was reduced by over 5 magnitudes. In other words, the light of the star on May 1 was reduced to less than one-hundredth of its light on April 1. If oursun were to behave in this way nearly all life would soon be destroyed on the face of the earth.
M. H. E. Lau finds that the short-period variable star δ Cephei varies slightly in colour as well as in light, and that the colour curve is parallel to the light curve. Near the minimum of light the colour is reddish yellow, almost as red as ζ Cephei; a day later it is pure yellow, and of about the same colour as the neighbouring ε Cephei.[332]But it would not be easy to fully establish such slight variations of tint.
A remarkably bright maximum of the famous variable Mira Ceti occurred in 1906. In December of that year it was fully 2nd magnitude. The present writer estimated it 1·8, or nearly equal to the brightest on record—1·7 observed by Sir William Herschel and Wargentin in the year 1779. From photographs of the spectrum taken by Mr. Slipher at the Lowell Observatory in 1907, he finds strong indications of the presence of the rather rare element vanadium in the star’s surroundings. Prof. Campbell finds with the Mills spectrograph attached to the great 36-inch telescope of the Lick Observatory that Mira is receding from the earth at the apparently constant velocity of about 38 miles a second.[333]This, of course, has nothing to do with the variation in the star’s light. Prof. Campbell failed to see any trace of the green lineof hydrogen in the star’s spectrum, while two other lines of the hydrogen series “glowed with singular intensity.”
Mr. Newall has found evidence of the element titanium in the spectrum of Betelgeuse (α Orionis); Mr. Goatcher and Mr. Lunt (of the Cape Observatory) find tin in Antares (and Scorpii). If the latter observation is confirmed it will be the first time this metal has been found in a star’s atmosphere.[334]
It is a curious fact that Al-Sufi (tenth century) does not mention the star ε Aquilæ, which lies closely north-west of ζ Aquilæ, as it is now quite conspicuous to the naked eye. It was suspected of variation by Sir William Herschel. It was first recorded by Tycho Brahé about 1590, and he called it 3rd magnitude. Bayer also rated it 3, and since his time it has been variously estimated from 3½ to 4. If it was anything like its present brightness (4·21 Harvard) in the tenth century it seems difficult to explain how it could have escaped Al-Sufi’s careful scrutiny of the heavens, unless it is variable. Its colour seems reddish to me.
Mr. W. T. Lynn has shown—and I think conclusively—that the so-called “new star” ofA.D.389 (which is said to have appeared near Altair in the Eagle) was really a comet.[335]
Near the place of Tycho Brahé’s great new starof 1572 (the “Pilgrim Star”), Hind and W. E. Plummer observed a small star (No. 129 of d’Arrest’s catalogue of the region) which seemed to show small fluctuations of light, which “scarcely include a whole magnitude.” This may possibly be identical with Tycho Brahé’s wonderful star, and should be watched by observers. The place of this small star is (for 1865) R.A. 0h17m18s, N.P.D. 26° 37′·1. The region was examined by Prof. Burnham in 1890 with the 36-inch telescope of the Lick Observatory. “None of the faint stars near the place presented any peculiarity worthy of remark, but three double stars were found.”[336]
With reference to the famous Nova (T) Coronæ—the “Blaze Star” of 1866—Prof. Barnard finds from careful comparisons with small stars in its vicinity that “the Nova is now essentially of the same brightness it was before the outburst of 1866 ... there seems to be no indication of motion in theNova.”
With reference to the cause of “temporary” stars, ornovæ, as they are now called by astronomers—the late Prof. H. C. Vogel said—