  Variable Stars In that interesting work A 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 the Instabilities 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; 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 century A.D.) there is no star of the 2nd magnitude now missing. It is true that Al-Sufi (tenth century) mentions a star of the third magnitude mentioned by Ptolemy in the constellation of the Centaur (about 2° east of the star e 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 Coelo duÆ stellÆ SecundÆ Magnitudinis in Puppi Navis ejusve Transtris Bayero et ?, prope Canem Majoris, À me et aliis, occasione prÆsertim CometÆ A. 1664 observatÆ et recognitÆ. Earum Disparitionem cui Anno debeam, non novi; hoc indubium, quod À die 10 April, 1668, ne vestigium quidem illarum 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 stars 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 be enforcer une porte ouverte.” According to Dr. A. W. Roberts, the components of the following “Algol variables” “revolve in contact”: V Puppis, X CarinÆ, LyrÆ, and ? Pegasi. Of those V Puppis and 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, the 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 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, 30m in 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 maximum 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 ? and ? 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. 3h 49m 8s, Dec. N. 30° 46', or about one degree south-east of the star ? Persei. On the night of August 20, 1886, Prof. Colbert, of Chicago, noticed that the star ? 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 “? LyrÆ became, for a moment, very bright, and then again faint.” (The words in his original observing book are: “? LyrÆ wurde einen Moment sehr hell und 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 its 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 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] 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 our M. H. E. Lau finds that the short-period variable star d 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 ? Cephei; a day later it is pure yellow, and of about the same colour as the neighbouring e 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 line Mr. Newall has found evidence of the element titanium in the spectrum of Betelgeuse (a 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 e AquilÆ, which lies closely north-west of ? 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” of A.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 star 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 the Nova.” With reference to the cause of “temporary” stars, or novÆ, as they are now called by astronomers—the late Prof. H. C. Vogel said— “A direct collision of two celestial bodies is not regarded by Huggins as an admissible explanation of the Nova; a partial collision has little probability, and the most that can be admitted is perhaps the mutual penetration and admixture of the outer It will be noticed that this hypothesis agrees with the fundamental assumption of the “Planetesimal Hypothesis” advocated by Professors Chamberlin and Moulton (see my Astronomical Essays, p. 324). The rush of a comparatively small body through a mass of gaseous matter seems also a very plausible hypothesis. This idea was originally advanced by Prof. Seeliger, and independently by Mr. Monck. With reference to the nebula which was observed round the great new star of 1901—Nova Persei—Prof. Lewis Bell supports the theory of Seeliger, which accounts for the apparent movements of the brightest portions of the nebula by The rapid diminution in the light of some of these “new stars” is very remarkable. Thus the new star which suddenly blazed out near the nucleus of the great nebula in Andromeda in August, 1885, faded down in 5 months from “the limit of visibility to the naked eye to that of a 26-inch telescope”! A large body could not cool in this way. Mr. Harold K. Palmer thinks that the “complete and astonishingly rapid changes of spectral type observed in the case of Nova Cygni and Nova AurigÆ, and likewise those observed in Nova NormÆ, Nova Sagittarii and Nova Persei, leave little doubt that the masses of these objects are small.”[339] No less than 3748 variable stars had been discovered up to May, 1907. Of these 2909 were The star 14. 1904 Cygni has a period of only 3 hours 14 minutes, which is the shortest period known for a variable star. A very interesting discovery has recently been made with reference to the star Herculis. It has been long suspected of variable light with a period of 35 or 40 days, or perhaps irregular. Frost and Adams now find it to be a spectroscopic binary, and further observations at Harvard Observatory show that it is a variable of the Algol (or perhaps LyrÆ) type. The Algol variation of light was suggested by MM. Baker and Schlesinger. The period seems to be about 2·05 days.[341] The northern of the two “pointers” in the Plough (so called because they nearly point to the Pole Star) is about the 2nd magnitude, as Al-Sufi rated it. It was thought to be variable in colour by Klein, Konkoly, and Weber; and M. Lau has recently found a period of 50 days with a maximum of “jaune rougeÂtre” on April 2, 1902. The famous variable star ? Argus did “not exceed the 8th magnitude” in February, 1907, according to Mr. Tebbutt.[342] This is the faintest ever recorded for this wonderful star. This “new star” of 1866—T CoronÆ, as it is now called—was, with the possible exception of Nova Persei (1901), the only example of a nova which was known to astronomers as a small star previous to the great outburst of light. It is the brightest of the novÆ still visible. It was the first of these interesting objects to be examined with the spectroscope. It was observed by Burnham in the years 1904-1906 with the great 40-inch telescope of the Yerkes Observatory (U.S.A.). He found its colour white, or only slightly tinged with yellow. In August and September, 1906, he estimated its magnitude at about 9·3, and “it would seem therefore that the Nova is now essentially of the same brightness it was before the outburst in 1866.” It shows no indication of motion. Burnham found no The following details of the great new star of 1572—the “Pilgrim Star” of Tycho BrahÉ—are given by Delambre.[344] In November, 1572, it was brighter than Sirius, Vega, and Jupiter, and almost equal to Venus at its brightest. During December it resembled Jupiter in brightness. In January, 1573, it was fainter and only a little brighter than stars of the 1st magnitude. In February and March it was equal to 1st magnitude stars, and in April and May was reduced to the 2nd magnitude. In June and July it was 3rd magnitude; in September of the 4th, and at the end of 1573 it was reduced to the 5th magnitude. In February, 1574, it was 6th magnitude, and in March of the same year it became invisible to the naked eye. From this account it will be seen that the decrease in light of this curious object was much slower than that of Nova Persei (1901) (“the new star of the new century”). This would suggest that it was a much larger body. There were also changes in its colour. When it was of the brightness of Venus or Jupiter it shone with a white light. It then became golden, and afterwards reddish like Mars, Aldebaran, or Temporary stars were called by the ancient Chinese “Ke-sing,” or guest stars.[345] A temporary star recorded by Ma-tuan-lin (Chinese Annals) in February, 1578, is described as “a star as large as the sun.” But its position is not given.[346] About the middle of September, 1878, Mr. Greely, of Boston (U.S.A.), reported to Mr. E. F. Sawyer (the eminent observer of variable stars) that, about the middle of August of that year, he had seen the famous variable star Mira Ceti of about the 2nd magnitude, although the star did not attain its usual maximum until early in October, 1878. Mr. Greely stated that several nights after he first saw Mira it had faded to the 4th or 5th magnitude. If there was no mistake in this observation (and Sawyer could find none) it was quite an unique phenomenon, as nothing of the sort has been observed before or since in the history of this famous star. It looks as if Mr. Greely had observed a new or “temporary” star near the In the so-called Cepheid and Geminid variables of short period, the principal characteristics of the light variation are as follows:— “1. The light varies without pause. “2. The amount of their light variation is usually about 1 magnitude. “3. Their periods are short—a few days only. “4. They are of a spectral type approximately solar; no Orion, Sirian or Arcturian stars having been found among them. “5. They seem to be found in greater numbers in certain parts of the sky, notably in the Milky Way, but exhibit no tendency to form clusters. “6. All those stars whose radial velocities have been studied have been found to be binaries whose period of orbital revolution coincides with that of their light change. “7. The orbits, so far as determined, are all small, a sin i being 2,000,000 kilometres or less. “8. Their maximum light synchronizes with their maximum velocity of approach, and minimum light with maximum velocity of recession. “9. No case has been found in which the spectrum of more than one component has been bright enough to be recorded in the spectrograms.”[347] It is very difficult to find an hypothesis which will explain satisfactorily all these characteristics, and attempts in this direction have not proved very successful. Mr. J. C. Duncan suggests the On March 30, 1612, Scheiner saw a star near Jupiter. It was at first equal in brightness to Jupiter’s satellites. It gradually faded, and on April 8 of the same year it was only seen with much difficulty in a very clear sky. “After that date it was never seen again, although carefully looked for under favourable conditions.” An attempted identification of Scheiner’s star was made in recent years by Winnecke. He found that its position, as indicated by Scheiner, agrees with that of the Bonn Durchmusterung star 15°, 2083 (8½ magnitude). This star is not a known variable. Winnecke watched it for 17 years, but found no variation of light. From Scheiner’s recorded observations his star seems to have reached the 6th magnitude, which is considerably brighter than the Durchmusterung star watched by Winnecke.[348] With reference to the colours of the stars, the supposed change of colour in Sirius from red to white is well known, and will be considered in the chapter on the Constellations. The bright star Arcturus has also been suspected of variation in colour. About the middle of the nineteenth century Dr. Julius Schmidt, of Athens, the well-known observer of variable stars, thought it one of the reddest stars in the sky, especially in the year Mr. Ennis speaks of Capella as “blue” (classing it with Rigel), and comparing its colour with that of Vega![350] But the present writer has never seen it of this colour. To his eye it seems yellowish or orange. It was called red by Ptolemy, El Fergani, and Riccioli; but Al-Sufi says nothing about its colour. Of UrsÆ Minoris, Heis, the eminent German astronomer said, “I have had frequent opportunities of convincing myself that the colour of this star is not always equally red; at times it is more or less yellow, at others most decidedly red.”[351] Among double stars there are many cases in which variation of colour has been suspected. In some of these the difference in the recorded colour may possibly be due to “colour blindness” in some of the observers; but in others there seems to be good evidence in favour of a change. The following may be mentioned:— ? Trianguli. Magnitudes 5½ and 7. Secchi estimated them as white or yellow and blue; but Webb called them yellow and green (1862). ? Leonis, 2 and 3½. Sir William Herschel noted them white and reddish white; but Webb, light orange and greenish yellow. 12 Canum Venaticorum, 2½ and 6½. White and red, Sir William Herschel; but Sir John Herschel says in 1830, “With all attention I could perceive no contrast of colours in the two stars.” Struve found them both white in 1830, thus agreeing with Sir John Herschel. Sestini saw them yellow and blue in 1844; Smyth, in 1855, pale reddish white and lilac; Dembowski, in 1856, white and pale olive blue; and Webb, in 1862, flushed white and pale lilac. On October 13, 1907, Nova Persei, the great new star of 1901, was estimated to be only 11·44 magnitude, or about 11½. When at its brightest this famous star was about zero magnitude; so that it has in about 6 years faded about 11½ magnitudes in brightness; in other words, it has been reduced to 1/40000 of its greatest brilliancy! |
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