Astronomical Curiosities: Facts and Fallacies

CHAPTER XVI

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; and his account of the wonder will be found – in good set Latin – in page 2202 of the Philosophical Transactions for 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 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 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 stellæ Secundæ Magnitudinis in Puppi Navis ejusve Transtris Bayero β et γ, 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.”321 Now taking the number of stars visible to the naked eye at 6000, this would give about 200 variable stars visible 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 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, 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, the light decreasing by about 1 magnitude in half an hour.322

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.”323 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, 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.324

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 of light coincides with the greatest velocity of approach in the line of sight, and the minimum with the greatest velocity of recession. 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.”325

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.326 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 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.”327 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 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.328

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 range 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.”329

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.330

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 our sun 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.331 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.332 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 of 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.333

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” of A.D. 389 (which is said to have appeared near Altair in the Eagle) was really a comet.334

Near the place of Tycho Brahé’s great new star of 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. 0h 17m 18s, 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.”335

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 gaseous envelopes of the two bodies at the time of their closest approach. A more probable explanation is given by an hypothesis which we owe to Klinkerfues, and which has more recently been further developed by Wilsing, viz. that by the very close passage of two celestial bodies enormous tidal disturbances are produced and thereby changes in the brightness of the bodies. In the case of the two bodies which form the Nova, it must be assumed that these phenomena are displayed in the highest degree of development, and that changes of pressure have been produced which have caused enormous eruptions from the heated interior of the bodies; the eruptions are perhaps accompanied by electrical actions, and are comparable with the outbursts in our own sun, although they are on a much larger scale.”336

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 supposing that the various parts of the highly tenuous matter were successively lighted up by the effects of a travelling electro-magnetic wavefront, and he shows that this theory agrees well with the observed phenomenon.337 The “collision theory” which explained the sudden outburst of light by the meeting of two dark bodies in space, seems to be now abandoned by the best astronomers. The rapid cooling down of the supposed bodies indicated by the rapid decrease of light is quite inconsistent with this hypothesis.

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.”338

No less than 3748 variable stars had been discovered up to May, 1907. Of these 2909 were found at Harvard Observatory (U.S.A.) chiefly by means of photography.339

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 μ 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 β Lyræ) type. The Algol variation of light was suggested by MM. Baker and Schlesinger. The period seems to be about 2·05 days.340

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 η Argus did “not exceed the 8th magnitude” in February, 1907, according to Mr. Tebbutt.341 This is the faintest ever recorded for this wonderful star.

It is stated in Knowledge (vol. 5, p. 3, January 4, 1884) that the temporary star of 1876 (in the constellation of Cygnus) “had long been known and catalogued as a telescopic star of the 9th magnitude with nothing to distinguish it from the common herd.” But this is quite erroneous. The star was quite unknown before it was discovered by Schmidt at Athens on November 24 of that year. The remark apparently refers to the “Blaze Star” of 1866 in Corona Borealis, which was known previously as a star of about the 9th magnitude before its sudden outburst on May 12 of that year.

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 peculiarity about its telescopic image. A small and very faint nebula was found by Burnham a little following (that is east of) the nova.342