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 [Greek: 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 [Greek: b] et [Greek: g],
_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 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, [Greek: b] Lyræ, and
[Greek: u] Pegasi. Of those V Puppis and [Greek: b] 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 7-1/2-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.[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 [Greek: b] 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, 30{m} 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 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."[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
[Greek: th] and [Greek: u] 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 7-1/2 years! It is now known as X Persei, and its
position for 1900 is R.A. 3{h} 49{m} 8{s}, Dec. N. 30° 46', or about one
degree south-east of the star [Greek: z] 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 [Greek: z] 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 "[Greek: z] Lyræ
became, for a moment, _very bright_, and then again faint." (The words in
his original observing book are: "[Greek: z] 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 7-1/2, 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."[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 [Greek: b] 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 [Greek: ê] 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:--
1905, April 1 6·0 magnitude
" 11 7·3 "
" 12 8·4 "
May 1 11·4 "
" 7 12·5 "
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 [Greek: 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 [Greek: z] Cephei; a day later it is pure
yellow, and of about the same colour as the neighbouring [Greek: 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 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 ([Greek: 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 [Greek: e] Aquilæ, which lies closely north-west of [Greek: z]
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 3-1/2 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 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. 0{h}
17{m} 18{s}, 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 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."[337]
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.[338] 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."[339]
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.[340]
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 [Greek: m] 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 [Greek: b]
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 [Greek: ê] 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.
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_.[343]
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 Betelgeuse. It afterwards
became of a livid white colour like Saturn, and this it retained as long
as it was visible. Tycho Brahé thought that its apparent diameter might
have been about 3-1/2 minutes of arc, and that it was possibly 361 times
smaller than the earth(!) But we now know that these estimates were
probably quite erroneous.
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
place of Mira Ceti; but as the spot is far from the Milky Way, which is
the usual seat of such phenomena, this hypothesis seems improbable.
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 action of
an absorbing atmosphere surrounding the component stars.
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 (8-1/2 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 1841, when he found its colour comparable with that of the planet
Mars.[349] In 1852, however, he was surprised to find it yellow and devoid
of any reddish tinge; in colour it was lighter than that of Capella. In
1863, Mr. Jacob Ennis found it "decidedly orange." Ptolemy and Al-Sufi
called it red.
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 [Greek: b] 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:--
[Greek: ê] Cassiopeiæ. Magnitudes of the components about 4 and 7-1/2.
Recorded as red and green by Sir John Herschel and South; but yellow and
orange by Sestini.
[Greek: i] Trianguli. Magnitudes 5-1/2 and 7. Secchi estimated them as
white or yellow and blue; but Webb called them yellow and green (1862).
[Greek: g] Leonis, 2 and 3-1/2. Sir William Herschel noted them white and
reddish white; but Webb, light orange and greenish yellow.
12 Canum Venaticorum, 2-1/2 and 6-1/2. 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 11-1/2. When at its
brightest this famous star was about zero magnitude; so that it has in
about 6 years faded about 11-1/2 magnitudes in brightness; in other words,
it has been reduced to 1/40000 of its greatest brilliancy!