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 ([Greek: a] 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 became invisible in all telescopes in
the year 1890, owing to its near approach to its brilliant primary. It
remained invisible until August 20, 1896, when it was again seen by Dr.
See at the Lowell Observatory.[315] Since then its distance has been
increasing, and it has been regularly measured. The maximum distance will
be attained about the year 1922.
The star [Greek: b] Cephei has recently been discovered to be a
spectroscopic binary with the wonderfully short period of 4{h} 34{m}
11{s}. The orbital velocity is about 10-1/2 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 [Greek: g] Geminorum has the
comparatively long period (for a spectroscopic binary) of about 3-1/2
years. This period is comparable with that of the telescopic binary
system, [Greek: d] Equulei (period about 5·7 years). The orbit is quite
eccentric. I have shown elsewhere[317] that [Greek: g] 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 [Greek: e] 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 the mass 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 [Greek: a] 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 5-1/2 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 [Greek: a]
Phoenicis, 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). The bright star is of a
deep orange or reddish colour, and the great difference in brightness
between the component stars "renders the system both striking and
difficult." The second is [Greek: m] Velorum, a star of the 3rd magnitude,
which has a companion of the 11th magnitude, and only 2-1/2" from its
bright primary (1897). Dr. See describes this pair as "one of the most
extraordinary in the heavens." The third is [Greek: ê] Centauri, of 2-1/2
magnitude, with a companion of 13-1/2 magnitude at a distance of 5"·65
(1897); colours yellow and purple. This pair is "extremely difficult,
requiring a powerful telescope to see it." Dr. See thinks that these three
objects "may be regarded as amongst the most splendid in the heavens."
The following notes are from Burnham's recently published _General
Catalogue of Double Stars_.
The Pole Star has a well-known companion of about the 9th magnitude, which
is a favourite object for small telescopes. Burnham finds that the bright
star and its faint companion are "relatively fixed," and are probably only
an "optical pair." Some other companions have been suspected by amateur
observers, but Burnham finds that "there is nothing nearer" than the known
companion within the reach of the great 36-inch telescope of the Lick
Observatory (_Cat._, p. 299).
The well-known companion to the bright star Rigel ([Greek: b] 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).
[Greek: g] 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 25-1/2 years, and Burnham
says the orbit "is as accurately known as that of any known binary."
[Greek: s] 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." The periods
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 [Greek:
e] 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
faint stars, supposed to have been seen by various observers, are merely
"ghosts produced by reflection."
The binary star [Greek: z] 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 a _nova_, 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 [Greek: k] 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 double star
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]