DOUBLE STARS.
Interesting Stellar Objects--Stars Optically Double--The Great
Discovery of the Binary Stars made by Herschel--The Binary Stars
describe Elliptic Paths--Why is this so important?--The Law of
Gravitation--Special Double Stars--Castor--Mizar--The Coloured
Double Stars--b Cygni.
The sidereal heavens contain few more interesting objects for the
telescope than can be found in the numerous class of double stars. They
are to be counted in thousands; indeed, _many_ thousands can be found in
the catalogues devoted to this special branch of astronomy. Many of
these objects are, no doubt, small and comparatively uninteresting, but
some of them are among the most conspicuous stars in the heavens, such
as Sirius, whose system we have already described. We shall in this
brief account select for special discussion and illustration a few of
the more remarkable double stars. We shall particularly notice some of
those that can be readily observed with a small telescope, and we have
indicated on the sketches of the constellations in a previous chapter
how the positions of these objects in the heavens can be ascertained.
It had been shown by Cassini in 1678 that certain stars, which appeared
to the unaided eye as single points of light, really consisted of two or
more stars, so close together that the telescope was required for their
separation.[36] The number of these objects was gradually increased by
fresh discoveries, until in 1781 (the same year in which Herschel
discovered Uranus) a list containing eighty double stars was published
by the astronomer Bode. These interesting objects claimed the attention
of Herschel during his memorable researches. The list of known doubles
rapidly swelled. Herschel's discoveries are to be enumerated by
hundreds, while he also commenced systematic measurements of the
distance by which the stars were separated, and the direction in which
the line joining them pointed. It was these measurements which
ultimately led to one of the most important and instructive of all
Herschel's discoveries. When, in the course of years, his observations
were repeated, Herschel found that in some cases the relative position
of the stars had changed. He was thus led to the discovery that in many
of the double stars the components are so related that they revolve
around each other. Mark the importance of this result. We must remember
that the stars are suns, comparable, it may be, with our sun in
magnitude; so that here we have the astonishing spectacle of pairs of
suns in mutual revolution. There is nothing very surprising in the fact
that movements should be observed, for in all probability every body in
the universe is in motion. It is the particular character of the
movement which is specially interesting and instructive.
It had been imagined that the proximity of the two stars forming a
double must be only accidental. It was thought that amid the vast host
of stars in the heavens it not unfrequently happened that one star was
so nearly behind another (as seen from the earth) that when the two were
viewed in the telescope they produced the effect of a double star. No
doubt many of the so-called double stars are produced in this way.
Herschel's discovery shows that this explanation will not always answer,
but that in many cases we really have two stars close together, and in
motion round their common centre of gravity.
When the measurements of the distances and the positions of double stars
had been accumulated during many years, they were taken over by the
mathematicians to be treated by their methods. There is one peculiarity
about double star observations: they have not--they cannot have--the
accuracy which the computer of an orbit demands. If the distance between
the pair of stars forming a binary be four seconds, the orbit we have to
scrutinise is only as large as the apparent size of a penny-piece at the
distance of one mile. It would require very careful measurement to make
out the form of a penny a mile off, even with good telescopes. If the
penny were tilted a little, it would appear, not circular, but oval; and
it would be possible, by measuring this oval, to determine how much the
penny was tilted. All this requires skilful work: the errors, viewed
intrinsically, may not be great, but viewed with reference to the whole
size of the quantities under consideration, they are very appreciable.
We therefore find the errors of observation far more prominent in
observations of this class than is generally the case when the
mathematician assumes the task of discussing the labours of the
observer.
The interpretation of Herschel's discovery was not accomplished by
himself; the light of mathematics was turned on his observations of the
binary stars by Savary, and afterwards by other mathematicians. Under
their searching enquiries the errors of the measurements were disclosed,
and the observations were purified from the grosser part of their
inaccuracy. Mathematicians could then apply to their corrected materials
the methods of enquiry with which they were familiar; they could deduce
with fair precision the actual shape of the orbit of the binary stars,
and the position of the plane in which that orbit is contained. The
result is not a little remarkable. It has been proved that the motion of
each of the stars is performed in an ellipse which contains the centre
of gravity of the two stars in its focus. This has been actually shown
to be true in many binary stars; it is believed to be true in all. But
why is this so important? Is not motion in an ellipse common enough?
Does not the earth revolve in an ellipse round the sun? And do not the
planets also revolve in ellipses?
It is this very fact that elliptic motion is so common in the planets of
the solar system which renders its discovery in binary stars of such
importance. From what does the elliptic motion in the solar system
arise? Is it not due to the law of attraction, discovered by Newton,
which states that every mass attracts every other mass with a force
which varies inversely as the square of the distance? That law of
attraction had been found to pervade the whole solar system, and it
explained the movements of the bodies of our system with marvellous
fidelity. But the solar system, consisting of the sun, and the planets,
with their satellites, the comets, and a host of smaller bodies, formed
merely a little island group in the universe. In the economy of this
tiny cosmical island the law of gravitation reigns supreme; before
Herschel's discovery we never could have known whether that law was not
merely a piece of local legislation, specially contrived for the
exigencies of our particular system. This discovery gave us the
knowledge which we could have gained from no other source. From the
binary stars came a whisper across the vast abyss of space. That whisper
told us that the law of gravitation was not peculiar to the solar
system. It told us the law extended to the distant shores of the abyss
in which our island is situated. It gives us grounds for believing that
the law of gravitation is obeyed throughout the length, breadth, and
depth of the entire visible universe.
One of the finest binary stars is that known as Castor, the brighter of
the two principal stars in the constellation of Gemini. The position of
Castor on the heavens is indicated in Fig. 86, page 418. Viewed by the
unaided eye, Castor resembles a single star; but with a moderately good
telescope it is found that what seems to be one star is really two
separate stars, one of which is of the third magnitude, while the other
is somewhat less. The angular distance of these two stars in the heavens
is not so great as the angle subtended by a line an inch long viewed at
a distance of half a mile. Castor is one of the double stars in which
the components have been observed to possess a motion of revolution. The
movement is, however, extremely slow, and the lapse of centuries will be
required before a revolution is completely effected.
A beautiful double star can be readily identified in the constellation
of Ursa Major (_see_ Fig. 80, page 410). It is known as Mizar, and is
the middle star (z) of the three which form the tail. In the close
neighbourhood of Mizar is the small star Alcor, which can be readily
seen with the unaided eye; but when we speak of Mizar as a double star,
it is not to be understood that Alcor is one of the components of the
double. Under the magnifying power of the telescope Alcor is seen to be
transferred a long way from Mizar, while Mizar itself is split up into
two suns close together. These components are of the second and the
fourth magnitudes respectively, and as the apparent distance is nearly
three times as great as in Castor, they are observed with facility even
in a small telescope. This is, indeed, the best double star in the
heavens for the beginner to commence his observations upon. We cannot,
however, assert that Mizar is a binary, inasmuch as observations have
not yet established the existence of a motion of revolution. Still less
are we able to say whether Alcor is also a member of the same group, or
whether it may not merely be a star which happens to fall nearly in the
line of vision. Recent spectroscopic observations have shown that the
larger component of Mizar is itself a double, consisting of a pair of
suns so close together that there is not the slightest possibility of
their ever being seen separately by the most powerful telescope in the
world.
A pleasing class of double stars is that in which we have the remarkable
phenomenon of colours, differing in a striking degree from the colours
of ordinary stars. Among the latter we find, in the great majority of
cases, no very characteristic hue; some are, however, more or less
tinged with red, some are decidedly ruddy, and some are intensely red.
Stars of a bluish or greenish colour are much more rare,[37] and when a
star of this character does occur, it is almost invariably as one of a
pair which form a double. The other star of the double is sometimes of
the same hue, but more usually it is yellow or ruddy.
One of the loveliest of these objects, which lies within reach of
telescopes of very moderate pretensions, is that found in the
constellation of the Swan, and known as b Cygni (Fig. 91). This
exquisite object is composed of two stars. The larger, about the third
magnitude, is of a golden-yellow, or topaz, colour; the smaller, of the
sixth magnitude, is of a light blue. These colours are nearly
complementary, but still there can be no doubt that the effect is not
merely one of contrast. That these two stars are both tinged with the
hues we have stated can be shown by hiding each in succession behind a
bar placed in the field of view. It has also been confirmed in a very
striking manner by spectroscopic investigation; for we see that the blue
star has experienced a special absorption of the red rays, while the
more ruddy light of the other star has arisen from the absorption of the
blue rays. The contrast of the colours in this object can often be very
effectively seen by putting the eye-piece out of focus. The discs thus
produced show the contrast of colours better than when the telescope
exhibits merely two stellar points.
Such are a few of these double and multiple stars. Their numbers are
being annually augmented; indeed, one observer--Mr. Burnham, formerly on
the staff of the Lick Observatory, and now an observer in the Yerkes
Observatory--has added by his own researches more than 1,000 new doubles
to the list of those previously known.
The interest in this class of objects must necessarily be increased when
we reflect that, small as the stars appear to be in our telescopes, they
are in reality suns of great size and splendour, in many cases rivalling
our own sun, or, perhaps, even surpassing him. Whether these suns have
planets attending upon them we cannot tell; the light reflected from the
planet would be utterly inadequate to the penetration of the vast extent
of space which separates us from the stars. If there be planets
surrounding these objects, then, instead of a single sun, such planets
will be illuminated by two, or, perhaps, even more suns. What wondrous
effects of light and shade must be the result! Sometimes both suns will
be above the horizon together, sometimes only one sun, and sometimes
both will be absent. Especially remarkable would be the condition of a
planet whose suns were of the coloured type. To-day we have a red sun
illuminating the heavens, to-morrow it would be a blue sun, and,
perhaps, the day after both the red sun and the blue sun will be in the
firmament together. What endless variety of scenery such a thought
suggests! There are, however, grave dynamical reasons for doubting
whether the conditions under which such a planet would exist could be
made compatible with life in any degree resembling the life with which
we are familiar. The problem of the movement of a planet under the
influence of two suns is one of the most difficult that has ever been
proposed to mathematicians, and it is, indeed, impossible in the present
state of analysis to solve with accuracy all the questions which it
implies. It seems not at all unlikely that the disturbances of the
planet's orbit would be so great that it would be exposed to
vicissitudes of light and of temperature far transcending those
experienced by a planet moving, like the earth, under the supreme
control of a single sun.