Among the brighter stars at least one in four is double, and I shall
only mention a few which for different reasons are of special interest.
The brightest double in the sky has already been mentioned, α Centauri,
and we have also described another southern pair scarcely less
brilliant, α Crucis, Sirius with his very dim companion, Rigel and
Antares with theirs of contrasting colours. Other fine southern doubles
are:
β Piscis australis, a white star with reddish companion, visible in a
3-inch telescope.
δ Corvi, an unequal distant pair, pale yellow and bluish, easily
separated in a 4½-inch telescope.
σ Scorpii (near Antares), white and blue.
32 Eridani, yellow and blue-green—“magnifici, superbi,” according to
Secchi.
β Capricorni (close to the splendid naked-eye double α Capricorni),
orange-yellow and blue.
γ Crucis, orange-yellow, companion fifth magnitude, rather distant.
γ Leporis, companion crimson. There is also a third faint star, forming
a triple group.
β Capricorni, like Antares, besides its visible companion, has a close
invisible one, only known by the shifting of lines in the spectrum, and
this is not an uncommon case. β Crucis consists similarly of two bright
stars and a spectroscopic companion, and so also θ Eridani. κ Velorum
and α Pavonis have spectroscopic companions only, one revolving in a
period of 116½ days, the other of only 11¾ days, and the period of μ¹
Scorpii is counted in hours!—34 hours 42 minutes. The brief periods of
these spectroscopic binaries[6] are in striking contrast with those of
many visual binaries, such as ζ Sagittarii with a period of 19 years
and γ Centauri with 150 years; and this is what one would expect, since
the stars must be comparatively far apart and their orbits ample for
them to be visible separately. Sometimes the stars of a pair or of a
group are known to be moving together through space, though no movement
of revolution round a common centre has yet been detected, probably
because it is very slow.
[6] A “binary” is a system of two stars which are known to be
comparatively close together and influencing one another’s movements. A
“double star” may be a binary, or the two stars may really be very far
apart and have no connection, merely happening to lie one nearly behind
the other.
It is an interesting fact that most of the spectroscopic binaries,
which are such close and rapid pairs, are found among the blue and
white stars, the numbers steadily decreasing as we pass through the
yellow to the red stars.
A significant fact about visual binaries is that companions which
differ much in colour invariably differ much in brightness also.
This is probably to be explained by supposing that one was from the
beginning much larger than the other, and that there is consequently
a difference in the rapidity with which each runs through its
life-changes. Where the two are alike in colour and spectrum, like the
two solar stars of α Centauri and the two Orion stars of α Crucis, it
is found that they are also nearly equal in mass, so they keep the same
pace and grow old together.
Among multiple stars there are some very remarkable instances in
Scorpio. β Scorpii is a pair of bright stars (easily separated with a
3-inch telescope) with a third fainter companion, and besides these,
one of the bright components is a spectroscopic pair, and the whole
company is travelling together through space. The joint mass of the
spectroscopic pair is twenty-one times as great as that of our sun,
and they revolve about one another in seven days; but a very strange
feature is that _some_ lines in their joint spectrum, due to calcium
gas, behave differently from the rest, and it is thought that these two
revolving stars may be enveloped in a calcium cloud which travels with
them.
ξ Scorpii is a telescopic double which has been watched throughout
a complete revolution of ninety-six years. It was discovered by Sir
William Herschel in 1782. Here also there is a third star, much fainter
and more distant than the brighter companion, and all are travelling
together through space.
ν Scorpii is one of the “double-doubles,” of which a good many are
known, where a star that looks single to the naked eye is seen as a
pair with a telescope, and each of these becomes a pair with higher
powers. It has been described as “perhaps the most beautiful quadruple
in the heavens.” Both pairs journey together through the skies.
σ Orionis, the fourth-magnitude star just below Orion’s belt, separates
very easily into two unequal components. Herschel found each of these
to be triple, and called it a “double-treble.” Later it was found to be
“double-quadruple,” with more stars between the two groups.
In the same way the beautiful naked-eye double star α Capricorni is
seen in good telescopes to consist of two groups of stars, one (α¹)
triple, the other (α²) quadruple.
If a group like this forms a connected system, the motions of the
several stars must be highly complicated.