Double Stars
ATLAS OF THE UNIVERSE
L
ook at Mizar, the second star in the tail of the Great
Bear, and you will see a much fainter star, Alcor, close
beside it. Use a telescope, and Mizar itself is seen to be
made up of two components, one rather brighter than the
other. The two Mizars are genuinely associated, and make
up a physically connected or binary system, while Alcor is
also a member of the group even though it is a long way
from the bright pair. Binary systems are very common in
the Galaxy; surprisingly, they seem to be more plentiful
than single stars such as the Sun.
Many double stars are within the range of small tele-
scopes, and some pairs are even separable with the naked
eye; Alcor is by no means a difficult naked-eye object
when the sky is reasonably clear and dark. Yet not all
doubles are true binaries. In some cases one component is
simply seen more or less in front of the other, so that we
are dealing with nothing more significant than a line of
sight effect. Alpha Capricorni, in the Sea-Goat, is a good
case of this (Map 14 in this book). The two components are
of magnitudes 3.6 and 4.2 respectively, and any normal-
sighted person can see them separately without optical aid.
The fainter member of the pair is 1600 light-years away,
and over 5000 times as luminous as the Sun; the brighter
component is only 117 light-years away, and a mere 75
Sun-power. There is absolutely no connection between the
two. As so often happens, appearances are deceptive.
The components of a binary system move together
round their common centre of gravity, much as the two
bells of a dumbbell will do when twisted by the bar
joining them. If the two members are equal in mass, the
centre of gravity will be midway between them; if not,
the centre of gravity will be displaced towards the
‘heavier’ star. However, the stars do not show nearly so
wide a range in mass as they do in size or luminosity,
so that in general the centre of gravity is not very far from
the mid position. With very widely separated pairs, the
orbital periods may be millions of years, so that all we can
really say is that the components share a common motion
in space. This is true of Alcor with respect to the Mizar
pair, while the estimated period of the two bright Mizars
round their common centre of gravity is of the order of▲ Double stars (drawings
by Paul Doherty). (Top)
Mizar and Alcor, the most
famous of all naked-eye
doubles; telescopically
Mizar itself is seen to be
made up of two components.
(Centre) Albireo (Beta Cygni).
This is almost certainly the
most beautiful coloured
double in the sky; the
primary (magnitude 3.1)
is golden yellow, the
secondary (magnitude 5.1)
vivid blue. The separation
is almost 35” of arc.
(Right) Almaak, or Gamma
Andromedae. The primary
is an orange K-type star
of magnitude 2.2. The
companion is of magnitude
5.0, and is a white star
of type A. It is a close
binary with a period of
61 years, and a separation
of about 0”.5.E152- 191 UNIVERSE UK 2003mb 7/4/03 5:48 pm Page 176