THE STARS
10,000 years. The real separation seems to be about
60,000 million kilometres (over 37,000 million miles).
With the Mizar group there is another complication. In
1889 E. C. Pickering, at the Harvard Observatory, exam-
ined the spectrum of the brighter component (Mizar A),
and found that the spectral lines were periodically doubled.
At once he realized that he was dealing with a binary
whose two components were much too close to be seen
separately. The revolution period is 20.5 days, and the two
stars are about equal in brightness. There are times when
one component is approaching us, and will show a blue
shift, while the other is receding and will show a red shift;
therefore the lines will be doubled. When the orbital
motion is transverse, the lines will be single. Mizar A
was the first-known spectroscopic binary; later it was
found that both Mizar B and Alcor are also spectroscopic
binaries. The eighth-magnitude star between Alcor and the
bright pair is more remote, and not one of the group.
The position angle or P.A. of a double star – either a
binary or an optical pair – is measured according to the
angular direction of the secondary (B) from the primary
(A), reckoned from 000 degrees at north round by 090
degrees at east, 180 degrees at south, and 270 degrees at
west back to north. In general, it may be said that a 3-inch
(7.6-cm) telescope will separate a pair 1.8 seconds of arc
apart provided that the two components are equal; a 6-inch
(15.2-cm) will reach down to 0.8 second of arc, and a
12-inch (30.5-cm) will reach to 0.4 second of arc.
Arich or Gamma Virginis (Map 6) is a good example
of a binary which has changed its appearance over the
years. The components are exactly equal at magnitude 3.5,
and the orbital period is 171.4 years. Several decades ago
it was very wide and easy, but it is now closing up, and
by 2016 the star will appear single except with giant
telescopes. This does not mean that the components are
actually approaching each other, but only that we are see-
ing them from a less favourable angle. With Zeta Herculis
(Map 9) the period is only 34 years, so that both the
separation and the position angle alter quite quickly; so
also with Alpha Centauri, the brighter of the two Pointers
to the Southern Cross, where the period is 79.9 years.
In 1995 the separation is 17.3 seconds and the P.A. is
218 degrees; by 2005 the separation will have decreased
to 10.5 seconds and the P.A. will have increased to 230
degrees. (Alpha Centauri is the nearest bright star beyond
the Sun. The dim red dwarf Proxima, more than a degree
away from Alpha, is slightly closer to us; it has always
been regarded as a member of the group, though there are
suggestions that it is merely ‘passing by’.)
In many cases the two components of a binary are very
unequal. Sirius has its dwarf companion, only^1 /10,000as
bright as the primary – though it must once have passed
through the red giant stage and been much more luminous
than it is now. Then there are pairs with beautiful contrast-
ing colours; Albireo or Beta Cygni (Map 8) is a yellow
star with a companion which is vivid blue, while some red
supergiants, notably Antares (Map 11) and Alpha Herculis
(Map 9) have companions which look greenish by contrast.
Multiple stars are also found. A famous case is that
of Epsilon Lyrae, near Vega (Map 8). The two main
components are of magnitude 4.7 and 5.1 respectively, and
can be separated with the naked eye; a telescope shows
that each is again double, so that we have a quadruple
system. Theta Orionis, in the Great Nebula (Map 16), has
its four main components arranged in the pattern which
has led to the nickname of the Trapezium. Castor in
Gemini, the senior though fainter member of the Twins
(Map 17), is an easy telescopic pair; each component is aSELECTED DOUBLE STARS
Name Mag. Sep., ” P.A., ° Map Notes
ÁAndromedae 2.3, 5.0 9.4 064 12 Yellow, blue. B is double.
̇Aquarii 4.3, 4.5 2.0 196 14 Widening.
ÁArietis 4.8, 4.8 7.6 000 12 Very easy.
·Canum Venaticorum 2.9, 5.5 19.6 228 1 Yellow, bluish.
·Centauri 0.0, 1.2 17.3 218 20 Very easy. Period 80 years.
ÁCentauri 2.9, 2.9 1.2 351 20 Period 84 years.
‰Cephei var, 7.5 41 192 3 Very easy.
·Crucis 1.4, 1.9 4.2 114 20 Third star in field.
‚Cygni 3.1, 5.1 34.1 054 18 Yellow, blue.
ÁDelphini 4.5, 5.5 9.3 267 18 Yellowish, bluish.
ÓDraconis 4.9, 4.9 62 312 2 Naked-eye pair.
ıEridani 3.4, 4.5 8.3 090 22 Both white.
·Geminorum 1.9, 2.9 3.5 072 17 Widening.
·Herculis var, 5.4 4.6 106 9 Red, greenish.
̇Herculis 2.9, 5.5 1.4 261 9 Period 34 years.
ÂLyrae 4.7, 5.1 207 173 18 Both double.
̇Lyrae 4.3, 5.9 44 149 18 Fixed, easy.
‚Orionis 0.1, 6.8 9.5 202 16 Not difficult.
̇Orionis 1.9, 4.0 2.4 162 16 Split with 7.5 cm.
‚Phoenicis 4.0, 4.2 1.5 324 21 Widening.
·Scorpii 1.2, 5.4 2.7 274 11 Red, greenish.
ÓScorpii 4.3, 6.4 42 336 11 Both double.
ıSerpentis 4.5, 4.5 22 104 10 Very easy.
‚Tucanae 4.4, 4.8 27 170 21 Both double.
̇Ursae Majoris 2.3, 4.0 14.4 151 1 Naked-eye pair with Alcor.
ÁVirginis 3.5, 3.5 2.2 277 6 Period 171 years. Closing.▼ A binary star systemcan
be seen at the centre of the
planetary nebula NGC 3132
in this representative-colour
image taken by the Hubble
Space Telescope. The blue
light surrounding the binary
system is energized by the
fainter of the two stars.spectroscopic binary, and there is a much fainter member
of the group which also is a spectroscopic binary.
It used to be thought that a binary system was formed
when a rapidly spinning star broke up, but this attractive
theory has now fallen from favour, and it seems much
more likely that the components of a binary were formed
from the same cloud of material in the same region of
space, so that they have always remained gravitationally
linked. If their initial masses are different they will evolve
at different rates, and in some cases there may even be ex-
change of material between the two members of the pair.E152- 191 UNIVERSE UK 2003mb 7/4/03 5:48 pm Page 177