Virgo, Libra
V
irgo is one of the largest of all the constellations,
covering almost 1300 square degrees of the sky, though
it has only one star of the first magnitude (Spica) and two
more above the third. Mythologically it represents the
Goddess of Justice, Astraea, daughter of Jupiter and
Themis; the name Spica is said to mean ‘the ear of wheat’
which the Virgin is holding in her left hand. The main
stars of Virgo make up a Y-pattern, with Spica at the base
and Áat the junction between the ‘stem’ and the ‘bowl’.
The bowl, bounded on the far side by ‚Leonis, is crowded
with galaxies.
Spica can be found by continuing the curve from
the Great Bear’s tail through Arcturus; if sufficiently pro-
longed it will reach Spica, which is in any case brilliant
enough to be really conspicuous. It is an eclipsing binary,
with a very small magnitude range from 0.91 to 1.01;
the components are only about 18 million kilometres
(11 million miles) apart. Around 80 per cent of the total
light comes from the primary, which is more than ten
times as massive as the Sun and is itself intrinsically vari-
able, though the fluctuations are very slight indeed. The
distance from us is 257 light-years, and the combined
luminosity is well over 2000 times that of the Sun. Like
Regulus, Spica is so close to the ecliptic that it can at
times be occulted by the Moon or a planet; the only other
first-magnitude stars similarly placed are Aldebaran and
Antares.
The bowl of Virgo is formed by Â, ‰, Á, Ëand ‚. The
last two are much fainter than the rest; early catalogues
made them equal to the others, but one must be very
wary of placing too much reliance on these old records,
and neither star seems to be of the type expected to show
long-term changes in brightness. Of the other stars in the
main pattern, ‰(Minelauva) is a fine red star of type M;
Angelo Secchi, the great Italian pioneer of astronomical
spectroscopy, nicknamed it Bellissima because of its
beautifully banded spectrum. It is 147 light-years away, and130 times as luminous as the Sun. Â, named Vindemiatrix
or The Grape-Gatherer, is of type G; distance 104 light-
years, luminosity 75 times greater than that of the Sun.
Áhas three accepted proper names; Arich, Porrima and
Postvarta. It is a famous binary, whose components are
identical twins; the orbital period is 171.4 years, and a few
decades ago the separation was great enough to make
Arich one of the most spectacular doubles in the sky. We
are now seeing it from a less favourable angle, and by
2007 the star will appear single except in giant telescopes,
after which it will start to open out again; the minimum
separation will be no more than 0.3 of a second of arc.
The orbit is eccentric, and the real separation between the
components ranges from 10,500 million kilometres (6520
million miles) to only 450 million kilometres (280 million
miles). Arich is relatively near, at 36 light-years.
There are no bright variables in Virgo, but there is
one much fainter star, W Virginis, which is worthy of
special mention. Its position is R.A. 13h 23m.5, declina-
tion 03 degrees 07 minutes, less than four degrees away
from ̇, but it is not likely to be of much interest to the
user of a small telescope; the magnitude never rises above
9.5, and drops to 10.6 at minimum. The period is 17.3
days. Originally W Virginis was classed as a Cepheid, but
it belongs to Population II, and short-period stars of
this sort are considerably less luminous than classical
Cepheids. It was this which led Edwin Hubble to under-
estimate the distance of the Andromeda Galaxy; at the
time he had no way of knowing that there are two kinds
of short-period variables with very different period–
luminosity relationships. For a while the less luminous
stars were called Type II Cepheids, but they are now
known officially as W Virginis stars. The brightest of
them, and the only member of the class easily visible with
the naked eye, is ÎPavonis, described with Star Map 21;
unfortunately it is too far south in the sky to be seen from
Britain or any part of Europe. W Virginis itself peaks atATLAS OF THE UNIVERSE
The constellationsin this
map are best seen during
evenings around April to
June. Both Virgo and Libra
are in the Zodiac, and Virgo
is crossed by the celestial
equator; the ‘Y’ of Virgo
is unmistakable, and the
‘bowl’ of the Y is crowded
with rather faint galaxies.
Libra adjoins Virgo to the
one side and Scorpius to
the other. The conspicuous
quadrilateral of Corvus is
also shown here, but is
described in Star Map 7.MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterOpen cluster–1
0 1 2 3 4 5HERCULES
SERPENS CAPUTBOÖTES
COMA BERENICES
LEO
VIRGO
LIBRACORVUS
CRATEROPHIUCHUS
LUPUS
HYDRA
Antares‚ ‰Áı·ÈÛ
̆
ÙÌÙ̇·ÈÎı‰ÁË‚Ó109SpicaSSSM90
M89
M60
M59
M86
M84
M87
M58
M49
M61
M104
SCORPIUS Á
‰
Á‚Í
‚‰ÁÙ·
ÛÚ‰ˆ‚
ÓÊ̇ ÍÓ‰ÌÏ·‰Á ‚Î
ı16
̇
‚ÔGa Atl of Univ Phil'03stp 2/4/03 7:36 pm Page 228