1500 times the luminosity of the Sun, but this is not very
much compared with the 6000 Sun-power of ‰Cephei.
Of course, the main feature of Virgo is the cluster of
galaxies, which spreads into the adjacent constellations
of Leo and Coma. The average distance of the cluster
members is between 40 and 50 million light-years, and
since there are thousands of systems it makes our Local
Group seem very puny. In Virgo there are no less than 11
Messier objects, plus many more galaxies with integrated
magnitudes of 12 or brighter.
Pride of place must go to the giant elliptical M87, dis-
covered by Messier himself in 1781. A curious jet, several
thousands of light-years long, issues from it, and it is
attended by many globular clusters – perhaps as many as a
thousand. M87 is a very strong radio source, and is known
to radio astronomers as Virgo A or as 3C-274; it is also
a source of X-rays, and it is clear that tremendous activity
is going on there. There is considerable evidence that in
the heart of the galaxy there is a super-massive black hole.
M104 is different; it is an Sb spiral distinguished by
the dark dust-lane which crosses it and gives it the nick-
name of the Sombrero Hat. It too is associated with a
wealth of globular clusters. With a telescope of more than
30-centimetre (12-inch) aperture the dust-lane is not hard
to see, and there is nothing else quite like it, so that it is
also a favourite target for astro-photographers.
M49, which makes up an equilateral triangle with ‰
and Â, is another giant elliptical, strictly comparable with
M87 apart from the fact that it is not a strong radio emitter.
All the other galaxies in the catalogue are fairly easy to
locate, as also are various others not in Messier’s list; this
is a favourite hunting-ground for observers who concen-
trate upon searching for supernovae in external systems.
Libra, the Scales or Balance, adjoins Virgo and is one of
the least conspicuous of the Zodiacal groups. It is the only
constellation of the Zodiac named after an inanimate
object, but it was originally the Scorpion’s Claws; some
early Greek legends link it, rather vaguely, with Mochis,
the inventor of weights and measures. Its main stars (·,
‚, Áand Û) make up a distorted quadrilateral; Ûhas
been filched from Scorpius, and was formerly known as
ÁScorpii.
·Librae – Zubenelgenubi, the Southern Claw – makes
a very wide pair with 8 Librae, of magnitude 5.2; the
separation is so great that the pair can be well seen with
binoculars. The brighter member of the pair is a spectro-
scopic binary, 72 light-years away and 31 times as lumi-
nous as the Sun. Of rather more interest is ‚Librae or
Zubenelchemale, the Northern Claw. It is 121 light-years
away and 100 Sun-power; the spectral type is B8, and
it has often been said to be the only single star with a
decidedly greenish hue. T. W. Webb, a famous last-century
English observer, referred to its ‘beautiful pale green’
colour. This is certainly an exaggeration, and most people
will call it white, but it is worth examining. There have
been suggestions that it is yet another star to have faded in
historic times, and there is evidence that Ptolemy ranked it
as of the first magnitude, but – as with other similar cases- the evidence is very slender.
There is not much else of interest in Libra, and there
are no Messier objects. There is, however, one eclipsing
binary of the Algol type. This is ‰Librae, which makes
up a triangle with 16 Librae (magnitude 4.5) and ‚;
the range is from magnitude 4.8 to 6.1, so that it is never
conspicuous, and at minimum sinks to the very limit of
naked-eye visibility. With low-power binoculars it is in
the same field with ‚, and this is probably the best way to
locate it.
STAR MAP 6
▼ The Sombrero Galaxy,
M104, within Virgo. It is seen
almost edge-on, but its spiralstructure is just visible. The
galaxy is at a distance of
about 65 million light-years.BRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper name
hm s ° ‘ “
67 · 13 25 11.5 11 09 41 0.98 B1 Spica
29 Á 12 41 39.5 01 26 57 2.6 F0F0 Arich
47 Â 13 02 10.5 10 57 33 2.83 G9 Vindemiatrix
79 ̇ 13 34 41.5 00 35 46 3.37 A3 Heze
43 ‰ 12 55 36.1 03 23 51 3.38 M3 Minelauva
Also above magnitude 4.3: ‚(Zavijava) (3.61), 109 (3.72), Ì(Rijl al Awwa) (3.88), Ë(Zaniah) (3.89), Ó(4.03),
È(Syrma) (4.08), Ô(4.12), Î(4.19) and Ù(4.26).
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm °’ ° “
Á 12 41.7 01 27 287 3.0 3.5, 3.5
ı 13 09.9 05 32 343 7.1 4.4, 9.4
(Apami-Atsa)
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm °’ ’
49 4472 12 29.8 08 00 8.4 8.9 7.4 E3 galaxy
58 4579 12 37.7 11 49 9.8 5.4 4.4 SB galaxy
59 4621 12 42.0 11 39 9.8 5.1 3.4 E3 galaxy
60 4649 12 43.7 11 33 8.8 7.2 6.2 E1 galaxy
61 4303 12 21.9 04 28 9.7 6.0 5.5 Sc galaxy
84 4374 12 25.1 12 53 9.3 5.0 4.4 E1 galaxy
86 4406 12 26.2 12 57 9.2 7.4 5.5 E3 galaxy
87 4486 12 30.8 12 24 8.6 7.2 6.8 E1 galaxy
(Virgo A)
89 4552 12 35.7 12 33 9.8 9.5 4.7 E0 galaxy
90 4569 12 36.8 13 10 9.5 9.5 4.7 Sb galaxy
104 4594 12 40.0 11 37 8.3 8.9 4.1 Sb galaxy
(Sombrero Hat)BRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper name
hm s ° ‘ “
27 ‚ 15 17 00.3 09 22 58 2. 61 B8 Zubenelchemale
9 · 14 50 52.6 16 02 30 2.75 A3 Zubenelgenubi
20 Û 15 04 04.1 25 16 55 3.29 M4 Zubenalgubi
Also above magnitude 4.3: ̆(3.58), Ù(3.66), Á(Zubenelhakrabi) (3.91), È(4.15).
ÛLibrae was formerly included in Scorpius, as ÁScorpii.
VARIABLE
Star R.A. Dec. Range Type Period Spectrum
hm °’ (mags) (d)
‰ 15 01.1 08 31 4.9–5.9 Algol 2.33 B
DOUBLE
Star R.A. Dec. P.A. Sep. Mags
hm °’ ° “
· 14 50.9 16 02 314 231.0 2.8, 5.2VIRGOLIBRAGa Atl of Univ Phil'03stp 2/4/03 7:37 pm Page 229