Hercules
H
erculesis a very large constellation; it covers 1225
square degrees, but it is not particularly rich. The best
guide to it is Rasalhague, or ·Ophiuchi, which is of the
second magnitude – bright enough to be prominent – and
is also rather isolated. Not far from it is Rasalgethi or ·
Herculis, which is some way away from the other main
stars of the constellation. The main part of Hercules lies
inside the triangle bounded by Alphekka in Corona
Borealis, Rasalhague and Vega. With its high northern
declination, part of it is circumpolar from the latitudes of
Britain or the northern United States. Its main features are
the red supergiant Rasalgethi, a wide and easy binary ( ̇),
and two spectacular globular clusters (M13 and M92).
In 1759 William Herschel discovered that Rasalgethi
is variable. At that time only four variables had been found- Mira Ceti, Algol in Perseus, ̄Cygni and R Hydrae –
so that the discovery was regarded as very important.
Certainly there is no doubt about the fluctuations; it is said
that the extreme range is from magnitude 3.0 to 4.0,
though for most of the time the star remains between 3.1
and 3.7. Officially it is classed as a semi-regular with a
rough period of 90 to 100 days, but this period is by no
means well marked. The variations are slow, but can be
followed with the naked eye; suitable comparison stars are
ÎOphiuchi (3.20), ‰Herculis (3.14) and ÁHerculis (3.75).
‚Herculis (2.71), the brightest star in the constellation, is
always considerably superior to Rasalgethi.
The distance of Rasalgethi is 218 light-years; the
spectral type is M. What makes it so notable is its vast
size. It may be even larger than Betelgeux, in which case
its diameter exceeds 400 million kilometres (250 million
miles). It is relatively cool – the surface temperature is well
below 3000 degrees C – and its outer layers, at least, are very
rarefied. It is a very powerful emitter of infra-red radiation.
Rasalgethi is also a fine double. The companion is of
magnitude 5.3, and since the separation is not much short
of 5 seconds of arc a small telescope will resolve the pair.
The companion is often described as vivid green, though
this is due mainly, if not entirely, to contrast with the red-
ness of the primary. The companion is itself an excessively
close binary, with a period of 51.6 days, and there is every
reason to believe that both stars are enveloped in a huge,
rarefied cloud. Rasalgethi is indeed a remarkable system.
‰Herculis has an eighth-magnitude companion at a
separation of 9 seconds of arc (position angle 236
degrees), but this is an optical pair; there is no connection
between the two components, and the secondary lies well
in the background. ‰itself is an ordinary A-type star, 35
times as luminous as the Sun and 91 light-years away.
Of more interest is ̇Herculis, or Rutilicus, which is a
fine binary; its duplicity was discovered by William
Herschel in 1782. The magnitudes are 2.9 and 3.5; the
period is only 34.5 years, so that both separation and
position angle change quickly. In 1994, the separation was
1.6 seconds of arc, so that this is a very wide, easy pair.
The primary is a G-type subgiant, 31 light-years away and
rather more than five times as luminous as the Sun.
68 (u) Herculis is an interesting variable of the ‚Lyrae
type. The secondary minimum takes the magnitude down
to 5.0 and the deep minimum to only 5.3, so that the star is
always within binocular or even naked-eye range. Both
components are B-type giants, so close that they almost
touch; as with ‚Lyrae, each must be pulled out into the
shape of an egg. If the distance is around 600 light-years,
as seems possible, each star must be well over 100 times
as luminous as the Sun.
On 13 December 1934 the English amateur J. P. M.
Prentice discovered a bright nova in Hercules, near Èand
ATLAS OF THE UNIVERSE
its illumination to Deneb. It lies in the same field as red-
dish ÍCygni.
Aquila, the Eagle, commemorates the bird sent by Jupiter
to fetch a shepherd boy, Ganymede, who was destined
to become the cup-bearer of the gods. Altair, at a distance
of 16.6 light-years, is the closest of the first-magnitude
stars apart from ·Centauri, Sirius and Procyon; it is ten
times as luminous as the Sun, and is known to be rotating
so rapidly that it must be egg-shaped. It is flanked to either
side by two fainter stars, Á(Tarazed) and ‚(Alshain). Áis
an orange K-type star, much more powerful than Altair but
also much more remote.
ËAquilae is a Cepheid variable. The range is from
magnitude 3.4 to 4.4, so that ‰and ımake ideal compari-
son stars; when Ëis near minimum, a useful comparison
star is È(4.0).
Scutum is not an original constellation; it was one
of Hevelius’ inventions, and was originally Scutum
Sobieskii, Sobieski’s Shield. The variable R Scuti is the
brightest member of the RV Tauri class, and is a favourite
binocular target; there are alternate deep and shallow
minima, with occasional periods of irregularity. Of the
two Messier open clusters, much the more striking is
the fan-shaped M11, which has been nicknamed the Wild
Duck cluster and is a glorious sight in any telescope; it
contains hundreds of stars, and is easily identified, being
close to Ïand 12 Aquilae.
Sagitta, the Arrow – Cupid’s Bow – is distinctive; the
main arrow pattern is made up of the two bright stars ‰and
Á, together with ·and ‚(each of magnitude 4.37). There is
one Messier object, M71, which was formerly classified as
an open cluster, but is now thought to be a globular, though
it is much less condensed than other systems of this type. It
lies a little less than halfway from Áto ‰.
Vulpeculawas originally Vulpecula et Anser, the Fox and
Goose, but the goose has long since vanished from the
maps. The constellation is very dim, but is redeemed by
the presence of M27, the Dumbbell, probably the finest of
all planetary nebulae. There is no problem in finding it
with binoculars; it is close to ÁSagittae, which is the best
guide to it. A moderate power will reveal its characteristic
shape. Like all planetaries it is expanding; the present
diameter is of the order of two and a half light-years.
Delphinusis one of Ptolemy’s original constellations. It
honours a dolphin which carried the great singer Arion to
safety when he had been thrown overboard by the crew of
the ship which was carrying him home after winning
all the prizes in a competition. Delphinus is a compact
little group – unwary observers have been known to
confuse it with the Pleiades. Its two leading stars have
curious names: ·is Svalocin, ‚is Rotanev. These names
were given by one Nicolaus Venator, and the association
is obvious enough.
ÁDelphini is a wide, easy double, and the two red
semi-regular variables U and EU are good binocular
objects. Near them an interesting nova, HR Delphini,
flared up in 1967 and was discovered by the English
amateur G. E. D. Alcock; it reached magnitude 3.7, and
remained a naked-eye object for months. Its present
magnitude is between 12 and 13, and as this was also the
pre-outburst value it is unlikely to fade much further.
Equuleusrepresents a foal given by Mercury to Castor,
one of the Heavenly Twins. It is so small and dim that it
is surprising to find it in Ptolemy’s original list, but the
little triangle made up of ·, ‰(4.49) and Á(4.69) is not
hard to identify, between Delphinus and ‚Aquarii. Âis a
triple star, but otherwise Equuleus contains nothing of
interest.Ga Atl of Univ Phil'03stp 2/4/03 7:37 pm Page 234