Hydra, Corvus, Crater
H
ydra, with an area of 1303 square degrees, is the
largest constellation in the sky; it gained that distinc-
tion when the old, unwieldy Argo Navis was dismem-
bered. As a matter of casual interest, the only other con-
stellations with areas of more than 1000 square degrees
are Virgo (1294), Ursa Major (1280), Cetus (1232),
Eridanus (1138), Pegasus (1121), Draco (1083) and
Centaurus (1060). At the other end of the scale comes the
Southern Cross, with a mere 68 square degrees.
Despite its size, Hydra is very far from being conspic-
uous, since there is only one star above the third magnitude
and only ten above the fourth. The only reasonably well-
defined pattern is the ‘head’, made up of ̇, Â, ‰and Ë; it is
easy to find, more or less between Procyon and Regulus,
but there is nothing in the least striking about it. Myth-
ologically, Hydra is said to represent the multi-headed
monster who became yet another of Hercules’ victims, but
many lists relegate it to the status of a harmless watersnake.
The only bright star, ·(Alphard), is prominent enough.
The Twins, Castor and Pollux, point directly to it, but in
any case it is readily identifiable simply because it lies in
so barren an area; there are no other bright stars in the
region, and Alphard has been nicknamed the Solitary One.
It has a K-type spectrum, and is obviously reddish. It is 180
light-years away, and 430 times as luminous as the Sun.
During the 1830s Sir John Herschel, son of the dis-
coverer of Uranus, went to the Cape of Good Hope to
survey the far-southern stars. On the voyage home he
made some observations of Alphard, and concluded that it
was decidedly variable. This has never been confirmed,
and today the star is regarded as being constant in light;
however, it may be worth watching – though it is awkward
to estimate with the naked eye because of the lack of
suitable comparisons. If there are any fluctuations, they
cannot amount to more than a few tenths of a magnitude.
There is, however, one interesting variable in the con-
stellation. This is R Hydrae, close to Áand therefore ratherinconveniently low for observers in Britain, Europe
and the northern United States. At its maximum it can
attain the fourth magnitude, and never falls below 10, so
that it is always an easy object; but it is not a typical Mira
star, because there seems no doubt that the period has
changed during the last couple of centuries. It used to be
around 500 days; by the 1930s it had fallen to 425 days,
and the latest official value is 390 days, so that we seem to
be dealing with a definite and probably permanent change
in the star’s evolutionary cycle. Observations are of value,
because there is no reason to assume that the shortening
in period has stopped. U Hydrae, which forms a triangle
with Óand Ì, is a semi-regular variable which is worth
finding because, like virtually all stars of spectral type N,
it is intensely red.
ÂHydrae is a multiple system. The two main com-
ponents are easy to resolve; the primary is an extremely
close binary with an orbital period of 15 years, and there
is a third and probably a fourth star sharing a common
motion in space. ‚is also double, but is a difficult test
for a 15-centimetre (6-inch) telescope. Though given the
second Greek letter, ‚is below the fourth magnitude, and
therefore more than a magnitude fainter than Á, ̇or Ó.
There are three Messier objects in Hydra. M48 is an
open cluster on the edge of the constellation, close to the
boundary with Monoceros (in fact the fourth-magnitude
star ̇Monocerotis is the best guide to it). It is just visible
with the naked eye, but it is not too easy to identify. M68,
discovered by Pierre Méchain in 1780, is a globular cluster
about 39,000 light-years away, lying almost due south
of ‚Corvi and more or less between Áand ‚Hydrae.
M83, south of Áand near the border between Hydra and
Centaurus, is a fine face-on spiral galaxy, about 8.5 mil-
lion light-years away and therefore not far beyond the
Local Group; it is easy to locate with a telescope of 10-
centimetre (4-inch) aperture or larger, and is a favourite
photographic target, but it is of course best seen from theATLAS OF THE UNIVERSE
This map shows a
decidedly barren region.
Hydra is the largest of all
the constellations, but
contains only one fairly
bright star, ·(Alphard).
The ‘head’ lies near Cancer,
the ‘tail’ extends to the
south of Virgo. Corvus is
fairly prominent, though
none of its stars is as bright
as the second magnitude;
Crater is very obscure.Á
„‚Í̄Ó
ÌÏ
̆^2
̆^1·È Ùı̇Â
‰Ë Û112
Ë
̇‰
Á ·
Ï‚Ë
‰ Á‚Â
·VIRGO
CORVUS
CRATER
HYDRA
ANTLIA
CENTAURUS
PYXIS
LEO
SEXTANS
SpicaAlphardM83
M68
M48
Uı·ÈRÛ3242
Î ·̇‰Á Ë‚Ô
Ó
Û‚ ·
‰Â Á„ ˆÁ·
‚
ıÓÈ̄MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterOpen cluster–1
0 1 2 3 4 5Ga Atl of Univ Phil'03stp 2/4/03 7:37 pm Page 230