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The name comes from the Latin; cancer means crab. The crab in
question is the one sent by Hydra to attack Heracles. It was only a bit
part, but one which secured its immortality.
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Heracles' first labour had been to kill the Lion of Nemea. Nemea is a
fertile valley in the Peloponnesus (renowned for its red wine, actually).
It was a sacred place in antiquity, with a famous temple to Zeus. But one
of its residents was causing problems: a gigantic lion roamed the streets
and the hills, devouring everyone it came across.
Heracles tracked the animal down, but the lion had fur which was impervious
to iron or bronze. Thus Heracles' arrows bounced off the animal, and his
sword bent, and his club broke into pieces.
The only thing left was to wrestle the thing, so in a mighty contest Heracles
fought the beast. The lion managed to nip off a finger from our hero, but
eventually Heracles choked the life out of the Nemean Lion. Thus ended the
First Labour.
Since nothing else would cut the lion's pelt, Heracles cleverly used its
own claws to skin the animal, and fashioned the impervious pelt into his own
protective clothing. Thus attired, he set off to accomplish his Second
Labour: to kill the Lernaean Hydra.
Nearby lay the swamps of Lerna, home of the Hydra, an enormous dog-like
monster with nine heads (one of which was immortal), and with breath
that would kill on contact.
With the help of Athene Heracles located the monster's lair and the
ensuing struggle was a standoff: as one head was sliced off, another
appeared in its place. Then at Hydra's bidding, a giant crab emerged from
the swamp and bit into Heracles' foot. Heracles promptly killed the animal
then cut off the Hydra's immortal head, killing it as well. He then
dipped his arrows in the Hydra's gall; the slightest scratch from one of
these arrows would bring instant death to his enemies.
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Scholars have expressed the opinion that astrologers later added the crab
to the ancient myth in order to have the Twelve Labours of Heracles
reflect the Twelve Signs of the Zodiac. While it is difficult to associate
all of Heracles' labours with the zodiac, it is true that the crab figured
in Heracles' Second Labour, and is in fact the Second Sign of the Zodiac.
In any case, apparently for following its mistress' command and sacrificing
its life, the crab was awarded with a heavenly home.
Cancer is a faint constellation located just
east of Gemini and north of the head of the sprawling constellation
Hydra. Its stars are generally four
magnitude, beta Cancri being the brightest at 3.52.
While rather small, Cancer still has a number of fine objects, including
a splendid star cluster and several visual binaries.
Double stars:
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Zeta Cancri is a notable triple system comprised of a close binary with a period of 59.5 years and a more distant star, component C, with a much longer period of 1115 years. (These values are recently published revisions; formerly the two orbits were thought to be 59.7 and 1150 years.)
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This distant companion also has its own binary star, which revolves
about zetaC every 17.6 years. It has never been seen, and its
existence has only been discovered through a particular wobble of
zetaC. The unseen star is thought to be a white dwarf.
Phi Cancri is a binary of two identical white stars (5.5m, 6m):
the PA is 217º and separation is 5.1".
Iota Cancri is a wide binary (4.5, 6.5) with a striking colour contrast:
yellow and blue. PA 307º separation 30.5".
Finally, for the perseverant, there are a number of binary systems visible
in the Beehive Cluster (see below). We'll point out two of them, very
close to each other.
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The brightest is Struve 1254. The primary is a bright 6.5m, with a 9.0m
companion B at 54º, 20.5". Then there are two more components:
C: 8.0, 342º, 63.2"; D: 9.0, 43º, 82.6".
To find this group, first locate epsilon Cancri, which is near the centre
of the Beehive Cluster and the brightest star in this cluster. Just to
the northwest of this star, less than a minute's distance, you'll find
this binary system.
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In the same field (slightly west and less than a minute south of Struve 1254) is the nice quadruple called beta 584, comprised of 7.0, 12.0, 7.0, and 6.5 visual magnitudes.
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AB is the most difficult to find, for the companion is a faint 12m star
at 291º and separation of only 1.2". AC: 156º, 45"; AD: 241º, 93".
Variable stars:
Cancer has no outstanding variables, but there are two which might be of
some interest.
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Kappa Cancri is an alpha-CVn type variable: 5.22-5.27 every five
days.
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Alpha-Canum Venaticorum type stars are rotating variables which typically
evince very little change in visual magnitude. These stars are generally
A-type (that is, they have a spectrum range of B9-A5) but curiously enough
they show an unusual abundance of a number of heavy metals and a corresponding
lack in the more common elements.
These stars are divided into three groups: those with predominantly silicon
spectral lines, those with manganese, and those with chromium-strontium lines.
Kappa Cancri shows a strong manganese line.
R Cancri is a Mira-type variable with period of 361.6 days and a
magnitude change from 6.07 to 11.8. In the year 2000 the maximum should
arrive in the first week of October.
Deep Sky Objects:
Cancer has two Messier objects, M44 and M67.
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M44 (NGC 2632) is better known by the name the Beehive Cluster,
or the Latin equivalent: Praesepe, which not only means a hive
but also a crib, or manger.
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This is a bright open cluster clearly visible to the naked eye on a dark
enough night, and best appreciated with binoculars or small scope. One of
the largest clusters, its 1.5 degree size is equivalent to three full moons
end-to-end. Its distance is calculated at between 520-590 light years.
This grouping is so large it was well-known in antiquity, when it was
thought to be a nebula, or gaseous region of the sky. The cluster often
served to predict the weather: if not crystal clear inclement weather
might be on the way.
Galileo was the first to study its stars with a telescope. He counted over
forty members, putting to rest the idea of its nebulosity and introducing
the idea of star clusters.
There are over three hundred stars in the Beehive (the Webb Society
Handbook claims 2000). It has been estimated that over a hundred of its
stars are brighter than our Sun, and in fact (as Burnham points out) if
the Sun were a member of this group, it would be a very modest member
indeed, at about 10.9 magnitude.
M67 (NGC 2682) sits about two degrees west of alpha Cancri and
south of the Beehive about nine degrees.
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Visually unremarkable, yet this deep sky object is renowned for its
venerable age: it is now believed that the cluster is approximately 10
billion years old. Its estimated distance is 2500 light years and there
are about five hundred stars in the cluster, tightly packed.
Being so old, many of its stars have nearly completed their life-cycle,
having passed through the red giant stage and now having "jumped off"
the main sequence and entering another phase. Indeed, this is how the
age of such clusters is determined.
It is assumed that all members of a star cluster evolved out of the
same gas cloud at roughly the same time (give or take a few million years).
These stars spend a given length of time on the main sequence, relative
to their mass. For example, stars equal to one solar mass will spend about
ten billion years on the main sequence. Since the stars of the Beehive
Cluster are rather similar to the sun, its age has therefore been
calculated to be at least 10 billion years.
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A printed version of this web site ["The Constellations
Pocket Guide"] is available, covering all 88 constellations and their graphics.
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