Cancer


Transit Date of principal star:
4 February


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.

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.

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:

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.)

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 217 and separation is 5.1".

Iota Cancri is a wide binary (4.5, 6.5) with a striking colour contrast: yellow and blue. PA 307 separation 30.5".

Finally, for the perseverant, there are a number of binary systems visible in the Beehive Cluster (see below).

The most accessible is Struve 1254, found near the centre of M44. Check the Binocular Section (at the bottom of this page) for more details.


Variable stars:

Cancer has no outstanding variables, but there are two which might be of some interest.

Kappa Cancri is an alpha-CVn type variable: 5.22-5.27 every five days.

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.

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.

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.

Visually unremarkable, yet this deep sky object is renowned for its venerable age: it is now believed that the cluster is approximately 3.5 to 5 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.


For a closer appreciation of Cancer, visit the Binocular Section.


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