Cancer - The Crab

This zodiacal constellation lies to the west of Leo and east of Gemini. Cancer is a poorly defined constellation consisting only of rather faint stars. It is best seen in the spring in the Northern Hemisphere and in autumn in the Southern Hemisphere.

History and Mythology

Cancer is an ancient constellation dating from early Greek times. Though it is the least conspicuous constellation of the zodiac, Cancer is one of the best known.

In Greek Mythology, the Crab was said to be one of the many beasts that battled the hero Hercules. The second labor of Hercules was to kill the many-headed monster Hydra. During the ferocious struggle between Hercules and Hydra, Hercules would cut off one head of the monster and two heads grew back in its place. Finally he overcame the monster.

The goddess Hera was an enemy of Hercules, and she sent a crab to pinch the warrior so he would lose. But Hercules destroyed both the crab and Hydra, tossing both into the sky. Hera rewarded the crab's bravery by giving it a permanent place in the heavens.

Notable Stars

The brightest star in Cancer is Acubens. Its name comes from the Arabic word for "claws" of the crab. It is a white star 175 light years away.

Zeta Cancri is a multiple system containing at least four stars. The system contains two binary pairs, 5 arcseconds apart, which orbit each other every 1100 years. The brighter pair consists of two white 6th magnitude stars separated by 0.8 arcseconds; the fainter is a 6th-magnitude sunlike star with a 10th magnitude red dwarf companion.

Another red dwarf in Cancer is the nearby but very dim Giclas 51-15. It is only 11.8 light years away, but is only 15th magnitude, and is among the least luminous stars known.

Clusters, Nebulae, and Galaxies

Although Cancer is a relatively inconspicuous constellation, it does contain an impressive open cluster, called the Praesepe, the Beehive Cluster, or M 44. To the naked eye, the Beehive looks like a fuzzy star, and in binoculars it resembles a swarm of bees. With a small telescope you can count at least 75 stars, and with a larger instruments perhaps 300. M 44 is a group of relatively young stars which were born about 600 million years ago. The cluster is about 520 light-years away.

M 44 may be related to another well known cluster, the Hyades in Taurus. Both clusters are about the same age, and share the some common motion through space, suggesting that they actually formed together - even though they are on opposite sides of the sky today.

M 67 is a fainter, 6th-magnitude open cluster in Cancer. It contains about 65 stars and is about 2,700 light-years away. M 67 is one of oldest open clusters known, with an estimated age greater than 4 billion years.

NGC 2775 is a spiral galaxy in Cancer that has been host to 5 supernova explosions in the past 30 years - this is a galaxy to keep an eye on.