Observing the Constellation Cancer

Learn the stars, clusters, and mythology of The Crab.

Cancer the Crab holds one of the best open clusters in the sky: the Beehive Cluster, M44.

In the quiet space between Leo the Lion and Gemini the Twins lies the zodiac constellation Cancer the Crab. Cancer looks a bit like the inner spokes in a wheel. A central star or cluster usually is the midpoint from which the lines radiate outward to the few bright stars in this constellation.

The brightest star in Cancer is Beta Cancri at magnitude 3.53. Beta Cancri lies in the southwestern area of the constellation. Sometimes called Altarf, this star is an orange giant and lies 290 light-years from Earth.

The alpha star in Cancer is actually a dimmer star at magnitude 4.26. In the southeastern section of the constellation, Alpha Cancri is sometimes called Acubens. It lies 173 light-years away.

A star that is brighter than Alpha Cancri is Iota Cancri. Iota is in the north of the constellation and shines at 4.03 magnitudes. It lies 298 light-years away. The two bright stars closest to the center of the constellation are Delta and Gamma Cancri. Delta is the second brightest in Cancer, at magnitude 3.94. Delta Cancri is sometimes called Asellus Australis. It lies 136 light-years away. Gamma Cancri is a magnitude 4.6 and lies 158 light-years away. It is sometimes called Asellus Borealis. The two “Asellus” stars figure into the mythology, as explained below.

The real showpiece in Cancer is M44, the Beehive Cluster. Located at the center of the constellation, the Beehive Cluster is a bright group of stars shining at magnitude 3.10. It is probably stargazers’ second favorite cluster after the Pleiades (M45) in Taurus. The cluster lies about 577 light-years away. M44 can be seen as a misty patch with the unaided eye and resolves into individual stars in binoculars. Through a telescope, the entire span of the Beehive Cluster may be lost. Using lower power is a better bet. M44’s nickname comes from the swarm of stars that can seem like bees around a hive.

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Another Messier object in Cancer is the open cluster M67. It lies not far from Alpha Cancri. M67 shines at magnitude 6.7. M67 is around 4 billion years old, which is quite old for an open cluster, which usually gets pulled apart rather quickly. A view through the telescope will start to reveal the hundreds of stars that make up this cluster.

In mythology, Cancer was the little crab sent by Hera to distract Hercules while he worked to defeat Hydra. Cancer was quickly crushed underneath Hercules’ feet, and Hera placed it in the stars as thanks for its attempt to battle the great Hercules. The Beehive Cluster in Cancer is also known as the Praesepe, which means “manger”. This manger is where the two donkeys, Asellus Australis and Asellus Borealis, ate from. The gods Dionysus and Silenus rode these two donkeys into battle against the Titans.

Cancer does not contain notable nebulae or galaxies.

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