Cancer, the Crab

This page was last amended on 30th November,  2007

Cancer is a faint constellation located just east of Gemini and is not easy to spot. Its brightest star is beta-Cancri at magnitude 3.52. In fact, Cancer's best known feature, M44, the Beehive Cluster, at magnitude 3.10 is brighter than any of the stars in its parent constellation!

The star 55 Cancri is magnitude 5.96, is the star that currently holds the record for the number of exoplanets (five) in orbit around it. It is therefore at the limits of naked-eye visibility, a degree or so from iota-Cancri, at magnitude of 4.03. 55 Cancri is shown in the star chart above at the point where the two thin red lines intersect.

55 Cancri's fifth planet to be discovered is allegedly "easily visible" with binoculars; even if true, it is likely to prove quite an observational challenge!

Iota Cancri is a  binary of magnitudes 4.5 and 6.5, coloured yellow and blue respectively

Cancer has no variables of any particular note, but does have two Messier objects, M44 and M67.

M44 was observed by Galileo using his telescope and he thought it looked like a swarm of bees, hence its modern name of "Beehive Cluster". Before that it was known to ancient Greek astronomers, who thought it looked like a pile of hay and consequently called it Praesepe (manger). Both names are in use to this day. Even earlier, Chinese astronomers were aware of it, and, inscrutable as ever, gave it a name that roughly translates to "the breath of a pile of corpses"! This name is no longer used!

It's a bright open cluster (some people consider it to be the most beautiful in the entire sky) easily seen with the naked eye on a dark night, but is better through binoculars.

It is one of the largest clusters at 1.5° across, three times the angular diameter of the Moon, and contains over 300 stars.

M67 on the other hand is not much to look at but its claim to fame is its great age - it's about 10 billion years old!