Cancer

Possessing no star brighter than the third magnitude, Cancer owes its relative prominence as a constellation to its location on the ecliptic and the presence of Praesepe, the 44th object in Charles Messier’s famous catalogue and one of the finest open star clusters in the heavens.

On clear, moonless nights Praesepe is visible to the naked eye as a faint misty patch in the centre of the constellation. It has been known about since ancient times and its stellar, rather than nebulous nature was established by Galileo using the newly invented telescope in 1610. Praesepe which lies at a distance of over 500 light years from us possesses several hundred stars, a few dozen of which can be made out with good binoculars.
Cancer has another excellent star cluster the somewhat more distant Messier 67, lying 2,500 light years away. M67 is readily observable with binoculars, but requires a telescope to resolve it into stars. There are thought to be around 500 members of M67 and it is remarkable for being one of the oldest galactic clusters known. Even “G” type stars similar to our own Sun in M67 have moved off the main sequence, indicating an age of at least 10 billion years.
Cancer possesses a number of interesting double and multiple stars, not least Zeta Cancri (also known as Tegmeni). A small telescope will show 5th and 6th magnitude stars with an easy separation of 6 arcseconds.
The brighter star is double again with the two components having magnitudes of 5.6 and 5.9 and a rather more challenging separation of about one arcsecond. The task of splitting this pair should get easier as they approach the point of widest separation in their 60 year mutual orbit in 2020.
Iota Cancri consists of a 4.5 magnitude yellow primary and a 6.5 magnitude blue secondary. The separation is 30 arcseconds – well within the range of good binoculars, although the colours will be better brought out by a telescope.