Categories: Deep-Sky Objects

Deep-Sky Objects in Cancer

The Beehive Cluster (M44) in Cancer

Cancer is the 31st biggest constellation, and in terms of zodiacal constellations is the faintest, as well as 9th smallest, taking up an area of 506 square degrees of the northern sky. The brightest star in Cancer is Al Tarf (Beta Cancri) which is located 290 light-years away, and shines with an apparent magnitude of just +3.5.

The constellation is visible from between latitudes of +90° and -60°, with the best time to view Cancer at about 9 PM Local Time in March, when it is highest in the sky. However, the constellation remains visible throughout April and May, before it sinks into the Sun’s glare by early June. Note, though, that Cancer in not conspicuous, and dark skies and good seeing conditions are generally required to see it at all between the bright stars Castor and Pollux in Gemini, and Leo’s brightest star, Regulus.

The constellation of Cancer contains only a few interesting deep sky objects, the most notable of which are explored in this list.

Beehive Cluster (Praesepe, Messier 44, NGC 2632, Cr 189)

Located about 577 light years away, the 3.7 magnitude Beehive Cluster is a naked eye object under clear and dark skies, and counted among the closest, biggest, and most densely populated open star clusters to Earth. All told, the cluster contains about 1,000 stars, of which around 63% are red dwarfs, with the rest being F-, G-, and K-type Sun-like stars. The most luminous stars in M44 are of the blue-white variety, with magnitudes that range between 6 and 6.5. Interestingly, when Galileo observed the cluster in 1609, he described it “[as] the nebulous mass in the breast of Cancer”, which description probably played a role in the cluster later being named “Praesepe”, meaning “the manger” in Latin.

Messier 67 (M67, NGC 2682)

With an estimated age of between 3.2 and 5 billion years, M67 is among the oldest open clusters discovered to date. This cluster shines with an apparent magnitude of 6.1, and contains about 100 or so stars that are similar to the Sun, and a number of red giants. Like the Pleiades (M45), all the stars in the cluster are at roughly the same distance from Earth, and have similar ages, which implies they all share a common point of origin. The exception to this is the 30 or so blue stragglers in the cluster. Although the distance to the cluster is still the subject of some debate, most investigators seem to agree on a distance value of between 815 parsecs (2,659 light years) and 855 parsecs (2,787 light years).

NGC 2775 (Caldwell 48)

Located about 55 million light years away, this magnitude 11.3 spiral galaxy is 70,000 light-years across, and estimated to contain around 100 billion stars. It sports multiple, finely interwoven spiral arms that encircle its central regions completely, with the bright blue knots of nebulosity in the spiral arms being evidence of recent star forming activity. In 1783, NGC 2775 was discovered by William Herschel lying close to the border between the constellations of Cancer and Hydra, and over the past 30 years there have been 5 supernovae explosions seen in the galaxy.

NGC 2535 & NGC 2536

Much of the interaction between the spiral galaxies NGC 2535 and NGC 2536 is not visible in optical wavelengths. In this enhanced, false-color image, NGC 2535 can be seen as an elongated structure that contains extensive star forming regions as a result of being pulled apart by NGC 2536. The pair of galaxies is listed in the Atlas of Peculiar Galaxies as an example of a spiral galaxy that is interacting with a high surface-brightness companion. Note that in the image above, mid-infrared emission is shown as red, H alpha emission at 694 nm is shown as green, while ultraviolet emission is shown as blue.

NGC 2608 (Arp 12)

Located about 93 million light years away, this grand design spiral galaxy is about 63,000 light years in diameter with spiral arms that wind moderately tightly around the core. When Halton Arp classified this galaxy in 1966, he noted the nucleus might be double, or a pair of superimposed nuclei, as can be seen in the image opposite. Recent studies, however, have confirmed that the double core seen here is the result of NGC 2608 having interacted violently with another galaxy in the distant past.

Peter Christoforou :