Cancer, or the Crab, is one of the constellations of the zodiac. It is a relatively small constellation with mostly faint stars that lies in the northern hemisphere. In Greek mythology, Cancer is identified with the crab that appeared while Heracles was fighting the many-headed Hydra. The crab bit Heracles on the foot, Heracles crushed it and then the goddess Hera, a sworn enemy of Heracles, placed the crab among the stars.
The Cancer constellation occupies an area of 506 square degrees and contains two stars with known planets. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of March.
The brightest star in the Cancer constellation is 
beta Cancri, also known as Tarf or Al Tarf, with an apparent magnitude of 3.50. The primary component is an orange K-type giant 290 light-years distant from Earth. The magnitude 14 companion is located 29 seconds away.
At magnitude 3.94, 
delta Cancri, is the second brightest star. It is an orange giant that also goes by the name Asellus Australis, or "southern donkey colt." The star also holds a record for the longest name, "Arkushanangarushashutu," derived from ancient Babylonian language, which translates to "the southeast star in the Crab." Delta Cancri also makes it easy to find X Cancri, the reddest star in the sky.
iota Cancri, with a magnitude of 4.03, is another star brighter than 
alpha Cancri. It is a binary star composed of a yellow G-type bright giant and an A-type main sequence dwarf. Both can easily be seen with a small telescope.
gamma Cancri or Asellus Borealis ("northern donkey colt") is a magnitude 4.6 white A-type subgiant about 158 light-years distant from Earth.
alpha Cancri or Acubens ("the claws") is a star system that lies 173 light-years away and has magnitude of 4.26. It is also sometimes known as Sertan ("the crab"). The star’s primary component is a white A-type main sequence dwarf, while the companion is a magnitude 11 star.
Zeta Cancri or Tegmine ("the shell") is another star system, one that contains at least four stars. 
zeta 1 Cancri consists of two yellow-white main sequence dwarfs (zeta Cancri A and B), while 
zeta 2 Cancri contains a yellow G-type star (zeta Cancri C) and a magnitude 10 companion that is either a red dwarf or a close pair of red dwarfs.
The Cancer constellation also has several notable deep sky objects. Praesepe, or the Beehive Cluster, is a very popular feature among astronomers. Also known as Messier 44, M44, NGC 2632 or Cr 189, the Beehive Cluster is located right in the centre of the Cancer constellation. It is an open star cluster, one of the nearest ones to our solar system. It is most easily observed when Cancer is high in the sky. North of the Equator, this period stretches from February to May. Ptolemy described the Beehive Cluster as "the nebulous mass in the breast of Cancer." It was one of the first objects Galileo observed with his telescope in 1609, spotting 40 stars in the cluster. Today, there are about 1010 high-probability members, most of them (68 percent) red dwarfs. The Greeks and Romans identified the nebulous object as a manger from which two donkeys, represented by the neighbouring stars 
Asellus Borealis and 
Asellus Australis, were eating. The stars represent the donkeys that the god Dionysus and his tutor Silenus rode in the war against the Titans. The ancient Chinese interpreted the object as a ghost or demon riding in a carriage, calling it a "cloud of pollen blown from under willow catkins."
Another notable feature in the Cancer constellation is 
rho-1 Cancri or 55 Cancri, a binary star approximately 40.9 light-years distant from Earth. 55 Cancri consists of a yellow dwarf and a smaller red dwarf, with five planets orbiting the primary star; one terrestrial planet and four gas giants. It is the only planetary system discovered to have five planets and possibly more. 55 Cancri A, classified as a rare "super metal-rich" star, is one of the top 100 target stars for NASA’s Terrestrial Planet Finder mission, ranked 63rd on the list. The red dwarf 55 Cancri B, a suspected binary, appears to be gravitationally bound to the primary star, as the two share common proper motion.
Cancer belongs to the Zodiac family of constellations, along with Leo
Constellations directly bordering Cancer are Lynx
, Canis Minor
and Leo Minor