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Cancer is one of the twelve of the zodiac and is located in the Northern celestial hemisphere. Its name is Latin for crab and it is commonly represented as one. Cancer is a medium-size constellation with an area of 506 and its stars are rather faint, its brightest star Beta Cancri having an apparent magnitude of 3.5. It contains ten stars with known exoplanet, including 55 Cancri, which has five: one Super-Earth and four , one of which is in the habitable zone and as such has expected temperatures similar to Earth. At the (angular) heart of this sector of our celestial sphere is Beehive Cluster (Messier 44), one of the closest to Earth and a popular target for amateur astronomers.
The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 3 main and 7 western edgework forming sides ( illustrated in infobox). Covering 506 square degrees or 0.921% of the sky, it ranks 31st of the 88 constellations in size. It can be seen at latitudes between +90° and -60° and is best visible at 9 p.m. during the month of March. Cancer borders the bright constellations of Leo, Gemini and Canis Minor. Under city skies, Cancer is invisible to the naked eye.
Also known as Altarf or Tarf, Beta Cancri is the brightest star in Cancer at apparent magnitude 3.5. Located 290 ± 30 from Earth, it is a binary star system, its main component an orange giant of spectral type K4III that is varies slightly from a baseline magnitude of 3.53—dipping by 0.005 magnitude over a period of 6 days. An aging star, it has expanded to around 50 times the Sun's diameter and shines with 660 times its luminosity. It has a faint magnitude 14 red dwarf companion located 29 arcseconds away that takes 76,000 years to complete an orbit. Altarf represents a part of Cancer's body.
At magnitude 3.9 is Delta Cancri, also known as Asellus Australis. Located 131±1 light-years from Earth, it is an orange-hued giant star that has swollen and cooled off the main sequence to become an orange giant with a radius 11 times and luminosity 53 times that of the Sun. Its common name means "southern donkey". 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. Known as Asellus Borealis "northern donkey", Gamma Cancri is a white-hued A-type subgiant of spectral type A1IV and magnitude 4.67, that is 35 times as luminous as of the Sun. It is located 181 ± 2 light-years from Earth.
Iota Cancri is a wide double star. The primary is a yellow-hued G-type bright giant star of magnitude 4.0, located 330 ± 20 light-years from Earth. It spent much of its stellar life as a B-type main sequence star before expanding and cooling to its current state as it spent its core hydrogen. The secondary is a white main sequence star of spectral type A3V and magnitude 6.57. Despite having different distances when measured by the HIPPARCOS satellite, the two stars share a common proper motion and appear to be a natural binary system.
Located 181 ± 2 light-years from Earth, Alpha Cancri (Acubens) is a multiple star with a primary component an apparent white main sequence star of spectral type A5 and magnitude 4.26. The secondary is of magnitude 12.0 and is visible in small amateur telescopes. Its common name means "the claw". The primary is actually two very similar white main sequence stars that are 5.3 AU distant from each other and the secondary is two small main sequence stars, most likely red dwarfs, that are 600 AU from the main pair. Hence the system is a quadruple one.
Zeta Cancri or Tegmine ("the shell") is a multiple star system that contains at least four stars located 82 light-years from Earth. The two brightest components are a binary star with an orbital period of 1100 years; the brighter component is a yellow-hued binary pair and the dimmer component is a yellow-hued star of magnitude 6.2. The brighter component is itself a binary star with a period of 59.6 years; its primary is of magnitude 5.6 and its secondary is of magnitude 6.0. This pair is at its greatest separation around 2019.
Ten star systems have been found to have planets. Rho1 Cancri or 55 Cancri (or Copernicus) is 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 low-mass planet that may be either a hot, water-rich world or a carbon planet and four gas giants. 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.
YBP 1194 is a sunlike star in the open cluster M67 that has been found to have three planets.
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 1213 Asellus Borealis and 1210 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."
The smaller, denser open cluster Messier 67 can also be found in Cancer, 2600 light-years from Earth. It has an area of approximately 0.5 square degrees, the size of the full Moon. It contains approximately 200 stars, the brightest of which are of the tenth magnitude.
QSO J0842+1835 is a quasar used to measure the speed of gravity in VLBI experiment conducted by Edward Fomalont and Sergei Kopeikin in September 2002.
OJ 287 is a BL Lacertae object located 3.5 billion light years away that has produced quasi-periodic optical outbursts going back approximately 120 years, as first apparent on photographic plates from 1891. It was first detected at radio wavelengths during the course of the Ohio Sky Survey. Its central supermassive black hole is among the largest known, with a mass of 18 billion solar masses, more than six times the value calculated for the previous largest object.
In the late 1890s, R.H. Allen asserted the following, with no supporting citation:
citating Allen (1898/1899).
Very few of Cancer's stars are visible to the naked eye, and its brightest stars are only 4th magnitude. Cancer was often considered the "Dark Sign", quaintly described as "black and without eyes". Dante Alighieri, alluded to its faintness in Paradiso, and mentioned it being visible for the whole night when it culmination at midnight in a Northern Hemisphere winter month:
Cancer was the backdrop to the Sun's most northerly position in the sky (the summer solstice) in ancient times, when the Earth's Sun-facing side was maximally tilted towards the south, in the Gregorian calendar kept within a few days of June 21. Equivalently, this is the date when the Sun is directly overhead as far north as 23.437° N. The northern-most parallel where the Sun is directly overhead is still called the Tropic of Cancer, even though the corresponding position on the sky now occurs in Taurus, due to the precession of the equinoxes.
The close conjunction of Jupiter and Saturn in 1563 – which was observed by Tycho Brahe and led him to note the inaccuracy of existing ephemerides and to begin his own program of astronomical measurements – occurred in Cancer not far from Praesepe.
In Greek mythology, Cancer is identified with the crab that appeared while Heracles fought the many-headed Lernaean Hydra. Hercules slew the crab after it bit him in the foot. Afterwards, the goddess Hera, an enemy of Heracles, placed the crab among the stars.
In the Egyptian records of about 2000 BC it was described as Scarabaeus (Scarab), the sacred emblem of immortality. In Babylonia the constellation was known as MUL.AL.LUL, a name which can refer to both a crab and a snapping turtle. On boundary stones, the image of a turtle or tortoise appears quite regularly and it is believed that this represents Cancer since a conventional crab has not so far been discovered on any of these monuments.
There also appears to be a strong connection between the Babylonian constellation and ideas of death and a passage to the underworld, which may be the origin of these ideas in later Greek myths associated with Hercules and the Hydra.
In the 12th century, an illustrated astronomical manuscript shows it as a water beetle. Albumasar writes of this sign in Flowers of Abu Ma'shar. A 1488 Latin language translation depicts cancer as a large crayfish, which also is the constellation's name in most Germanic languages. Jakob Bartsch and Stanislaus Lubienitzki, in the 17th century, described it as a lobster.
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