Cancer is a constellation which is part of the zodiac and is visible in both the Northern and Southern hemispheres. The name Cancer comes for the Latin for “the crab”, which the constellation traditionally depicts. It is actually quite difficult to make out any form of a crab within the constellation as most of the stars are quite faint. Only two of Cancer’s stars are above magnitude 4, making it the dimmest of the 13 constellations of the Zodiac.
| Cancer Constellation Fact File | ||
| Symbol | ♋ | |
| Depiction | Crab | |
| Abbreviation | Cnc | |
| Stars with Planets | 10 | |
| Messier Objects | 2 | |
| Brightest Star | Beta Cancri/ Altarf | |
| Meteor Shower | Delta Cancrids | |
History
Cancer was recognized by the Greek astronomer Ptolemy, who first recorded the constellation in the 2nd century. Though the constellation has been recognized for centuries, the creature it depicts has varied from one culture to the next, though it is usually described as an animal which lives in water and has an exoskeleton.
Cancer was recorded by the ancient Egyptians around 2000 BC. To the Egyptians, Cancer represented a Scarab; a sacred symbol of immortality.
The Babylonian name for the constellation could be translated to refer to a crab or a snapping turtle. The boundary stones monuments show images of a turtle or tortoise, and none of a crab.
Cancer was noted as a water beetle in a 12th century manuscript on astronomy, included with accompanying drawings. Latin translations dating back to 1488 describe the constellation as a large crayfish – the most common interpretation of Cancer in the majority of modern languages and traditions.
We still call the line of latitude marking the most northerly point at which the Sun can still be seen directly overhead the Tropic of Cancer (this occurs at the June Solstice). Due to the procession of the equinoxes, this line of latitude actually appears in Taurus now but gets the name from ancient times when it was in the constellation Cancer.
Out of 88 constellations, Cancer is the 31st largest in the sky.
Mythology
In Greek mythology, Hercules is the son of Zeus and stepson to the jealous Hera who was always out to kill him and who eventually drives him insane. To make up for his horrific actions during his bout of insanity, the young hero has to complete a series of tasks. In one such challenge, Hercules must engage in a battle against Lernaean Hydra (represented by the constellation Hydra), a snake-like monster with multiple heads and breath that is like poison. Hera sends the crab Karkinos to distract Hercules during the battle so he will fail.
In one account, Hercules notices the ploy and kicks the crab so hard that it lands in the sky. In an alternative version, the crab pinches Hercules on the toe before the hero crushes Karkinos under his foot. Hera rewards the crab for his efforts by placing him in the sky but makes sure that the crab had no bright stars due to his failure.
Locating Cancer
The constellation is part of the zodiac and can be found between two other constellations of the zodiac; namely Leo the lion and the twins, Gemini. The constellation is very faint because of its dim stars, and any form of a crab is truly indiscernible. As with all constellations of the zodiac, Cancer can be viewed from both the Northern and Southern Hemispheres.
Stars
Even though there are few bright stars in the constellation, most are still worth noting.
- Acubens– α Cancri (Alpha Cancri): Acubens is also sometimes called Al Zubanah or Sertan, which can be translated as ‘claws’ and ‘the crab’ respectively. It is a multi-star system which lies approximately 175 light years from Earth. Alpha Cancri A is the brighter star in the system; a white main sequence star very similar in radius to the Sun but double the Sun’s mass.
The star’s magnitude varies ever so slightly between 4.20 and 4.27. Acubens can be occulted by the Moon and the planets (though much more rarely) because of its close position to the ecliptic.
It has a twelfth magnitude companion, Alpha Cancri B, which is visible through a small telescope. The primary star, Alpha Cancri A, is actually two main sequence stars, while Alpha Cancri B likely consists of two red dwarfs. This makes Acubens a quadruple star system.
- Altarf– β Cancri (Beta Cancri): Despite its Beta designation, Altarf is the brightest star in the constellation Cancer. Beta Cancri has an apparent magnitude of 3.5. It is a binary star system. The companion is a magnitude fourteen star which takes 76 000 years to complete an orbit. The primary star is an orange giant over 500 times more luminous than the Sun, and 50 times larger in diameter. It is located some 290 light years away from Earth.
- Asellus Australis– δ Cancri (Delta Cancri): Located 180 light years away from Earth, Asellus Australis is an orange giant star of magnitude of 3.94. Though its designation is Delta Cancri, it is actually the second brightest star in Cancer. It also points to the location of the open star cluster Praesepe (Messier 44), colloquially called the Beehive Cluster. The star also has a less common, but no less interesting name: Arkushanangarushashutu (Babylonian for “the southeast star in the Crab”). It is the longest name of any star. Delta Cancri can be occulted by the Moon, and more rarely by the planets, due to its close proximity to the ecliptic.
- Asellus Borealis– γ Cancri (Gamma Cancri): Gamma Cancri is located approximately 158 light years away, and has an apparent magnitude of 4.66. Its name means “northern donkey colt.” The star lies close to the ecliptic and can be occulted by the Moon and planets.
- Tegmine– ζ Cancri (Zeta Cancri): Tegmine is a traditional name meaning “the shell of the crab”. Zeta Cancri is a multiple star system containing at least four stars that we know of. Collectively, the system has a magnitude of 4.67. There are two binary stars in the system. The first pair of binary stars is Zeta-1, consisting of Zeta Cancri A and Zeta Cancri B. The second pair, Zeta-2 is made up of stars Zeta Cancri C and Zeta Cancri D.
- ι Cancri (Iota Cancri): This is a binary star system consisting of a brighter white giant star and a main sequence star, both lying around 330 light years from Earth.
- λ Cancri(Lambda Cancri): Lambda Cancri is a main sequence dwarf star, located approximately 419 light years from Earth. The star has an apparent magnitude of 5.92.
- ξ Cancri(Xi Cancri): Xi Cancri, also sometimes called Nahn, is a binary star system located about 381 light years from Earth. The primary star, Xi Cancri A, has a magnitude of 5.16. Its companion, XI Cancri B, is only 0.1 arc seconds away from the primary star.
- ρ Cancri(Rho Cancri): Also called 55 Cancri, Rho Cancri is a double star system which lies relatively close to Earth at only 41 light years away. The system is made up of a yellow main sequence dwarf star and red dwarf of magnitude 13. Rho Cancri made headlines when astronomers confirmed that five exo-planets orbit the primary star, 55 Cancri A. This extra-solar system is one of four known planetary systems to have at least five planets.
Deep Sky Objects
The crab constellation is home to two Messier objects and several galaxies and is even the location of two supernovae.
- Beehive Cluster: Also known as Praesepe, Messier 44, M44, NGC 2632, and Cr 189, this open star cluster is situated about 590 light years from Earth. With an apparent magnitude of 3.7, it is definitely the standout feature of the constellation Cancer. It has been known since ancient times. The Beehive Cluster is clearly visible to the naked eye and is beautiful to behold through binoculars and small telescopes with a wide field of view. In fact, it was one of the first objects Galileo observed with his telescope, noting around 40 stars.
The Beehive Cluster contains over a thousand stars. Most of the stars within the cluster are red dwarfs, followed by Sun-like stars. It is relatively young compared to the age of the Solar System, estimated to be around 600 – 700 million years old.
- Messier 67(also known as M67 and NGC 2682): The second Messier object in Cancer is M67; an open cluster 2600 light years from Earth. M67 was first discovered in 1779 by the German astronomer Johann Gottfried Koehler. The cluster has an overall apparent magnitude of 6.1. It is smaller and denser than the Beehive cluster, containing over 200 stars. At least 100 of these stars are quite Sun-like.
The cluster is always being studied and observed by professional astronomers. Almost all of its stars are the same age and equally spaced from one another, but not all of the stars are at the same stage in their stellar evolution.
NGC 2682 is also one of the oldest known open clusters, with estimates of its age as high as 5 billion years old.
- NGC 2775: Also called Caldwell 48, NGC 2775 is a spiral galaxy found in Cancer approximately 55.5 million light years from Earth. The 11.3 magnitude galaxy was discovered by Sir William Herschel in 1783. Its many spiral arms and few HII regions suggest there was recent star-forming activity within the galaxy.
- NGC 2535 & NGC 2536: These two spiral galaxies in Cancer are seen interacting with each other. NGC 2535 is a magnitude 16.9 unbarred spiral. NGC 2536 is a barred spiral galaxy and has a magnitude of 14.6.
- NGC 2500: This barred spiral galaxy was discovered in the 18th Century by Sir William Herschel. It is located some 33 million light years away. It has an apparent magnitude of 12.2 and can be observed with a medium sized amateur telescope.
- NGC 2608: NGC 2608 is a barred spiral galaxy about 93 million light years from Earth. The galaxy is approximately 62,000 light years across, and has an apparent magnitude of 13.01.
- Other Deep Space Objects: Cancer is the location of two relatively recent supernovae. SN 1920A reached its peak magnitude of 11.7 in December 1920. In May 2001, SN 2001bg was observed in Cancer, peaking at magnitude 13.7.
Another object of interest within the borders of Cancer is OJ 287. It is a BL Lacertae object, a type of active galactic nucleus located 3.5 billion light years away. It is home to one of the largest known central supermassive black holes, with a mass of 18 billion solar masses.
Exo-planets
Cancer is a bust constellation for extrasolar planets. Ten stars within the constellation are known to host exoplanets. Rho Cancri or 55 Cancri is one such star; a binary system about 40.9 light-years from Earth. The primary star of the system, 55 Cancri A, is a yellow dwarf star with five planets in its orbit. Four of the planets orbiting 55 Cancri A are gas giants, and the fifth is a low mass planet which may either be a carbon filled planet, or a water planet.
Three exo-planets were discovered orbiting the Sun-like star YBP 1194 in the open cluster M67.
Cancer is also home to one of the most famous exo-planets, 55 Cancri e. The planet has been described as a super-Earth and as a ‘diamond world’, due to observations at the time that hinted at large quantities of carbon in its interior. Astronomers also believed that that the planet had a graphite surface concealing a large layer of diamond. Further studies revealed that the amount of carbon present is far less than scientists originally thought, but also showed that the planet is scorching hot and may be producing a sort of lava.
Meteor Showers
The only meteor shower associated with the constellation Cancer is the Delta Cancrids; a shower of medium strength seen from December 14 to February 14. Only four meteors per hour are expected at the shower’s peak on January 17 each year. Astronomers are uncertain of the shower’s source.
Conclusion
Even though Cancer is the dimmest of the constellations of the zodiac, there are several deep sky objects worth observing with binoculars or a small telescope. The Beehive open cluster alone makes a trip over to the crab constellation worth your while.
Clear skies!