The Cancer Constellation

Posted on Friday, 8 December, 2017Friday, 8 December, 2017 by Zheluo Cai

The Cancer Constellation.

Cancer is one of the twelve constellations of the zodiac. Its name is Latin for crab and it is commonly represented as one. Cancer is a medium-size constellation with an area of 506 square degrees that is bordered by Gemini to the west, Lynx to the north, Leo Minor to the northeast, Leo to the east, Hydra to the south, and Canis Minor to the southwest. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 10 sides. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 07^h 55^m 19.7973^s and 09^h 22^m 35.0364^s, while the declination coordinates are between 33.1415138° and 6.4700689°. 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 is the dimmest of the zodiacal constellations, having only two stars above the fourth magnitude, with its brightest star Beta Cancri or Altarf having an apparent magnitude of 3.5. Located 290 ± 30 light-years from Earth, it is a binary star system; its main component an orange giant of spectral type K4III. An ageing 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 a luminosity 53 times that of the Sun. 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.

Ten star systems have been found to have planets. Rho-1 Cancri or 55 Cancri 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. YBP 1194 is a sunlike star in the open cluster M67 that has been found to have three planets.

Cancer is best known among stargazers as the home of Praesepe (Messier 44), an open cluster also called the Beehive Cluster, located right in the centre of the constellation. Located 577 light-years from Earth, it is one of the nearest open clusters to our Solar System. M 44 contains about 50 stars, the brightest of which are of the sixth magnitude. Epsilon Cancri is the brightest member at magnitude 6.3. Praesepe is also one of the larger open clusters visible; it has an area of 1.5 square degrees or three times the size of the full Moon. The smaller, denser open cluster Messier 67 is 2500 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. NGC 2775 is a spiral galaxy, approximately 55.5 million light years from Earth. It has an apparent magnitude of 11.03. The galaxy was discovered by William Herschel in 1783. It has multiple spiral arms with few HII regions, which means there was recent star-forming activity occurring in it. NGC 2500 is a barred spiral galaxy, about 33 million light years distant, with an apparent magnitude of 12.2. It has an H II nucleus. The galaxy was discovered in the late 18th century by Friedrich Wilhelm Herschel. It belongs to the NGC 2841 group of galaxies, along with NGC 2537, NGC 2541, and NGC 2552 in the constellation Lynx.

Two supernovae were observed in the area in recent history. SN 1920A peaked at magnitude 11.7 in December 1920 and SN 2001bg was spotted in May 2001 and later peaked at magnitude 13.7.

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.