Cancer (space science)
Cancer is a constellation in the night sky, recognized for its representation of a crab, and is part of the zodiac. It is best observed in the Northern Hemisphere during March, although it remains visible from December to June. Positioned between the constellations Leo and Gemini, Cancer is relatively dim and is more easily viewed with a telescope. The brightest star in this constellation is beta Cancri, also known as Al Tarf, followed by alpha Cancri, or Acubens.
Cancer is notable not only for its stars but also for containing two Messier objects—M44, known as the Beehive Cluster, and M67. The constellation has deep historical roots in mythology; in Greek tales, it is associated with Hercules and his encounter with a crab sent by Hera during his labors. This constellation serves as a point of interest for astronomers, who study its stars and deep-sky objects, including the binary star system 55 Cancri, which is home to five exoplanets. Although these planets are similar to those in our solar system, they are not located in the habitable zone, making them unlikely to support life.
Cancer (space science)
FIELDS OF STUDY: Astronomy; Observational Astronomy
ABSTRACT: Cancer is a constellation and part of the zodiac. This constellation represents a crab, and the word cancer means "crab" in Latin. The star pattern in the constellation resembles an upside-down Y. The constellation is located between the other zodiacal constellations Leo and Gemini. Cancer is the dimmest constellation in the zodiac. It includes a number of stars and other objects, including stars clusters and a planetary system. The crab in the constellation is linked to a crab from Greek mythology that was killed while fighting Hercules. Scientists study constellations such as Cancer to learn about their stars, deep sky objects, and exoplanets. Cancer contains a planetary system that scientists are studying because it has some similarities to Earth’s solar system.
Constellations and the Zodiac
Cancer is a constellation that is interpreted to be in the shape of a crab—hence its name, which means "crab" in Latin. It represents the crab that, according to ancient Greek mythology, Heracles (romanized as Hercules) fought during his twelve labors.
For thousands of years, people from many different cultures have identified constellations in the night sky. Often, constellations are related to mythologies, stories, or religions. The International Astronomical Union (IAU) recognizes eighty-eight official constellations. The Cancer constellation has been recognized for thousands of years, though it is faint and hard to see, with only two stars above the fourth magnitude of brightness. Its shape resembles an upside-down Y.
Cancer is also part of the zodiac, which is a group of thirteen constellations. Ancient cultures believed this group of constellations was particularly important because the sun seemed to pass through them throughout the year, traveling along an imaginary line called the ecliptic, as a result of Earth's orbit around the sun. People remain interested in the constellations of the zodiac because they are well-known constellations and some of them are easily recognized.
Attributes of Cancer
The Cancer constellation is best seen in the Northern Hemisphere during March, though it remains visible between December and June. When viewed from Earth, it appears to be located between the zodiacal constellations Leo and Gemini. Other constellations located close to Cancer include Hydra, Lynx, and Canis Minor. People who want to view Cancer can first look for Regulus, the brightest star in the Leo constellation, and the stars Castor and Pollux from the Gemini constellation. If a line is drawn between Regulus and the twin stars, Cancer will be in the middle. However, Cancer is very dim and is best viewed with a telescope.
Cancer is made up of a number of different stars and objects. The brightest star in the constellation is beta Cancri, also known as Al Tarf, which is an orange star located approximately 290 light years from Earth. This star is in the leg of the crab and has a magnitude of 3.59. Another main star in the constellation is alpha Cancri, also known as Acubens. Even though alpha Cancri is only the fourth-brightest star in the constellation, with a magnitude of about 4.2, it was most likely named the alpha star because it represents one of the crab’s claws. Iota Cancri is the star that represents Cancer’s other claw.
The constellation Cancer also contains two Messier objects, Messier 44 (M44) and Messier 67 (M67). Messier objects are 110 deep-sky objects, such as galaxies, star clusters, and nebulae, that were catalogued by French astronomer Charles Messier (1730–1817) in the 1700s. Object M44 is a cluster of stars sometimes called the Beehive Cluster or Praesepe, which means "manger" or "crib" in Latin. The Beehive Cluster is made up of approximately one thousand stars and somewhat resembles a swarm of bees. It is the only part of Cancer that is easily visible with the naked eye or with binoculars. The Beehive is an open cluster, which means that its stars are gravitationally bound together and were formed from the same cloud or nebula. It is one of the closest open clusters to Earth and is located about six hundred light-years away.
M67 is another open cluster. It contains more than five hundred gravitationally bound stars. The stars in this cluster are approximately four billion years old, which is quite old for a star cluster, as clustered stars usually separate from one another over time. This star cluster is approximately 2,700 light-years from Earth and is roughly 12 light-years across.
Cancer in Mythology
The Cancer constellation has its origins in ancient cultures. Most of the constellations recognized by the IAU were created by humans thousands of years ago. Some ancient cultures used constellations to tell stories about their religions. Others used them to identify seasons. Since the stars appear in the same part of the sky at the same time each year, farmers could observe the sky to find out when they should plant and harvest their crops.
Different stories are told about constellations over time. Many of the modern constellations are still related to Greek and Roman mythology. The Greek astronomer Ptolemy (ca. 100–170) wrote about the Cancer constellation. The pattern was most likely identified before his lifetime.
In Greek mythology, Hercules encountered Cancer the crab during the twelve labors he had to undertake. One of his labors was to fight the sea serpent Hydra. While the two fought, Zeus’s wife, Hera, sent a crab to help defeat Hercules. In one version of the legend, Hercules kicked the crab so hard that it flew into the sky. In another, Hercules crushed the crab, and Hera placed it among the stars as a reward for its efforts. The dimness of its stars was said to be because it failed to defeat Hercules.
Studying Cancer and Other Constellations
In the past, humans used the constellations to tell stories and to track the passage of seasons. Scientists use the constellations for other reasons. Since scientists can observe so many different stars and deep-sky objects, they use the constellations to identify and name some of the stars in the sky. For example, the stars beta Cancri and alpha Cancri received their names because they are part of the Cancer constellation.
Scientists also study Cancer and other constellations because they want to learn more about stars, deep-sky objects, and exoplanets. One binary star inside the Cancer constellation, called 55 Cancri, is orbited by five exoplanets. This star system is of interest to scientists because it is similar to Earth’s solar system in a number of ways. The central star, 55 Cancri A, is about the same age and has about the same mass as Earth’s sun. The planets farthest away from the star are giants, similar to the gas giants in Earth’s solar system. However, these five exoplanets are larger than the planets in Earth’s solar system. Furthermore, four of them are very close to 55 Cancri A, while the fifth is very far away. None of the planets are in the star’s habitable zone, which is the distance at which a planet could contain liquid water, considered necessary to support life. The planets are therefore considered unlikely to hold life-forms of any kind.
PRINCIPAL TERMS
- constellation: an apparent pattern of stars, identified by humans, that can be seen in the night sky from Earth.
- International Astronomical Union: a recognized and authoritative professional organization for astronomers, founded in 1919 and based in Paris, France.
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