Next to the head of the Hydra, sandwiched between the bright and distinctive figures of Leo, the lion, and Gemini, the twins, lies the very faint constellation Cancer (Latin for crab). It is one of the dimmest constellations in the sky, half concealed (like its earth-bound counterpart) in the deep sand of the cosmos. But if you have a nice dark night, and you look carefully enough, you will see it. The stars are faint, but they do form the outline of a large crab, and people have seen it as such since the birth of Christ.
Being an ancient constellation, Cancer is associated with not one but two myths. The first is the tale of Hercules. One of the labours of Hercules imposed by the gods was the killing of Hydra, the sea monster. As Hercules was battling the Hydra, and beginning to get the upper hand, the goddess Hera sent a crab to harass and distract Hercules. But Hercules stepped on the crab, and crushed it. The gods put both the Hydra and the Crab beside each other in the sky to commemorate the occasion.
The other myth involves the two eyes of the crab, the stars Asellus Borealis, and Asellus Australis, Latin terms for the Northern Donkey, and the Southern Donkey. In between the two donkeys is a patch of blurry light called Praesepe, the manger (M44), from which the donkeys are feeding. The beasts were put in the sky to honour the animals ridden by the Olympian gods in their great battle with the Titans. It is said the loud braying of the donkeys confused the Titans (who had never heard such a noise), and helped win the battle for the Olympians.
The stars of Cancer may be faint, but the ancients considered them important enough to put names to five of them.
Alpha Cancri is named Acubens, according to R. H. Allen derived from the Arabic "Al Zubanah", the claws. Although designated Alpha, it is only the fourth brightest star in the constellation. Acubens is a binary star system, with the primary star an A5V white main sequence star with a magnitude of 4.2. The companion is a faint 11th magnitude star, and there is evidence that both stars may be binaries themselves, composing a quadruple star system, all residing about 174 light years away.
Beta Cancri is named Altarf, Arabic for the end. At magnitude 3.54 it is the brightest star in the constellation, classified as a K4III orange giant. In May, 2014, an enormous gas planet eight times larger than Jupiter was discovered in orbit around Altarf. The planet is designated Beta Cancri B, and the system is about 290 light years away.
Gamma Cancri is named Asellus Borealis, Latin for the northern donkey feeding at the Praesepe (manger). It is an A1IV white subgiant, magnitude 4.66, 158 light years away.
Delta Cancri is named Asellus Australis, Latin for the southern donkey, also feeding at the manger. At magnitude 3.94, it is the second brightest star in the constellation. Classified as a G9III yellow/orange giant, it is about 136 light years away.
The last of the named stars is Zeta Cancri, known as Tegmine, Latin for shell of the crab, a quadruple star system, consisting of two binary pairs orbiting each other. The brighter, dominant pair are both F9V yellow main sequence stars, not too dissimilar to our own Sun. At a distance of about 84 light years, the system has a combined magnitude of 4.67.
Possibly the most interesting and exciting star in the constellation is designated 55 Cancri (Rho01 Cancri, HD 75732). It is a K0V yellow/orange main sequence star, slightly smaller and cooler than our Sun. With a magnitude of 5.95, it lies at the very limits of naked eye visibility, but is quickly found in binoculars or a small scope. After monitoring minute spectroscopic fluctuations in the star's light for more than eighteen years, scientists were able to confirm no less than five planets in orbit around this star, with at least one of these planets occupying the habitable zone, where temperatures allow the existence of liquid water. Unfortunately this planet, 55 Cancri f, is a gas giant unlikely to harbour life. There is one rocky planet, 55 Cancri e, about eight times larger than Earth, but it orbits very close to the star and is also unlikely to support any kind of life as we know it. Nevertheless, the system is only 40 light years away, quite close in astronomical terms, and one to keep an eye on.
Besides Beta Cancri and 55 Cancri, there are ten other stars in Cancer hosting planetary systems, but they are all very far away and too dim to be seen with the naked eye, and all their planets are gas giants. For more information on these and other extrasolar planets, visit NASA's New Worlds Atlas, and The Open Exoplanets Catalogue.
M44 (NGC 2632), is known by the traditional name Praesepe, (Latin for manger,) and in modern times as the Beehive Cluster. It is the first thing you notice when you look at Cancer, easily visible to the naked eye and the most prominent feature of the constellation. It is an open star cluster about 575 light years away with an overall magnitude of 3.7, and an exceptional sight through a small telescope.
Cancer contains another open star cluster, M67 (NGC 2682), much farther away and fainter at a distance of 2,700 light years and a magnitude of 6.1.
The beauty of NGC 2623 belies the cosmic scale of the violence that's occurring as hundreds of billions of stars crash into each other in this merger of galaxies. Fortunately, all this chaos is safely 300 million light years away. NGC 2623 has a magnitude of 13.2.
Cancer is one of the thirteen constellations that reside in the ecliptic (the path the Sun, Moon and planets appear to take across the sky), and one of the twelve traditional constellations and signs of the astrological zodiac. In tropical (traditional) astrology the Sun is in Cancer from June 22 to July 22. In sidereal (modern) astrology the Sun is in Cancer from July 21 to August 9.
The Tropic of Cancer is an imaginary line circling the globe at 23.5 ° north latitude. It is where the Sun is exactly overhead at the moment of the summer solstice, June 22, a date when the Sun has traditionally resided in the constellation Cancer. Now, due to precession (the wobble of Earth's axis) the Sun is actually in Taurus on June 22. No-one has suggested the line be renamed The Tropic of Taurus however, because in approximately 20,000 years (the length of a precession cycle), the Sun will have passed through all the 13 constellations of the ecliptic (the path the Sun, Moon and planets follow amongst the stars), and be right back in Cancer again anyway.
|Winter: Orion Canis Major Canis Minor Monoceros Lepus Eridanus Taurus Auriga Camelopardalis Lynx Gemini Cancer|
|Spring: Hydra Sextans Crater Corvus Leo Leo Minor Ursa Major Ursa Minor Canes Venatici Coma Berenices Virgo Bootes|
|Summer: Draco Corona Borealis Hercules Ophiuchus Serpens Libra Scorpius Sagittarius Scutum Aquila Sagitta Vulpecula Lyra Cygnus|
|Autumn: Andromeda Perseus Pegasus Cassiopeia Cepheus Cetus Lacerta Delphinus Equuleus Capricornus Aquarius Pisces Aries Triangulum|
|Southern Skies: Centaurus Crux Lupus Corona Australis Piscis Australis Sculptor Tucana Fornax Dorado Columba Puppis Carina Vela|