Star Constellation Facts: Cancer, the Crab

Star Constellation Facts: Cancer, the Crab
Alexander Jamieson’s Celestial Atlas (1822)

Cancer (“crab”) is a zodiac constellations, meaning it lies within the ecliptic path that the Sun, Moon and planets travels in the sky each year. It is arguably the constellation that looks the least like the object it depicts, and is also the faintest of the 12 zodiacal constellations, with its brightest star, Al Tarf, just fourth magnitude.

Location: A Northern Constellation

Cancer is a northern constellation that can be seen by observers from between latitudes +90 and -60 degrees. The easiest way to find Cancer is to look for it between the two striking constellations of Gemini to the west, and Leo to the east. A number of other constellations also border Cancer, including Lynx to the north, and Hydra and Canis Minor to the south.

Best Seen: Winter

Cancer is a winter constellations that in the northern hemisphere is visible from late autumn to spring, although best seen in March. In the southern hemisphere, it can be viewed in the summer and autumn months.

Mythology: Represents a Crab

In Greek mythology, the constellation Cancer is associated with Heracles, and the giant crab his step-mother, the goddess Hera, sent to harass him during his fight with the many-headed Hydra. The crab was meant to kill Heracles but instead of killing the hero, it got crushed underfoot, and so Hera placed it in the heavens for its valiant efforts.

Cancer: The Crab

Shape: Inverted ‘Y’

Cancer is only the 31st biggest constellation, taking up an area of 506 sq/deg. As far as its shape goes, however, it looks nothing like a crab, but appears more like an inverted ‘Y’, representing the crab’s back. The famous Beehive Cluster (M44) located in the centre of the constellation also helps in identifying the constellation proper.

Meteor Showers: Delta Cancrids

Only one meteor shower, the Delta Cancrids, is associated with Cancer, with this long-duration shower active from December 14th to February 14th, and peaking on the 17th of January each year. Its maximum meteor count, however, rarely exceed 4-6 sightings per hour. The source of the meteors is unknown, but it has been suggested that meteors from this shower share a similar orbit to the asteroid 2001 YB5.

Notable Stars: None Above 4th Magnitude

Cancer is the faintest of the 12 zodiac constellations, with all its stars of magnitude 3.5 or fainter, and as such is often hard to identify.

The Constellation Cancer– Al Tarf (Beta Cancri) is an orange K-type giant star located about 290 light years distant, and with an apparent magnitude of 3.5 making it the constellation’s most luminous star. It is the primary component of a binary system whose red dwarf companion, Beta Cancri, is a faint, 14th magnitude star. The star’s name “Al Tarf”, is thought to derive from the Arabic meaning “the glance [of Leo]”.

– Asellus Australis (Delta Cancri) is an orange giant star located about 180 light years away, and with an apparent magnitude of 3.94, making it the second most luminous star in Cancer. It marks the location of the Beehive Cluster (M44, or Praesepe), but since it lies on the ecliptic, it can be occulted by the Moon, and somewhat infrequently, by the planets.

– Acubens (Alpha Cancri) is the fourth most luminous star in Cancer, located 174 light years distant and of magnitude 4.20. It is, in fact, a binary star system composed of white dwarf star that is 23 times as luminous as the sun, and a faint companion star of just 11th magnitude. Acubens marks the lower end of the crab’s left “leg”.

Notable Objects: Beehive Cluster

– Beehive Cluster (Praesepe, M44) is a beautiful open cluster around 600 million years old, and at about 577 light years distant is among the closest, most densely populated open clusters to Earth. It has a magnitude of 3.7, and contains at least 1,000 stars, of which about 63% are red dwarfs, with the remainder all Sun-like stars (F, G, K). M44 appears to the naked eye as a misty cloud, and it was also a familiar sight to the ancients, with Ptolemy (2nd Century AD) calling it “the nebulous mass in the breast of Cancer.”

– Messier 67 (M67) is one of the oldest known open clusters with an estimated age of between 3.2 and 5 billion years. It contains more than 100 Sun-like stars, and a large number of red giants, providing students of stellar evolution with a “snap-shot” of stars in different stages of evolution. Apart from 30 or so blue stragglers, nearly all the stars in the cluster are roughly of the same age, and located around 2,500 light years away. You can make M67 out with the naked eye, but since its stars are all magnitude 8 to 13, it looks more like a bright patch rather than a collection of stars.

Planets: 10+ Planets

Two stars with confirmed planets have thus far been discovered in Cancer, with 55 Cancri being the most notable with five planets. It is a suspected binary star, consisting of a yellow star and a red dwarf star, with all five planets orbiting the primary, yellow star (55 Cancri A). At least four of its planets are known to be gas giants, and the other a super-earth. The other star with known planets is YBP 1194, located in the open cluster M67.

Astrology: June 22 to July 22

In astrology, the Sun passes into Cancer on 21st June, coinciding with the summer solstice, the northern hemisphere’s longest day after which the days grow shorter. Due to precession, however, the summer solstice now occurs when the Sun enters Taurus, and does not enter Cancer until around one month later. Other astrological associations include the following:

Date of Birth: June 22nd to July 22nd
Sign Ruler: Moon
Element: Water
Birth Stone: Ruby
Metal: Silver
Color: White, Yellow
Characteristics: sympathetic, caring, intuitive, moody, defensive,
Compatibility: Pisces, Scorpio

Star Lore: Gateway for Incarnation

In ancient star lore, Cancer was known as the “Gate of Men”, since it was believed to be the portal through which souls descended from Heaven to take up their abodes in the bodies of new-born babies. Likewise, Capricorn was known as ‘the Gate of the Gods’, through which the souls of the dead ascended back into Heaven.

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