Tell Me About Cancer, the Crab

Cancer is one of the twelve constellations of the zodiac. For those who put any stock in astrology*, if your birthday lies between June 22nd and July 22nd this is your sign. That's all the astrology you're getting in this post: we're concentrating on reality, here, and we'll also be focusing on the asterism rather than the constellation.

How to find it

Cancer is visible in the Northern Hemisphere in early spring, looks almost entirely nothing like a crab, and has no particularly bright stars so it's a tricky one to find even with clear, dark skies. You could try a planetarium app, or the following might give you half a chance:

Find Cancer in the Sky by T. Briggs is licensed under CC BY-NC-SA 4.0

With reference to the diagram above:

1. Find Polaris (the "pole star", a really good star to learn how to find if you want to be able to visually navigate the night sky).
2. Find Ursa Major (The Great Bear), known as "the plough" or "the big dipper" to many.
3. The most famous part of Ursa Major is marked out with brighter lines in the diagram. Three more stars from that constellation have been added**.
4. Find the furthest of these three stars from the main constellation***
5. Draw an imaginary straight line from Polaris that passes through this star, and carry it on for a bit more than the same distance again.
6. You should now be somewhere in the general vicinity of Cancer.

The Stars in Cancer

There are 121 stars within the boundaries of Cancer, constellation, but we'll concentrate on the ones that form the asterism itself (and a couple of bonus features).

The Stars in Cancer by T. Briggs is licensed under CC BY-SA 4.0

α Cancri

Also known as "Acubens," Alpha Cancri is just about visible to the naked eye in perfect conditions, and is actually a quadruple star system made up of two white main-sequence white stars orbiting each other at about 5AU and two much smaller stars (possibly red dwarfs) orbiting at about 600AU from the centre of the system. It's about 173 light years away.

β Cancri

Also known as "Tarf," Beta Cancri is the brighest star in the Cancer asterism and is actually a binary system made up of an orange giant star with a red dwarf star orbiting about 2,600AU away. In 2014 it was discovered to have an exoplanet 7.8 times the mass of Jupiter, and orbiting at a distance of around 1.7AU.

δ Cancri

Assellus Australis ("Southern Donkey"), Arkushanangarushashutu**** (Ancient Babylonian for "the southeast star in the crab"), or Delta Cancri, is Cancer's second brightest star. It's an orange giant about 131 light years away from us.

Bonus: The reddest star in the sky, X Cancri is very close to Delta Cancri. So close, in fact, that you can get them both in the same view if you're using binoculars. X Cancri is about 2,900 light years away from us.

ɣ Cancri

Gamma Cancri, also known as Asellus Borealis, is another binary system. It's about 181 light years away and the primary is a white subgiant.

ɩ Cancri

Iota Cancri, or "Decapoda," is a double star (assumed to be binary, but not yet confirmed). It's about 330 light years from Earth. The primary is a yellow giant; the secondary a white dwarf. Due to the composition of the primary star it is assumed that there is an as-yet undiscovered, further evolved, white dwarf in the system, closer to the primary and feeding it with material.

Bonus Objects

There are two Messier objects to be found close to main stars of the Cancer asterism: M44 and M67.

M44

Known by the slightly more interesting name "The Beehive Cluster," M67 is an open cluster containing more than a thousand stars. It is about 610 light years away more than 23 light years across, and about 600 million years old. In the right location and with keen eyesight it is possible to see with the naked eye.

M67

M67 is a particularly old (about 4 billion years) open cluster about two and a half thousand light years away. It measures about 20 light years across and consists of more than 500 stars. It's near Alpha Cancri and possible to view with the naked eye if your eyes are REALLY good, the sky is REALLY clear, and your surroundings are REALLY dark.

 

* I am not such a person, but I will concede that the industry has done a lot to make a few constellations a bit more famous.

** These three stars form the Great Bear's Behind, if you get my drift. This is still not the entire asterism. Maybe I'll do a post about the entire Ursa Major one day.

*** This star is called "Omicron Ursae Majoris".

**** Bless you. This lovely collection of letters puts Delta Cancri at the number one spot in the "longest name of a star" chart.