A guide to the constellation Cancer
A guide to Cancer and some of the best objects to observe within the constellation.
As a constellation, it has to be said that Cancer isn’t one of the most impressive.
Yes, it contains the open cluster M44, also known as the Beehive Cluster (pinpointed by the asterisk in our chart above) and there’s another fainter open cluster near Acubens (Alpha (α) Cancri) called M67, but the stars that form the Zodiacal Crab of Cancer are pretty faint.
Nevertheless, there are still exciting targets to observe in Cancer, including one star that hosts its own planetary system.
Read on for our guide to some of the best things to see in Cancer through your telescope.
What to observe in the constellation Cancer
The brightest star in Cancer isn’t Acubens as you might think, but Altarf (Beta (β) Cancri) which marks the southwest point of the constellation’s inverted Y-shaped pattern.
The name is from the Arabic ‘Al Tarf’ meaning ‘the End’, and it represents one of the Crab’s legs.
Altarf has a spectral type of K4III Ba1, a cool orange K4 class giant with abundances of barium (chemical symbol Ba).
Its effective temperature is 3,717˚C, which is much cooler than our Sun at 5,505˚C.
Located at a distance of 290 lightyears, Altarf shines at mag. +3.5.
Despite its diminutive status, it should be borne in mind that Altarf is still 61 times larger and 870 times more luminous than the Sun.
A faint red-dwarf companion accompanies the main star. Shining at 14th magnitude, it is estimated
to lie around 2,600 AU from Altarf; that’s around 65 times the Sun-Pluto distance.
The orbital period is estimated to be 76,000 years, but no evidence for any mutual orbital motion has been confirmed yet.
Altarf is also believed to have a planet. Discovered in 2014, this body is estimated to be nearly eight times the mass of Jupiter, orbiting the star once every 605 days at an average distance of 1.7 AU.
55 Cancri (ρ1 Cancri) is a mag. +5.9 star on the threshold of naked-eye visibility from a typical dark sky site.
The best way to locate it is to first look for mag. +4.0 Iota (ι) Cancri, marking the northern end of the inverted Y-shape of Cancer.
Two degrees west of Iota sits mag. +5.2 ρ2 Cancri, more likely to be seen with the naked eye than ρ1.
Using binoculars, our target star is the northernmost of a close pair of stars located two-thirds of the way from Iota towards ρ2.
This pairing is false as ρ1, at 40.3 lightyears, is just a fraction of the 894 lightyear distance of its ‘pair’, the mag. +6.3 53 Cancri.
55 Cancri is a true binary, consisting of a G-type star, 55 Cancri A, with a 13th magnitude red dwarf companion, 55 Cancri B.
The spectral type of 55 Cancri A is G8V, slightly cooler and redder than the G2V classification of our own Sun.
Physically, there are further similarities as 55 Cancri A has a diameter and mass equal to 96% of the Sun.
Its luminosity is around 59%, with a temperature of 5,165K compared to 5,778K for the Sun.
55 Cancri A’s age is difficult to determine, with estimates ranging between 7.4–12.7 billion years – far older than the Sun’s 4.6 billion years.
Most excitingly, the star has a planetary system consisting of five or possibly more planets, several within its habitable zone.
Any moons existing around them, if sizeable enough, could play host to simple microbial life.
In 2014 the International Astronomical Union (IAU) asked the public to help name certain exoplanets and their parent stars.
As a result, 55 Cancri A is also known as Copernicus and the five known planets, in order of distance from Copernicus, are Janssen (e), Galileo (b), Brahe (c), Harriot (f) and Lipperhey (d).
A message beamed to the system on 6 July 2003 is set to arrive in May 2044.
This guide originally appeared in the April 2018 and March 2022 issues of BBC Sky at Night Magazine.