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21 Cancri, HD70734, HIP41163, HR3290

21 Cancri is a red giant star that can be located in the constellation of Cancer. The description is based on the spectral class. The star can be seen with the naked eye, that is, you don't need a telescope/binoculars to see it.

21 Cancri's Alternative Names

The Id of the star in the Yale Bright Star Catalogue is HR3290. HIP41163 is the reference name for the star in the Hipparcos Star Catalogue. The Id of the star in the Henry Draper catalogue is HD70734.

Flamsteed designations are named after the creator, Sir John Flamsteed. Sir John numbered the stars in the constellation with a number and the latin name, this star's Flamsteed designation is 21 Cancri with it shortened to 21 Cnc.

BD number is the number that the star was filed under in the Durchmusterung or Bonner Durchmusterung, a star catalogue that was put together by the Bonn Observatory between 1859 to 1903. The star's BD Number is BD+11 1830.

More details on star alternative names can be found at Star Names .

Location of 21 Cancri

The location of the star in the night sky is determined by the Right Ascension (R.A.) and Declination (Dec.), these are equivalent to the Longitude and Latitude on the Earth. The Right Ascension is how far expressed in time (hh:mm:ss) the star is along the celestial equator. If the R.A. is positive then its eastwards. The Declination is how far north or south the star is compared to the celestial equator and is expressed in degrees. For 21 Cancri, the location is 08h 23m 55.21 and +10° 37` 55.6 .

Proper Motion of 21 Cancri

All stars like planets orbit round a central spot, in the case of planets, its the central star such as the Sun. In the case of a star, its the galactic centre. The constellations that we see today will be different than they were 50,000 years ago or 50,000 years from now. Proper Motion details the movements of these stars and are measured in milliarcseconds. The star is moving -24.48 ± 0.48 miliarcseconds/year towards the north and 1.05 ± 0.80 miliarcseconds/year east if we saw them in the horizon. The Radial Velocity, that is the speed at which the star is moving away/towards us is 4.03000 km/s with an error of about 0.15 km/s .

Physical Properties (Colour, Temperature) of 21 Cancri

21 Cancri has a spectral type of M2III. This means the star is a red giant star. The star has a B-V Colour Index of 1.5 which means the star's temperature has been calculated using information from Morgans @ Uni.edu at being 3,989 Kelvin.

21 Cancri Radius has been calculated as being 40.10 times bigger than the Sun.The Sun's radius is 695,800km, therefore the star's radius is an estimated 27,904,553.19.km. If you need the diameter of the star, you just need to multiple the radius by 2. However with the 2007 release of updated Hipparcos files, the radius is now calculated at being round 32.00. The figure is derived at by using the formula from SDSS and has been known to produce widely incorrect figures.

21 Cancri Apparent and Absolute Magnitudes

21 Cancri has an apparent magnitude of 6.11 which is how bright we see the star from Earth. Apparent Magnitude is also known as Visual Magnitude. If you used the 1997 Parallax value, you would get an absolute magnitude of -1.56 If you used the 2007 Parallax value, you would get an absolute magnitude of -1.07. Magnitude, whether it be apparent/visual or absolute magnitude is measured by a number, the smaller the number, the brighter the Star is. Our own Sun is the brightest star and therefore has the lowest of all magnitudes, -26.74. A faint star will have a high number.

Distance to 21 Cancri

Using the original Hipparcos data that was released in 1997, the parallax to the star was given as 2.93 which gave the calculated distance to 21 Cancri as 1113.19 light years away from Earth or 341.30 parsecs. It would take a spaceship travelling at the speed of light, 1113.19 years to get there. We don't have the technology or spaceship that can carry people over that distance yet.

In 2007, Hipparcos data was revised with a new parallax of 3.67 which put 21 Cancri at a distance of 888.73 light years or 272.48 parsecs. It should not be taken as though the star is moving closer or further away from us. It is purely that the distance was recalculated. 21 Cancri brightness ranges from a magnitude of 6.228 to a magnitude of 6.180 over its variable period. The smaller the magnitude, the brighter the star. Its variable/pulsating period lasts for 0.0 days (variability).

Source of Information

The source of the information if it has a Hip I.D. is from Simbad, the Hipparcos data library based at the University at Strasbourg, France. Hipparcos was a E.S.A. satellite operation launched in 1989 for four years. The items in red are values that I've calculated so they could well be wrong. Information regarding Metallicity and/or Mass is from the E.U. Exoplanets. The information was obtained as of 12th Feb 2017.

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21 Cancri Facts

Visual Facts


Primary / Proper / Traditional Name21 Cancri
Alternative NamesHD 70734, HIP 41163, HR 3290, 21 Cnc, BD+11 1830
Spectral TypeM2III
Multiple Star SystemYes
Star TypeGiant Star
Colour red
GalaxyMilky Way
ConstellationCancer
Absolute Magnitude-1.56 / -1.07
Visual / Apparent Magnitude6.11
Naked Eye VisibleYes - Magnitudes
Right Ascension (R.A.)08h 23m 55.21
Declination (Dec.)+10° 37` 55.6
Galactic Latitude25.32 degrees
Galactic Longitude213.68 degrees
1997 Distance from Earth2.93 Parallax (milliarcseconds)
 1113.19 Light Years
 341.30 Parsecs
2007 Distance from Earth3.67 Parallax (milliarcseconds)
 888.73 Light Years
 272.48 Parsecs
Proper Motion Dec.-24.48 ± 0.48 milliarcseconds/year
Proper Motion RA.1.05 ± 0.80 milliarcseconds/year
B-V Index1.50
Radial Velocity4.03 ± 0.15 km/s

Companions (Multi-Star and Exoplanets) Facts


Exoplanet CountNone/Unaware

Variable Star Details


Mean Variability Period in Days0.030
Variable Magnitude Range (Brighter - Dimmer)6.180 - 6.228

Estimated Facts


Calculated Effective Temperature3,989 Kelvin

Sources and Links


SIMBAD SourceLink

Multi-Star System

The star has been identified as being a multi-star system, one in which there is at least one star in close orbit to another star or two or more stars orbiting a central point. The stars may be of equal mass, unequal mass where one star is stronger than the other or be in groups orbiting a central point which doesn't necessarily have to be a star. More information can be found on my dedicated multiple star systems page. The source of the info is Simbad. The file is dated 2000 so any differences between this and any other source will be down to the actual source from where the information came from.


Proper Motion mas/yr
H.D. IdB.D. IdStar CodeMagnitudeR.A.Dec.SpectrumColourYear
70734+11 1830.0A6.300004.00000-22.00000K5Orange
B11.300001915

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