21 Cancri
21 Cancri is a red star, among the coolest, in the constellation Cancer. It is a giant: a star that has used up the hydrogen in its core and swollen up.
How far
890 light years
The light you see from 21 Cancri tonight set off about 890 years ago. Everything we know about it is that much out of date.
Distances this great are hard to measure, and this one could be out by a fifth or more.
How bright
230 Suns
It gives out 230 times as much visible light as the Sun. It can just be seen by eye from a really dark place.
Astronomers put its brightness in our sky at magnitude 6.11. Smaller numbers are brighter, and the faintest stars the eye can see are about 6.
How hot
About 3,800 K
That is the temperature of its surface, worked out from its colour, so it is a guide and not a measurement. It could be out by a few hundred degrees. The Sun's is 5,772 K. Hotter stars are bluer and cooler stars are redder, the same way a poker in a fire goes from red to white.
21 CancriThe Sun
In your sky now
Working it out…
Its names
It has no name of its own. Its name by constellation is a number and the constellation it is in, from the catalogue of John Flamsteed, the first Astronomer Royal.
- By constellation (Flamsteed)
- 21 Cancri
- Hipparcos catalogue
- HIP 41163
- Henry Draper catalogue
- HD 70734
- Spectral type
- M2III
Where this comes from
Distance, brightness, colour and spectral type are from the HYG star database, version 4.4, by David Nash, shared under a Creative Commons Attribution-ShareAlike 4.0 licence. Its distances are those measured by the Hipparcos satellite. The temperature is worked out here from the colour with the formula of Ballesteros (2012), and the words are written for this site.
To read more: Colour and brightness, How stars live and die, Measuring distance.