Castor
Castor is a white star, hotter than the Sun, in the constellation Gemini. It is in the long, steady part of its life, making helium from hydrogen in its core, as the Sun is.
How far
51 light years
The light you see from Castor tonight set off about 51 years ago. Everything we know about it is that much out of date.
How bright
49 Suns
It gives out 49 times as much visible light as the Sun. It is one of the hundred brightest stars in the night sky. It is easy to see by eye on a clear night.
Astronomers put its brightness in our sky at magnitude 1.58. Smaller numbers are brighter, and the faintest stars the eye can see are about 6.
How hot
About 9,700 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.
CastorThe Sun
In your sky now
Working it out…
Its names
- Its own name
- Castor
- By constellation (Bayer)
- Alpha Geminorum
- Hipparcos catalogue
- HIP 36850
- Henry Draper catalogue
- HD 60179
- Spectral type
- A2Vm
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.