Arcturus
Arcturus is an orange star, cooler than the Sun, in the constellation Boötes. It is a giant: a star that has used up the hydrogen in its core and swollen up.
How far
37 light years
The light you see from Arcturus tonight set off about 37 years ago. Everything we know about it is that much out of date.
How bright
120 Suns
It gives out 120 times as much visible light as the Sun. From the Earth it is the third brightest star in the night sky. It is one of the brightest stars in the whole sky, easy to see even from a town.
Astronomers put its brightness in our sky at magnitude −0.05. Smaller numbers are brighter, and the faintest stars the eye can see are about 6.
How hot
About 4,200 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.
ArcturusThe Sun
In your sky now
Working it out…
Its names
- Its own name
- Arcturus
- By constellation (Bayer)
- Alpha Boötis
- Hipparcos catalogue
- HIP 69673
- Henry Draper catalogue
- HD 124897
- Spectral type
- K2IIIp
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.