Sigma Ophiuchi
Sigma Ophiuchi is an orange star, cooler than the Sun, in the constellation Ophiuchus. It is a bright giant: a star that has left the steady part of its life and swollen up.
How far
900 light years
The light you see from Sigma Ophiuchi tonight set off about 900 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 Everything we know about it is that much out of date.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
1,200 Suns
It gives out 1,200 times as much visible light as the Sun. It can be seen by eye away from bright lights.
Astronomers put its brightness in our sky at magnitude 4.34. 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.
Sigma OphiuchiThe Sun
In your sky now
Working it out…
Its names
It has no name of its own. Its name by constellation is a Greek letter and the constellation it is in, a way of naming begun by Johann Bayer in 1603.
- By constellation (Bayer)
- Sigma Ophiuchi
- Hipparcos catalogue
- HIP 85355
- Henry Draper catalogue
- HD 157999
- Spectral type
- K3IIvar
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.