Tau Octantis
Tau Octantis is an orange star, cooler than the Sun, in the constellation Octans. It is a giant: a star that has used up the hydrogen in its core and swollen up.
How far
490 light years
The light you see from Tau Octantis tonight set off about 490 years ago. Everything we know about it is that much out of date.
At this distance the figure could be out by a tenth or so.
How bright
120 Suns
It gives out 120 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 5.50. 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.
Tau OctantisThe 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)
- Tau Octantis
- Hipparcos catalogue
- HIP 115836
- Henry Draper catalogue
- HD 219765
- Spectral type
- K2III
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.