Kappa Hydrae
Kappa Hydrae is a blue-white star, far hotter than the Sun, in the constellation Hydra. It is a subgiant: a star just beginning to leave the long, steady part of its life.
How far
440 light years
The light you see from Kappa Hydrae tonight set off about 440 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 Everything we know about it is that much out of date.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
150 Suns
It gives out 150 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 5.07. Smaller numbers are brighter, and the faintest stars the eye can see are about 6.
How hot
About 16,000 K
That is the temperature of its surface, worked out from its spectral type, B4IV/V, with a standard table of how hot stars of each type are. It is a guide and not a measurement, and could be out by a tenth or so. For stars as hot as this, colour is a poor thermometer, because most of their light is ultraviolet, so the type is the better guide. 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.
Kappa HydraeThe 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)
- Kappa Hydrae
- Hipparcos catalogue
- HIP 47452
- Henry Draper catalogue
- HD 83754
- Spectral type
- B4IV/V
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 read from the spectral type with the table of Pecaut and Mamajek (2013), and the words are written for this site.
To read more: Colour and brightness, How stars live and die, Measuring distance.