Omicron-2 Canis Majoris

Omicron-2 Canis Majoris is a blue-white star, far hotter than the Sun, in the constellation Canis Major. It is a supergiant: a very heavy star, swollen to an enormous size and near the end of its life.

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A drawing in the star's own colour, worked out from its measured colour index. It is not a photograph.

How far

2,800 light years

The light you see from Omicron-2 Canis Majoris tonight set off about 2,800 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 Everything we know about it is that much out of date.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

39,000 Suns

It gives out 39,000 times as much visible light as the Sun. It is easy to see by eye on a clear night.

Astronomers put its brightness in our sky at magnitude 3.02. Smaller numbers are brighter, and the faintest stars the eye can see are about 6.

How hot

About 17,000 K

That is the temperature of its surface, worked out from its spectral type, B3Ia, 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.

Omicron-2 Canis MajorisThe 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): Omicron-2 Canis Majoris
Hipparcos catalogue: HIP 33977
Henry Draper catalogue: HD 53138
Spectral type: B3Ia

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.