WR 21a is an

eclipsing binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

in the constellation Carina. It includes one of the most massive known stars and is one of the most massive binaries. WR 21a lies near the

Westerlund 2 Westerlund 2 is an obscured compact young star cluster (perhaps even a super star cluster) in the Milky Way, with an estimated age of about one or two million years. It contains some of the hottest, brightest, and most massive stars known. The ...

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, an ...

and likely to be an ejected member. The distance of WR 21a was not definitively known until

Gaia mission ''Gaia'' is a space observatory of the European Space Agency (ESA), launched in 2013 and expected to operate until 2025. The spacecraft is designed for astrometry: measuring the positions, distances and motions of stars with unprecedented preci ...

. There have been estimates from 2.85 kpc to around 8 kpc, with consequent uncertainties in the system luminosity. The larger distance was preferred because of consistency with the derived orbital parameters. Every 31 days and 16 hours the two stars in this system revolve around each other. The inclination of the orbit means that only very shallow eclipses are observed and the brightness dips by only about 0.05 magnitudes. There are also even smaller brightness variations attributed to the heartbeat effect where the closest passage of the stars in their eccentric orbits creates brightness changes as the two stars illuminate each other. There may also be tidally-excited oscillations producing further small variations. The colliding winds of the two stars produce extremely high temperatures and luminous x-ray emission. The system is also bright at radio wavelengths.

References

{{DEFAULTSORT:WR 21a Wolf–Rayet stars Spectroscopic binaries Carina (constellation) J10255650-5748435 O-type main-sequence stars