WR 21a

WR 21a is an eclipsing binary star 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 open cluster and likely to be an ejected member.

The distance of WR 21a was not definitively known until Gaia mission. 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.