WR 21a
WR 21a is a 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 may be an ejected member.
The distance of WR 21a is not definitely known. There have been estimates from 2.85 kpc to around 8 kpc, with consequent uncertainties in the system luminosity. The larger distance is preferred because of consistency with the derived orbital parameters, but it is still uncertain whether the star is nearer or further than Westerlund 2.
Every 31 days and 16 hours days the two stars in this system revolve around each other. The inclination of the orbit means that we do not see eclipses and the visual brightness of the system is quite stable.
The colliding winds of the two stars produce extremely high temperatures and luminous x-ray emission. The system is also bright at radio wavelengths.
