V838 Monocerotis

V838 Monocerotis (Nova Monocerotis 2002) is a cataclysmic binary star in the constellation Monoceros about 19,000 light years (6 kpc) from the Sun. The previously unremarked star was observed in early 2002 experiencing a major outburst, and was one of the largest known stars for a short period following the outburst. Originally believed to be a typical nova eruption, it was then identified as the first of a new class of eruptive variables known as luminous red novae. The reason for the outburst is still uncertain, but is thought to have been a merger of two stars within a triple system.

The eruption occurred on one of two B3 main sequence stars in a close binary orbit. The erupting star appeared as an unusually cool supergiant and for a while engulfed its companion. By 2009 the temperature of the supergiant had increased (since 2005) to 3,270 K and its luminosity was 15,000 times solar (L☉), but its radius had decreased to 380 times that of the Sun (R☉), although the ejecta continues to expand.