V838 Monocerotis (V838 Mon) is a red variable star in the constellation Monoceros about 20,000 light years (6 kpc) from the Sun, and possibly one of the largest known stars for a short period following the outburst. The previously unknown star was observed in early 2002 experiencing a major outburst. Originally believed to be a typical nova eruption, it was then realized to be something completely different. The reason for the outburst is still uncertain, but several conjectures have been put forward, including an eruption related to stellar death processes and a merger of a binary star or planets.
The remnant is evolving rapidly. As of 2009 it has increased in both temperature (now class M6.3I and 3,270K) and luminosity (now 15,000 times solar), but decreased in radius (now 380 times the sun) although the ejecta continues to expand. The opaque ejected dust cloud has completely engulfed the B-type companion.
Read more about V838 Monocerotis: Outburst, Progenitor Star, Light Echo, Models