Bright Merger

For a star system in the constellation Monoceros, everything changed in a flash. V838 Monocerotis is about 20,000 light-years away, on the rim of the Milky Way galaxy. In January of 2002, it staged a dramatic outburst — it flared a million times brighter than the Sun. It faded for a few weeks, than flared twice more. This wasn’t like most stellar outbursts, though. It was brighter than a nova — an eruption on the surface of a small star — but not bright enough to be a supernova, which blasts a star to bits. And while novae and supernovae happen at the end of a star’s life, V838 appears to be at the start of its life. The most popular explanation for the outburst says that a star about eight times as massive as the Sun merged with a star that was lighter than the Sun. The merger caused the outer layers of the combined stars to swell to enormous proportions — perhaps 1200 times the diameter of the Sun. Since then, the star has been shrinking. As seen today, it’s only about 750 times the