V838 Monocerotis [V838 Mon] is a variable star in the constellation Monoceros about 20,000 light years from the Sun. The star experienced a major outburst in early 2002.
On 10 January, a previously unknown star was seen to brighten up in Monoceros. Being a new variable star, it was designated V838 Monocerotis. The initial light curve resembled that of a nova, an eruption that occurs when enough hydrogen gas has accumulated on the surface of a white dwarf from its close binary companion. V838 Monocerotis reached maximum visual magnitude of 6.75 on 6 February 2002 after which it started to dim rapidly. In early March the star started to brighten again, this time mostly in infrared wavelengths. Another brightening in infrared occurred in early April after which the star returned to near its original brightness before the eruption.
The star brightened to about a million times solar luminosity ensuring that at the time of maximum it was one of the most luminous stars in the Milky Way Galaxy. The brightening was caused by a rapid expansion of the outer layers of the star. The expansion took only a couple of months meaning the expansion speed was phenomenal.
The Hubble Space Telescope's latest image of the star V838 Monocerotis [V838 Mon] reveals dramatic changes in the illumination of surrounding dusty cloud structures. The effect, called a light echo, has been unveiling never-before-seen dust patterns ever since the star suddenly brightened for several weeks in early 2002.