V838 Monocerotis (V838 Mon) is a red variable star about 20,000 light-years away in the constellation Monoceros. It spans about 14 light-years, what makes it one of the largest known stars. It did not expel its outer layers; instead it grew enormously in size.
The previously unknown star was observed on January 6, 2002 experiencing a major outburst. The initial light curve resembled that of a typical nova eruption, it was then realized to be something completely different. The reason for the outburst is still uncertain.
V838 Monocerotis reached maximum visual magnitude on February 6, 2002, after which it started to dim rapidly, as expected. However, in early March the star started to brighten again, this time mostly in infrared wavelengths. Yet another brightening in infrared occurred in early April, after which the star returned to near its original brightness before the eruption. The light curve produced by the eruption is unlike anything previously seen.
It appears that the progenitor star is considerably more massive and luminous than the Sun, but at the time of maximum V838 Mon was one of the most luminous stars in the Milky Way. The brightening was caused by an abnormal rapid expansion of the outer layers of the star. The laws of thermodynamics dictate that expanding gas cools. Therefore the star became extremely cool and deep red. In fact, some astronomers argue that the spectra of the star resembled that of L-type brown dwarfs. If that is the case, V838 Monocerotis would be the first known L-type supergiant.
Rapidly brightening objects are known to produce a light echo. The light continues propagating outward through a cloud of dust surrounding the star. The light reflects or “echoes” off the dust and then travels to Earth.
In the case of V838 Monocerotis, the light echo produced was unprecedented. While the photos taken by Hubble appear to depict an expanding spherical shell of debris, they are actually formed by reflecting dust that is mostly ‘behind’ the star, not in ‘front’ of it.
There is strong evidence that the V838 Monocerotis system is very young and still embedded in the nebula from which it formed.