V838 Monocerotis - Outburst

Outburst

On January 6, 2002, an unknown star was seen to brighten up in Monoceros, the Unicorn. Being a new variable star, it was designated V838 Monocerotis, the 838th variable star of Monoceros. 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. Therefore it was also designated Nova Monocerotis 2002. V838 Monocerotis reached maximum visual magnitude of 6.75 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, magnitude 15.6. The light curve produced by the eruption is unlike anything previously seen.

The star brightened to about a million times solar luminosity and absolute manitude of −9.8, ensuring that at the time of maximum V838 Monocerotis 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 star was observed using the Palomar Testbed Interferometer, which provided a radius of 1,570 ± 400 solar radii (comparable to Jupiter's orbital radius), confirming the earlier indirect calculations. The expansion took only a couple of months, meaning that its speed was abnormal. The laws of thermodynamics dictate that expanding gases cool. 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. However, current estimates of the distance, and hence of the radius, are about 25% lower than assumed in those papers.