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V838 Monocerotis
V838 Monocerotis (V838 Mon) is a red variable star in the constellation Monoceros about 20,000 light years (6 kpc) from the Sun. The previously unknown star was observed in early 2002 experiencing a major outburst, and was possibly one of the largest known stars for a short period following the 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. By 2009 its temperature had increased to 3,270 K and its luminosity was 15,000 times solar, but its radius had decreased to 380 times that of the Sun although the ejecta continues to expand. The opaque ejected dust cloud has completely engulfed the B-type companion.
==Outburst==
On January 6, 2002, an unknown star was seen to brighten 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 magnitude 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. At the currently accepted distance of 6,100pc, the measured angular diameter corresponds to a radius of 1,200 ± 150 solar radii. 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 spectrum 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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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