V838 Monocerotis (V838 Mon) is a red variable star in the constellation Monoceros about 20,000 light years (6 kpc) from the Sun, and possibly one of the largest known stars. The previously unknown star was observed in early 2002 experiencing a major 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.
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 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 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.
Other possibly similar events
There are a handful of outbursts that resemble the one which occurred on V838 Monocerotis. In 1988 a red star was detected erupting in the Andromeda Galaxy. The star, designated M31-RV, reached the absolute bolometric magnitude of −9.95 at maximum (corresponding a luminosity of 0.75 million times solar) before dimming beyond detectability. A similar eruption occurred in 1994 in the Milky Way (V4332 Sagittarii).
Progenitor star
Some details are emerging on the nature of the star that experienced the outburst. Based on an incorrect interpretation of the light echo the eruption generated, the distance of the star was first estimated to be 1,900 to 2,900 light years. Combined with the apparent magnitude measured from pre-eruption photographs, it was thought to be an underluminous F-type dwarf not much unlike our Sun which posed a considerable enigma.
More accurate measurements gave a much larger distance, 20,000 light years (6 kpc). It appears that the star is considerably more massive and luminous than the Sun. The star probably has a mass of from 5 to 10 times solar, and a luminosity of from 550 to 5,000 times solar. The star may have originally had a radius roughly 5 times solar and temperature of 4,700–30,000 K. Munari et al. (2005) suggest that the progenitor star is in fact a very massive supergiant with a mass of about 65 times solar. They also conclude that the system may be only about 4 million years old.
The spectrum of V838 Monocerotis reveals a companion, a hot blue B-type main sequence star probably not much different from the progenitor star. It is also possible that the progenitor was slightly less massive than the companion and only just entering the main sequence.
Based on the photometric parallax of the companion, Munari et al. get a greater distance, 36,000 light years (10 kpc).
Light echo
Rapidly brightening objects like novae and supernovae are known to produce a phenomenon known as light echo. The light that travels directly from the object arrives first. If there are clouds of interstellar matter around the star, some light is reflected from the clouds. Because of the longer path, the reflected light arrives later, producing a vision of expanding rings of light around the erupted object. In addition, the rings appear to travel faster than the speed of light.
In the case of V838 Monocerotis, the light echo produced was unprecedented and is well documented in images taken by the Hubble Space Telescope. While the photos appear to depict an expanding spherical shell of debris, they are actually formed by the illumination of an ever-expanding ellipsoid with the progenitor star at one focus and the observer at the other. Hence, despite appearances, the structures in these photos are actually concave toward the viewer. In other words, the light is reflecting dust that is mostly 'behind' the star, not in 'front' of it.
It is not yet clear if the surrounding nebulosity is associated with the star itself. If that is the case, they may have been produced by the star in earlier eruptions which would rule out several models that are based on single catastrophic events. However, there is strong evidence that the V838 Monocerotis system is very young and still embedded in the nebula from which it formed.
Interestingly, the eruption initially emitted at shorter wavelengths (i.e. was bluer), which can be seen in the light echo: the outer border is bluish in the Hubble images.
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