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V838 Mon: A Stellar Mystery

Michael Szpir

In January 2002 an unassuming star in the constellation Monoceros briefly became one of the most luminous stars in the Milky Way galaxy—shining 600,000 times more brightly than our Sun. Dubbed V838 Monocerotis (Mon), the star has since faded to its previous levels, but it continues to attract attention because scientists don’t fully understand the enigmatic shifts in the star's original outburst. In the span of just a few weeks, the star’s brightness increased and then decreased three times—resulting in a "light curve" that has never been seen before.

The usual astrophysical suspects don't quite match the modus operandi of V838 Mon. The eruption of a nova, for example, is quite different from the January 2002 outburst. A typical nova is produced by a thermonuclear explosion when matter from a normal star is dumped onto a hot white-dwarf companion. The powerful explosion blows off the outer layers of accreted material, sending a hot, dense gas into space. But V838 Mon did not eject its outer layers, and its temperature remained very cool throughout the outburst.

An expanding light echoClick to Enlarge Image

The puzzle has inspired scientists to devise some exotic astrophysical mechanisms. Noam Soker and Romuald Tylenda of the Copernicus Astronomical Center in Torun, Poland have suggested that the outburst was caused by the release of gravitational energy when two stars merged (The Astrophysical Journal, January 10, 2003). In this view, the three brightness peaks occurred as first the outer layers of the stars, then the stellar cores, made contact and collided; the third peak is attributed to the attainment of a new stellar equilibrium. Another proposal came from Alon Retter and Ariel Marom at the University of Sydney, who suggest that the outburst was caused by the expansion of a red giant star that swallowed in succession three giant planets in very close orbits (Monthly Notices of the Royal Astronomical Society, October 10, 2003). Still others suggest that the outburst may represent a rarely seen and brief stage in the life of a star.

Although few astronomers agree on the exact mechanism that caused the outburst, many seem inclined to accept the possibility that a new class of astrophysical phenomenon has been discovered. At least two other stars that may belong to this class are currently being investigated, including M31-RV (a red variable star in the Andromeda galaxy), which erupted in 1989, and V4332 Sagittarii (a red giant in the constellation Sagittarius) that had an outburst in 1994.

As an added bonus, V838 Mon is putting on a pretty show that is being captured by the Advanced Camera for Surveys on the Hubble Space Telescope (above). The images represent a "light echo" from the January 2002 outburst as the original flash of light moved through the shell of dust that surrounds the supergiant star. The dust shell is believed to be material leftover from previous outbursts. Astronomers think that the light echo will continue to propagate through the dust till the end of the decade. (You can also see the video on the Internet:

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