Weekly Science Update

Wednesday, August 15, 2007
The Orion Nebula Monster
The Orion Nebula, shining brightly in the "sword" of the constellation Orion, is one of the most famous sights in the sky. The nebula is actually several clusters of hot young stars surrounded by gas and dust that glow from the intense ultraviolet radiation from the stars. The nebula is the closest nursery to the Earth of massive stars; it is only about 1300 light-years away. But despite its fame, brightness, and relative proximity, astronomers still do not understand it very well. For example, no one even knows whether a single star or a cluster of stars drive the dramatic flows that are seen. The reason for this ignorance is in part because the nebula is so crowded with stars, and in part because the dust there obscures many regions from optical view.

The brightest object in the nebula shines with as much light as 100,000 Suns. About ten years ago, astronomers found that this source was itself comprised of several smaller ones. Radio wavelength measurements in particular suggested that one of these smaller objects, an intense radio source called "Orion I" was the dominant young star, with the other cluster sources perhaps seen in its reflected light. But Orion I was an enigmatic monster: it is seen to be moving in space as it would if it had been ejected from another system a few hundred years ago, and it is surrounded by natural masers (masers are the radio wavelength analogues of lasers) that may have come from a rotating disk of material.

SAO astronomers Mark Reid and Lincoln Greenhill, together with two of their colleagues, have used very high angular resolution radio images, the best such images ever obtained, to discover that Orion I is apparently a single, very massive young star surrounded by an edge-on, ionized disk of material. The results are striking because they suggest that the star is responsible for nearly all of the huge luminosity, but this value is too large to be consistent with any known star. The scientists suggest as one possibility that accretion onto the star from the disk could be contributing substantially to the total luminosity. This in turn challenges other ideas, however, since conventional wisdom holds that infall onto a young massive star will be inhibited by winds from the star. The new paper, while resolving several long-standing puzzles about the Orion Nebula itself, also takes tentative steps towards clarifying the processes involved in massive star formation.

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