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Massive Star Formation

© Wesley Colley

I discussed the basic story of star formation in a previous article ("Star Formation Eludes Explanation"), and there is little doubt that the basic picture of a collapsing molecular cloud as the progenitor for stars is correct. As I mentioned in the previous article, the microscopic details of how accreting gas really becomes a star remain elusive, because of the daunting difficulty of calculating gas dynamics over such a wide range of spatial and temporal scales (from the interactions of individual dust grains up to magnetic field lines the size of many solar systems). However, many important questions also remain in the more macroscopic realm.

One of the most basic, but, still very difficult questions to address is what kind of star is made in what situation. Namely, stars range in mass from about a tenth of a solar mass up to perhaps one hundred solar masses. The range by a factor of a thousand is too large to dismiss as "details." Furthermore, we observe stars forming in significantly different circumstances, from fairly quiescent molecular clouds, to starbursting galaxy collisions.

The trend that seems to apply is that the more dynamically disturbed the region, the more likely it is for there to be very massive stars forming. For instance, the Large Magnellanic Cloud (LMC), our nearest galaxian neighbor, is constantly torqued and distorted by the tidal forces of the Galaxy, and not coincidentally there is one of the brightest star formation regions in any of the local galaxies. Seen here, 30 Doradus contains hundreds of very massive, very bright, very hot, very young stars. In fact, if 30 Doradus were as close as the Orion Nebula, it would occupy the entire constellation of Orion in our sky, and exhibit hundreds of stars as bright as the belt stars in Orion. There are still more extreme examples, such as M82 where the entire galaxy is virtually exploding with star formation.

Because it is so nearby, the Orion Nebula is possibly the best massive star factory to study, even if it is a tad wimpy compared to M82 or 30 Doradus. The Orion Nebula presumably started off as a giant molecular cloud, which was tweaked by some dynamical phenomenon that stirred up the gas (a nearby supernova, a gravitational shock by a spiral arm, etc.). Some regions became highly excited and produced several massive and bright stars (4 of which, "the Trapezium," are seen here). These very bright stars emit so much light that they disturb the gas inside the molecular cloud and form shocks (much as do supersonic jets), which are essentially boundary

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