November 18, 2005 | David F. Coppedge

A New Way to Make Stars, Or One Old Way Discredited?

Several news sources sounded a rather triumphant note that astronomers are figuring out how stars form.  In actuality, the paper by Krumholz, McKee and Klein in Nature1 did more to discredit a competitive theory than to establish their own.  That competitive theory, ironically, is called “competitive accretion” and posits that clumps of material add up as they collapse.  The astronomers claim that this theory, popular since the 1990s especially in Europe, made unrealistic assumptions about initial masses, collapse rates, and turbulence.  Also, the competing model predicts ejected brown dwarfs moving at high speeds, which are not observed.
    For these reasons, the authors feel the competitive accretion model is a “dead theory.”  That leaves the other model standing by default: core accretion.  A press release from Lawrence Livermore Labs about this paper states that much remains to be learned: “Star formation is a very rich problem, involving questions such as how stars like the sun formed, why a very large number of stars are in binary star systems, and how stars 10 to 100 times the mass of the sun form,” McKee said (emphasis added).
    Meanwhile, the Spitzer Space Telescope has been showing itself a worthy competitor to Hubble in photography skill.  A press release from Jet Propulsion Laboratory showed a blazing region in Cassiopeia captioned, “Cosmic Mountains of Creation.”  They claim it is a region of active star formation similar to that in the Eagle Nebula famously imaged by Hubble (but see 05/15/2002 entry).

1Krumholz, McKee and Klein, “The formation of stars by gravitational collapse rather than competitive accretion,” Nature 438, 332-334 (17 November 2005) | doi:10.1038/nature04280.

We haven’t heard from the opponents yet whether they will be able to support their view and shoot holes in the core accretion model.  When you think about it, diffuse gas and dust have a lot to overcome in forming a star.  Gravity and shock waves are the only formative forces.  On the way in, the dust heats up, pushing outward, magnetic fields and turbulence oppose collapse, and increasing pressure, increasing rotation, and angular momentum fight the formation process.  Getting the supermassive stars is hard for both models, and why do so many binaries form?  Many articles glibly speak of “regions of active star formation” with confidence vastly exceeding theoretical and observational understanding.  If scientists would just learn to admit more often what they don’t know and speak a little more humbly, it would be a worthy development.

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