Small stars midwife to big stars

Contributed by
Feb 27, 2008

We know of a handful of very massive stars in the galaxy. And by massive, I mean whoppers: some with 100 times the Sun's mass. These are enormous stars that are millions of times brighter than the Sun and live very short lives, ending in titanic supernova explosions.

The problem is, how do they form?

Simulation of the formation of a 100 solar mass star from a cloud of gas, courtesy Mark Krumholz

Stars form from clouds of dust and gas. These are generally pretty cold, just a few degrees above absolute zero (-450 Fahrenheit, -273 Celsius). The Sun formed from just such a cloud, we think. It's not hard to form lots of stars like the Sun from a typical cloud, but massive stars are tougher. Cold gas tends to fragment into smaller clumps, and these clumps can't make really big stars. So how do the monster ones get their start?

New work just published by Mark Krumholz at Princeton and Christopher McKee at Berkeley indicates that the big stars get a helping hand from small ones.

Imagine a giant cloud of gas, a light year or so across and containing a couple of hundred times the Sun's mass. It's cold inside, and the gas starts to collapse here and there under its own gravity. Small stars like the Sun start to form. When they do, they heat up the environment around them. This heat input suppresses the further fragmentation of the cloud, allowing it to collapse more than it would otherwise. So if a big star starts to form, there is a constant supply of gas to feed it, letting it grow larger. If the smaller stars weren't there, the insides of the cloud would fragment and cut off the supply of mass to the nascent massive star.

This may sound a little contradictory: a warmed up gas expands, right? Well, usually, but we're not talking about a major heating here. The sun-like stars raise the temperature of the cloud just a few degrees. That's enough to stop the fragmentation, but not enough to counteract the cloud's own gravity causing it to collapse. Note to creationists: when you talk about all gas expanding, and therefore stars cannot form, you look a little silly. You might want to look into some actual science once in a while.

Anyway, this new model explains something that has been seen but not well understood: massive stars only seem to form in clusters. The vast majority of truly huge stars we see are members of clusters, indicating that there is something in the cluster environment they need to get so big. It looks like Krumholz and McKee have found it: there have to be other stars around to help them grow.

Of course, this work is just beginning. There are lots and lots of things that can affect star formation, and they have only modeled a few. But this is an excellent start in solving one of the bigger -- literally! -- mysteries in star formation science today.

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