New model explains how giant stars form

By Robert Sanders, Public Affairs



Stellar “eggs” — the seeds of stars — are emerging from this vast molecular cloud, which resembles a serpent’s head, in the Eagle Nebula 7,000 light years away. Berkeley astronomers say these hot celestial seedlings grow big by coalescence — accumulating more and more gas from the surrounding molecular cloud — rather than by colliding with other stars.
Jeff Hester, Paul Scowen, ASU/Hubble Space Telescope photo

13 March 2002 | Dense clouds of gas and dust like the Orion Nebula give birth to stars 10 to 100 times bigger than the sun, but astronomers are still debating how these giant stars form.

Now a team of astronomers at Berkeley claims they have resolved the issue using a new model of massive star formation.

By extending the widely accepted theory of low-mass star formation, they have calculated that stars about 100 times the mass of the sun would form in about 100,000 years. To put that in perspective, Earth’s sun is thought to have formed in a much less dense molecular cloud in, roughly, several hundred thousand years.

The model also suggests that protostars — embryonic stars in the early stages of formation— most likely grow big by the coalescence (accretion) of gas falling inward from the surrounding molecular cloud, rather than by the collision (merging) of a number of smaller stars, as some astronomers have proposed.

“These massive stars are very important, because they produce most of the heavy elements from which we are made,” says Christopher McKee, professor of astronomy and physics and chair of Berkeley’s physics department. McKee is one of three authors of a paper describing the model, which appears in the March 6 issue of the British journal Nature.

“Previous theories have …suggested formation times ranging from thousands of years to millions of years,” he says. “Some of these theories were saying that it would take the entire lifetime of the star for it to form.”

McKee and his colleagues were able to show, by extension of a theory developed many years ago by former Berkeley astronomer Frank Shu, that it is possible to predict how long it will take a massive star to form. McKee and his associates put the theory of massive star formation on a firmer footing.

Co-author Jonathan Tan, a former Berkeley graduate student who is now a postdoctoral fellow at Princeton University Observatory, says that one of the problems in modeling the formation of massive stars is that protostars are very hot — so hot, in fact, that radiation pressure pushes the infalling gas and dust away.

Because they burn their nuclear fuel so fast, these young stars have relatively short life spans — as short as 3 million years, compared to 10 billion years for our sun.

As a result, some scientists have concluded that massive stars would never be able to grow big enough by accretion. They propose, instead, that massive stars form from the collision of several smaller stars, even though the density of protostars in star clusters would seem to make this a rare event.

By contrast, McKee and Tan found that the pressure of the infalling gas is more than sufficient to overcome the radiation pressure from the hot young protostar.

“The very high pressures of the star-forming regions need to be considered,” Tan says. The densities and pressures of infalling gas are strong enough to overcome other forces, such as radiation pressure, and allow the young star to gradually grow and enlarge.


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