A UC Berkeley astronomer has a new theory on the birth of stars that may provide the explanation for dramatic but puzzling discoveries made over the past decade by scientists scrutinizing the gas clouds where stars form.
The theory, by Frank Shu, chairman of the astronomy department at University of California, Berkeley, depicts the events that occur from the time a dense pocket of gas collapses to form a "protostar" until the point, about 10 million years later, that intense solar winds blast away the enveloping gas cloud to reveal a burning star.
The theory provides an answer to the question of why most young stars are much smaller in mass than the gas clouds that they are derived from. Most such stars are about the size of our sun.
Some of the ideas in Shu's theory have been talked about before, according to astronomer Philip Myers of the Center for Astrophysics in Cambridge, Mass. "What makes Frank's work different," Myers said, "is that he has a comprehensive picture of the four major stages of the birth process. . . . It's a much better integrated theory than had previously been suggested."
Shu was not available for comment last week, and is scheduled to present a paper on his theory today at a meeting of the International Astronomical Union in Heidelberg, West Germany.
Shu's star-birth scenario--as described in a synopsis of the paper--begins with the formation of small, dense pockets of cold dust and gas that slowly condense out of a much larger complex of clouds. This stage, Shu said, can last as long as 10 million years.
As individual pockets grow larger, their gravity causes them to collapse upon themselves--with the center regions collapsing first and the outer regions falling inward in successive waves. The protostar at the center grows larger for perhaps 100,000 years as gravity draws more gas and dust inward.
Calculations Match Well
Radiation emitted by the protostar during this period would be absorbed and re-emitted many times by the dust particles surrounding the core, until the majority of the radiation would be in the infrared region of the spectrum.
Shu and graduate student Frank Adams have calculated the distribution of infrared wavelengths that would be expected to emerge. Their calculations match very well with actual measurements--taken by the Infrared Astronomical Satellite--of the infrared spectra of objects embedded in interstellar clouds.
Previously, Myers said, "there was no good set of theoretical predictions that matched the emissions from the youngest stars that we know about. This match is not proof (of Shu's theory), but it makes a much stronger case than we had before."
The most controversial aspect of Shu's theory involves the third stage of development, when atoms within the protostar begin combining in a fusion reaction. This initial reaction, Shu said, generates powerful blasts of hot gas that are expelled from the star's poles at a speed of about 500,000 m.p.h.
Jets Have Been Detected
Because the core of the star is still embedded in the gaseous cloud, these jets of high-temperature gases are the only parts of the star that can be seen at interstellar distances. Such jets have been detected by astronomers, who have not been able to identify their sources.
A key point, Shu said, is that the volume of gases expelled by the jets eventually reaches an equilibrium with the amount of gas and dust being drawn into the protostar by gravity. This equilibrium limits the eventual size of the star.
Ultimately, as the fusion reaction in the protostar grows stronger, the jets widen and get less dense, until a uniform stellar wind blows outward in all directions. This wind blows the rest of the dust cloud away from the star, making further growth impossible and exposing the star to view. This phase, from ignition of fusion to clearance of the dust cloud, may last from 10,000 to 100,000 years, Shu said.
Evidence for this latter phase of development comes from observations of very young stars. Astronomers have previously noted a stellar "birth line," a relationship between a young star's mass and its radius that was not previously explained.
Stellar Birth Line Defined
Shu's calculations, however, show that for any given mass of dust, there is a unique radius at which a temperature sufficient for the ignition of fusion occurs. These calculated radii, Shu said, actually define the stellar birth line.
Shu's scenario "certainly sounds very reasonable," according to physicist Ed Saltpetar of Cornell University, and it matches up well with the observational data. One of the most interesting things about the description, he added, is that the areas where these very young stars are forming today "must, in some sense, look today like our galaxy must have looked a very, very long time ago."