Dense clouds of gas and dust like the Orion Nebula give
birth to stars 10 to 100 times bigger than the sun, but astronomers
still debate how these giant stars form.
A new model of massive star formation by astrophysicists at the
University of California, Berkeley, finally resolves the issue. 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.
For comparison, our sun is thought to have formed in a much less
dense molecular cloud in about several hundred thousand years.
The model also suggests that protostars most likely grow big by the
infall of gas from the surrounding molecular cloud, rather than by
the collision 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,” said Christopher McKee,
professor of astronomy and physics and chair of physics at UC
Berkeley and one of two co-authors of a paper describing the model
that appears this week in the British journal Nature. “But previous
theories have been mostly phenomenological and suggested formation
times ranging from thousands of years to millions of years. Some of
these theories were saying that it would take the entire lifetime of
the star for it to form.
“We were able to show, by proper extension of the theory (former UC
Berkeley astronomer) Frank Shu had developed many years ago, that you
could predict how long it would take a massive star to form. We put
the theory of massive star formation on a firmer footing.”
One of the problems in modeling the formation of massive stars, said
co-author Jonathan C. Tan, a former graduate student at UC Berkeley
who now is a postdoctoral fellow at Princeton University Observatory,
is that protostars are so hot that the radiation pressure pushes the
infalling gas and dust away. Because they burn their nuclear fuel so
fast, they have relatively short life spans: as short as 3 million
years, compared to 10 billion years for our sun.
As a result, some have concluded that massive stars would never be
able to grow big enough by accretion. They proposed, 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.
What McKee and Tan found, however, is that the pressure of the
infalling gas is more than sufficient to overcome the radiation
pressure from the protostar.
“The very high pressures of the star-forming regions need to be
considered,” Tan said. “The densities and ram pressures associated
with the infall of gas are strong enough to overcome the radiation
pressure and boost the accretion rate onto the star.”
Interestingly, the actual formation time doesn’t depend very strongly
on the mass of the star. While a 100-solar-mass star forms in about
100,000 years, a star the mass of the sun – 100 times smaller – would
form only three times faster – in about 30,000 years.
“This helps us understand how clusters form, because there is no
direct evidence, for example in the Orion Nebula, one of the nearest
clusters, that massive stars formed at a different time from the low
mass stars,” Tan said.
The accepted theory of low-mass star formation was laid out some 30
years ago by Frank Hsia-San Shu, a UC Berkeley astronomer who early
this year left to become president of National Tsing Hua University
in Taiwan. He calculated that interstellar clouds of atomic and
molecular hydrogen gas, helium and dust would begin to collapse under
their own weight, swirling and flattening into a disk. As material
fell inward, the pressure and temperature would rise as the
gravitational energy is converted to heat.
A protostar eventually would form at the center of the collapsing
accretion disk, heated by its own gravitational energy, and continue
to draw more matter onto it until it was large enough to trigger
nuclear fusion at the core.
McKee and Tan applied this theory to the much more extreme conditions
observed in the densest regions of giant molecular clouds, where
massive stars are observed to form. The model will help them
understand other processes in massive star formation, such as the
production of high-powered jets as matter accretes onto a star, and
the protostar mass at which nuclear burning in the core produces
enough radiation to outshine the glowing accretion disk.
“This is certainly going to be important in understanding star
clusters,” said Tan.
McKee agreed, noting that massive stars are hard to study because
their early stages are hidden behind a veil of gas and dust.
“Massive star research is way behind research on low-mass stars,” he
said. “It’s definitely going to be a very active area of research
during the coming decade.”
The research was supported by the National Science Foundation and the
Center for Star Formation Studies, which is funded by the National
Aeronautics and Space Administration.
NOTE: Chris McKee can be reached at (510) 642-3316 or
cmckee@socrates.berkeley.edu. Jonathan Tan is at (609) 258-7529 or
jt@astro.princeton.edu.
