The
existence of massive stars — up to a whopping 120 times the mass of the sun — has
long perplexed astronomers. The big question was how these stellar behemoths
reached their enormous sizes without blowing off all the gas that feeds them.
A new
computer simulation of star
formation has found a surprisingly simple solution to how these stars might
get around this problem.
The new
findings, detailed in the Jan. 16 issue of the journal Science, also
explain why these
giants tend to occur in binary or multiple star systems.
"We
didn't' set out to solve that question, so it was a nice side benefit of the
study," said study leader Mark Krumholz of the University of California, Santa Cruz.
Balancing
forces
When a star
begins to form, two opposing forces are at play. One is the pull of gravity
creating by the rotating gas cloud from which the star is born. Gravity pulls
the gaseous material in, feeding the protostar.
The other
force, called radiation pressure, is generated by the growing star itself. This
pressure is the force exerted by electromagnetic radiation on the surfaces it
strikes. For ordinary light, this force is nearly negligible, but it becomes
significant in the interior of stars because of the intensity of their
radiation.
For massive
stars, radiation pressure is the dominant outward-flowing force counteracting
gravity's inward pull to prevent the further collapse of the star. Previous
studies had suggested that radiation pressure would blow away a star's gas
cloud before the star could grow much larger than 20 times the mass of the sun.
"When
you apply the radiation pressure from a massive star to the dusty interstellar
gas around it, which is much more opaque than the star's internal gas, it
should explode the gas cloud," Krumholz explained.
Yet plenty
of these massive stars have been spotted by astronomers (though they are rarer
than small stars).
Surprise
solution
Krumholz
and his colleagues solved the dilemma with a three-dimensional computer
simulation of the collapse of a giant interstellar gas cloud to form a massive
star. Their research was funded by the National Science Foundation, NASA and
the U.S. Department of Energy.
As the
dusty gas collapsed, onto the star's growing core, instabilities developed that
resulted in channels where radiation blew out through the cloud into
interstellar space, while gas continued falling inward through other channels.
"You
can see fingers of gas falling in and radiation leaking out between those
fingers of gas," Krumholz said. "This shows that you don't need any
exotic mechanisms; massive stars can form through accretion processes just like
low-mass stars."
The disk of
the collapsing gas also did something unexpected: it clumped to form a series
of small secondary stars, most
of which collided into the primary star, but some of which came to be stars
in their own right and formed a multiple star system.
"I
think now we can consider the mystery of how massive stars are able to form to
be solved," Krumholz said.
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