Small "Helper" Stars
Needed For Formation Of Massive Stars, UC Berkeley, Princeton Researchers
Report
(27 February 2008) In order for a
rare, massive star to form inside an interstellar cloud of gas and dust, small
"helper" stars about the size of the sun must first set the stage, according to
a new theory proposed by astrophysicists at the University of California,
Berkeley, and Princeton University.
Massive stars between 10 and
150 times the mass of the sun are few in number but produce the bulk of the
heavy elements in a galaxy when they explode in supernovas. Their extreme
brightness makes them signposts of star formation in distant
galaxies.
Astrophysicist Christopher F. McKee, professor of physics and
astronomy at UC Berkeley, and Mark R. Krumholz, a Hubble postdoctoral fellow in
the Department of Astrophysical Sciences at Princeton, have been modelling the
formation of these stars for nearly 10 years. Recently, they looked at the
conditions inside cold clouds of molecular hydrogen that favour formation of
massive stars over low-mass stars like the sun.
In a report this week in
Nature, Krumholz and McKee argue that early formation of a few low-mass stars
in a cloud paves the way for later formation of a stellar big brother instead
of fragmentation of the cloud into a hundred smaller clouds, which would
produce only low-mass siblings.
"It's only the formation of these
low-mass stars that heats up the cloud enough to cut off the fragmentation,"
McKee said. "It is as if the cold molecular cloud starts on the process of
making low-mass stars but then, because of heating, that fragmentation is
stopped and the rest of the gas goes into one large star."
"What it
comes down to is that if a cloud is cold, it tends to break up into many small
pieces that become low-mass stars," added Krumholz, who recently accepted a
faculty position with the astronomy department at UC Santa Cruz. "As the cloud
gets warmer, though, it can make bigger and bigger objects."
The cloud
temperatures are still cold, however. A typical interstellar hydrogen cloud is
10-20 degrees Celsius above absolute zero (10-20 Kelvin, or about -430 degrees
Fahrenheit), while low-mass stars can heat the cloud to double or triple this
temperature. To stop the entire cloud from collapsing, the temperature would
have to increase to many hundreds of degrees above absolute zero, McKee
said.
According to Krumholz, each small star within a hydrogen cloud has
a zone of influence where it warms up the gas and prevents it from collapsing
into small fragments. In low density clouds, each zone of influence is small
and contains very little mass, so this effect is unimportant.
As the
density increases, however, the gas and small stars get packed closer and
closer together. Eventually, said Krumholz, the zones of influence of the few
low-mass stars encompass the entire cloud, preventing the cloud from
fragmenting and forcing it to collapse to make a massive star.
McKee
noted that this collapse occurs within an even larger interstellar cloud that
may contain more than a million times the mass of the sun. Therefore, as in our
galaxy's Orion Nebula, many massive stars may be forming simultaneously inside
a giant molecular cloud.
The density above which massive stars can form
is about a million hydrogen molecules per cubic centimetre, which is a very
extreme vacuum on Earth, he said, but nevertheless dense enough to collapse
into a massive star over hundreds of thousands of years. The particle density
in Earth's atmosphere is 10 trillion times greater.
According to McKee,
one implication of the density limitation is that in the outer reaches of
galaxies, where the density may not reach this threshold in a sufficiently
large region of space, low-mass stars may be forming in the absence of any
massive stars. Because we can see only the big, bright stars from Earth, he
said, astronomers may be underestimating the amount of star formation going on
in distant galaxies.
"In fact, there may be many stars forming in the
outer reaches of distant galaxies, just not the bright ones we can see," McKee
said. "Star formation could be occurring that is essentially
invisible."
He noted that a recent satellite collecting ultraviolet
light from distant galaxies has seen evidence of star formation in the very
outer regions of galaxies, and that this may confirm their
prediction.
McKee and Krumholz are involved in large-scale computer
simulations of star formation inside cold molecular clouds to confirm the
researchers' mathematical theory that low-mass star formation is necessary for
formation of high-mass stars.
The work was supported by the National
Science Foundation and NASA's Hubble Fellowship program.
(source:
University of California Berkeley)
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