Massive stars need small "helper" stars for their formation

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Washington, Feb 28 : A new theory proposed by astrophysicists has suggested that 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 are needed.

Astrophysicists at the University of California, Berkeley, and Princeton University put this theory forward.

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 modeling the formation of these stars for nearly 10 years.

Recently, they looked at the conditions inside cold clouds of molecular hydrogen that favor formation of massive stars over low-mass stars like the sun.

According to Krumholz and McKee, 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," said McKee. "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," he added.

"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," said Krumholz. "As the cloud gets warmer, though, it can make bigger and bigger objects," he added.

A typical interstellar hydrogen cloud is 10-20 degrees Celsius above absolute zero, 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," said McKee.

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, 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," said Krumholz.

ANI

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