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Magnetic assist helps big stars form

Image: Source I
This artist's conception shows a disk of gas and dust surrounding the massive young stellar object called Source I. A wind of gas flows toward the surface of the disk (colored arrows) and is sculpted into an hourglass shape by magnetic field lines (thin blue lines). Gas also flows away from the disk. (Redder colors indicate motion away from observer and blue colors indicate material moving toward an onlooker.) Bill Saxton
/ Source:

Massive stars in the process of forming likely rely on magnetic fields to steer gas onto their surfaces and help them grow into adults, according to new images.

The findings come from radio observations of a young protostar called Source I (pronounced "Source Eye") next to the Orion nebula, which sits in the constellation's sword. The star has been around no more than 100,000 years. Our sun, by comparison, is 4.6 billion years old and middle-aged.

Scientists know a thing or two about how low-mass stars like the sun form. But they have been puzzled over the birth of high-mass stars that weigh in at eight solar masses and greater, in part because the massive stars are rare and spend their youths enshrouded by a veil of dust and gas.

"We know how these stars die, but not how they are born," said study researcher Lincoln Greenhill of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The new findings will be published in the January issue of the Astrophysical Journal.

To cut through the veil, Greenhill and his colleagues used the National Science Foundation's Very Long Baseline Array. The array collects radio waves, which have much greater wavelengths than visible light and pass more freely through gas and dust.

The astronomers looked at both the inflowing and outgoing gas for Source I, noticing some gasses flowed along curved paths rather than a straight journey.

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"It means something must be exerting a lateral force on it, and we think that logically might be a magnetic field," Greenhill said.

In general, stars form as a swirl of gas and dust collapses inward due to gravity and continues to grow until it becomes massive enough to ignite nuclear fusion, at which point a full-fledged star is born. At the same time, some of that inflowing material escapes into space.

As the cloud collapses, it spins faster and faster, just as an ice skater rotates faster as he pulls in his arms. In order for star formation to proceed, there needs to be both an inflow and an outflow.

"The material does much the same thing [as an ice skater], and if you don't get rid of some of that spin the material would never make it down to the star," Greenhill told "The star's gravity would not be enough to continue pulling it in and it would just fly off."

They found that magnetic fields may be the key to steering the material to the star's surface.

Next the team hopes to figure out how star formation is affected by radiation, which can counteract gravity by pushing stuff away from a new star.

"The energy output [of a star] which is 10 times the mass of the sun is actually 10,000 times the radiation output of the sun," Greenhill said. "That makes everything much more difficult to understand."