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Magnetic Fields Guide Star Birth

The pictureof star formation just got a little more complicated: Cosmic magnetic fields,which can channel condensing interstellar gas, play a more important role inthe birth of stars that previously thought, a new study suggests.

Thesimplified story of stellar birth involves giantclouds of gas and dust collapsing inward due to gravity, growing denser andhotter until nuclear fusion ignites a newborn star.

But inreality, there?s much more to the story: When a molecular cloud collapses, onlya small fraction of the cloud?s material forms stars, and scientists haven?tbeen sure why that is.

Sincegravity favors star formation because it draws material together, some otherforce must be hindering the process, scientists reason. The two leadingcandidates are turbulenceand magneticfields.

Magneticfields (produced by moving electrical charges and present around stars and mostplanets, including Earth) channel flowing gas, making it hard to draw the gasin from all directions. Turbulence stirs the gas and induces and outwardpressure that counteracts gravity.

?Therelative importance of magnetic fields versus turbulence is a matter of muchdebate,? said astronomer Hua-bai Li of the Harvard-Smithsonian Center for Astrophysics.?Our findings serve as the first observational constraint on this issue.?

Li and histeam studied 25 dense patches, or cloud cores, each one about a light-year insize. The cores, which act as seeds fromwhich stars form, were located within molecular clouds as much as 6,500light-years from Earth. (A light-year is the distance light travels in a year,or 6 trillion miles.)

Theresearchers studied polarized light, which has electric and magnetic componentsthat are aligned in specific directions. From the polarization, they measuredthe magnetic fields within each cloud core and compared them to the fields inthe surrounding, tenuous nebula.

Themagnetic fields tended to line up in the same direction, even though therelative size scales (1 light-year cores versus 1,000 light-year nebulas) anddensities were different by orders of magnitude. Since turbulence would tend tochurn the nebula and mix up magnetic field directions, their findings show thatmagnetic fields dominate turbulence in influencing star birth.

?Our resultshows that molecular cloud cores located near each other are connected not onlyby gravity but also by magnetic fields,? Li said. ?This shows that computersimulations modeling star formation must take strong magnetic fields intoaccount.?

The studywill be detailed in an upcoming issue of the Astrophysical Journal.

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