Magnetic Fields Guide Star Birth

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

The simplified story of stellar birth involves giant clouds of gas and dust collapsing inward due to gravity, growing denser and hotter until nuclear fusion ignites a newborn star.

But in reality, there?s much more to the story: When a molecular cloud collapses, only a small fraction of the cloud?s material forms stars, and scientists haven?t been sure why that is.

Since gravity favors star formation because it draws material together, some other force must be hindering the process, scientists reason. The two leading candidates are turbulence and magnetic fields.

Magnetic fields (produced by moving electrical charges and present around stars and most planets, including Earth) channel flowing gas, making it hard to draw the gas in from all directions. Turbulence stirs the gas and induces and outward pressure that counteracts gravity.

?The relative importance of magnetic fields versus turbulence is a matter of much debate,? 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 his team studied 25 dense patches, or cloud cores, each one about a light-year in size. The cores, which act as seeds from which stars form, were located within molecular clouds as much as 6,500 light-years from Earth. (A light-year is the distance light travels in a year, or 6 trillion miles.)

The researchers studied polarized light, which has electric and magnetic components that are aligned in specific directions. From the polarization, they measured the magnetic fields within each cloud core and compared them to the fields in the surrounding, tenuous nebula.

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

?Our result shows that molecular cloud cores located near each other are connected not only by gravity but also by magnetic fields,? Li said. ?This shows that computer simulations modeling star formation must take strong magnetic fields into account.?

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

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