Credit: Hua-bai Li (CfA/MPIA); Darren Dowell (JPL/Caltech); Alyssa Goodman (CfA); Roger Hildebrand (U. Chicago) ;
and Giles Novak (Northwestern U.)
About this image: Hua-Bai Li and his colleagues measured magnetic fields in the Orion molecular cloud region, which includes the famous Orion Nebula.
The background image shows the far-infrared map in representative color and logarithmic scale. Superposed on it are the magnetic field directions inferred from optical polarimetry data (blue lines) tracing the diffused regions.
The average of all the optical data is shown as the thick gray line. Magnetic fields within eight high-density cloud cores (labels A through H on the background map) are mapped using submillimeter polarimetry and the results are shown as insets, using red lines on individual representative-color intensity maps.
The average direction of each core is shown as a white line superposed on the core map; these white lines also are plotted on the background map. Even though the core separations exceed the core sizes by as much as a factor of 100, they are for the most part "magnetically connected," i.e. the cores' average field directions are similar. Moreover, these directions are close to the average field direction seen in the diffused regions.
Cambridge, MA - The simple picture of star formation calls for giant clouds of gas and dust to collapse inward due to gravity, growing denser and hotter until igniting nuclear fusion. In reality, forces other than gravity also influence the birth of stars. New research shows that cosmic magnetic fields play a more important role in star formation than previously thought.
A molecular cloud is a cloud of gas that acts as a stellar nursery. When a molecular cloud collapses, only a small fraction of the cloud's material forms stars. Scientists aren't sure why.
Gravity favors star formation by drawing material together, therefore some additional force must hinder the process. Magnetic fields and turbulence are the two leading candidates. (A magnetic field is produced by moving electrical charges. Stars and most planets, including Earth, exhibit magnetic fields.) Magnetic fields channel flowing gas, making it hard to drawn the gas from all directions, while turbulence stirs the gas and induces an 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. (Some sunglasses work by blocking light with specific polarization.) 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 1000 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," said Li. "This shows that computer simulations modeling star formation must take strong magnetic fields into account."
In the broader picture, this discovery aids our understanding of how stars form and, therefore, how the universe has come to look the way it is today.
The paper detailing these findings has been accepted for publication in The Astrophysical Journal and is available online at http://arxiv.org/abs/0908.1549.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.
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Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
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Harvard-Smithsonian Center for Astrophysics
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