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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