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Do
not go gentle into that good night,...
Rage, rage against the dying of the light.
Oct.
21, 2008: There's a universal tendency to heed Dylan
Thomas's exhortation and go out with a bang instead of a whimper.
Nowhere is this more evident than deep in the cosmos.
When
their time is up, stars make their exits in a number of flamboyant
ways. The most massive stars leave with the greatest fanfare
of all – blasting out gamma-ray bursts (GRBs), tremendous
explosions that rock the Universe like nothing else. These
spectacular blasts, second in power only to the Big Bang,
occur when stars 50 to 100 times more massive than our sun
use up all their fuel and collapse. Most astrophysicists believe
that gamma-ray bursts herald the formation of a black hole.
All
types of galaxies--spirals, ellipticals, dwarfs and irregulars--contain
super-massive stars. Curiously, though, not all types of galaxies
produce gamma-ray bursts. This is one of the puzzles being
discussed today at the 2008 Gamma-Ray Burst Symposium held
in Huntsville, Alabama.
Above:
The Universe is populated by a variety of galaxy-types; a
sampling is shown in this Hubble image of Abell Cluster S0740.
Gamma-ray bursts prefer some and avoid others. [larger
image]
Andrew
Fruchter of the Space Telescope Science Institute is at the
Symposium to share what he knows.
First
of all, he clarifies, there are two types of gamma-ray bursts:
long ones, produced by the explosion of supermassive stars
as described above, and short
ones, produced by some other still-unknown process. "Short
GRBs aren't choosey about their hosts," says Fruchter.
"They're found in all types of galaxies. But the host
galaxies of long GRBs tend to be oddballs, small and irregular,
instead of 'regular' spiral galaxies like our own Milky Way."
Fruchter
believes he understands the discrepancy. Extreme supernova
explosions of the type that produce GRBs require stars of
both great mass and low metallicity.
(In astronomy, "metals" are any elements heavier
than hydrogen or helium.) "Larger galaxies tend to be
more metal-rich than smaller ones," he says. "So
GRBs avoid those larger galaxies."
The
underlying mechanism works like this:
"Metals
in a star produce strong stellar winds -- the metals' atoms
reflect the star's light and act like a solar sail, getting
an extra push that hydrogen and helium alone would not get,"
says Fruchter. "This activity causes some of the star's
mass to flow out into space."
So,
stars with high metallicity tend to lose a lot of their mass
before they explode. "[Metals] can cause such great mass
loss that instead of turning into black holes upon collapse,
some stars may only turn into neutron stars. It's quite possible
that a black hole may need to be present to create a gamma-ray
burst."
In
galaxies crowded with high-metallicity stars, gamma-ray bursts
are thus suppressed. Oddball galaxies of lower-metallicity
get all the best explosions!
Right:
In 1999, the Hubble Space Telescope watched as a GRB faded
away, revealing its oddly-shaped host galaxy in the background.
[more]
Great
mass. Low metallicity. "We should also add to that list
rapid spin," says Symposium participant
Chip Meegan of the NASA Marshall Space Flight Center.
It's
becoming clear that a star must spin rapidly to produce an
explosion with a GRB's power. "The general consensus
is that GRBs emit most of their energy in a jet. Jets in astrophysics
are usually formed by rotating objects," Fruchter says.
"If a slowly spinning star collapses into a black hole,
most of the energy just disappears into the black hole,"
explains Meegan. Rapidly rotating stars have a trick for allowing
some of that energy to escape: "The centrifugal force
of rotation causes the infalling material to form a torus
and makes a less dense region form along the axis of rotation.
This provides a channel for the some of the matter and energy
to blast out along the poles instead of being absorbed by
the black hole."
Mystery
solved? Maybe. Meegan thinks there are more surprises in the
offing:
"Gamma-ray
bursts have amazed us many times before, and I suspect that
they aren't out of surprises yet. The unpredictability is
what makes this such an interesting field."
The
Sixth Huntsville Gamma-Ray Burst Symposium 2008 is sponsored
by NASA's Fermi and Swift Projects and hosted by the Fermi
GBM Team based at the Marshall Space Flight Center in Huntsville.
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Author: Dauna Coulter
| Editor: Dr.
Tony Phillips | Credit: Science@NASA
|