Star Remnants Retain 'Memory' of Explosions (SPACE.com)

Like the
smoke left in the sky after a round of fireworks, debris remaining in the wake
of a supernova could reveal exactly how that star exploded even though hundreds
or thousands of years have passed.





That's what
scientists have determined from images of such leftovers taken by NASA's Chandra
X-ray Observatory.





"It's
almost like the supernova remnants have a 'memory' of the original explosion,"
said lead researcher Laura Lopez of the University of California at Santa Cruz. "This is the first time anyone has systematically compared the shape of
these remnants in X-rays in this way."





Astronomers
sort supernovas
into categories based on properties in the optical spectrum within days of the
star exploding. Such properties label a supernova in two main ways: Type Ia, meaning the progenitor was a binary star system in which one star accumulated
matter from its neighbor until a runaway nuclear reaction ignited; or a Type
II, which occurs when a very massive, young star collapses onto itself before
exploding.





Since
observed remnants of supernovas are leftovers from star
explosions that occurred long ago, other methods are needed to accurately
classify the original supernovas.





"In
the last 300 years we have not observed a supernova go off in the Milky
Way," Lopez told SPACE.com. "And so all of the ones we've observed
directly in the last 30 or 50 years are in other galaxies. The ones we know in
our galaxy are only from remnants."





Lopez and
her colleagues looked at supernova remnants in the Milky Way and a neighboring
galaxy, the Large Magellanic Cloud. Results showed that Type Ia supernovas left
behind relatively symmetric, circular remnants, while debris from Type II
supernovas was distinctly more asymmetric.





When the
stellar guts spew out into space, they also heat up the interstellar medium nearby,
and so Lopez thinks the symmetry could tell astronomers something about
that medium.





"It
seems that Type Ia supernovas probably go off in a very low-density medium that's
very homogenous whereas core-collapse supernovas probably go off in a very
dense environment that is not uniform," Lopez said.





One of the
remnants, known as SNR 0548-70.4, was a bit of an oddball, the researchers
found. Based on its chemical abundances, SNR 0548-70.4 was considered a Type Ia
supernova, but Lopez found it was asymmetric, suggesting a core-collapse
remnant.





"We do
have one mysterious object, but we think that is probably a Type Ia with an
unusual orientation to our line of sight," Lopez said. "But we'll
definitely be looking at that one again."





Even though
they studied supernova in our galaxy and a neighbor, the researchers think the
technique could be extended to remnants farther away.





The
research was published in the Nov. 20 issue of The Astrophysical Journal
Letters.




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Original Story: Star Remnants Retain 'Memory' of Explosions
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