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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