The apparent lack of shells had led to two classes of star explosions, sort of like molluscs with and without shells, from oysters to squid. However, Dr. Samar Safi-Harb, physics and astronomy, and her graduate student Heather Matheson, say that all star explosions likely create a shell; but some create a faint ('soft') shell, or only half of an easily detectable shell.

The Manitoba astronomers consolidated nearly 150 hours' worth of observations of one nearby star explosion with the NASA Chandra X-ray Observatory. Sure enough, a faint shell once thought missing has emerged. It was just a matter of looking hard and long enough.

"Most star explosions make well-defined and colourful shells, the signature of a classic star explosion," says Safi-Harb. "But even some famous explosions, such as the Crab Nebula, have no obvious shells. It could be that the Crab Nebula is like a soft-shell crab with a thin, barely visible shell."

Safi-Harb added that in astronomy vernacular, 'soft' usually means lower energy. But here the scientists use 'soft' to refer to a faint, softly glowing shell of high-energy x-ray light. Safi-Harb and Matheson studied a star explosion with the unappetizing scientific name of G21.5-0.9. They hope to apply the same type of observation technique to the Crab and other seemingly unshelled star explosions.

The shell marks a region where elements made in the star's core and during the explosion — such as nitrogen, oxygen, iron and all the building blocks of planets and life itself — collide with gas in the region surrounding the star. The explosion creates shockwaves that ram the star's fast-moving ejecta into slower-moving interstellar gas, producing x-rays in the process. Finding the lost shells will help scientists understand how chemical elements are created and ultimately distributed in the Universe.

The work of Safi-Harb and Matheson involves a specific kind of massive star explosion called a supernova. The aftermath of the explosion, called a supernova remnant (SNR), can linger for hundreds of thousands of years. SNR G21.5-0.9 is somewhat young, likely only few thousand years old. Stars that produce supernovae are at least several times more massive than our Sun.

The gas colliding in a SNR shell is millions of degrees and is hotter than the surface of the Sun. These shells radiate predominantly in x-ray light, not visible light; and they are therefore invisible to powerful optical telescopes, but they can be detected with the orbiting Chandra X-ray Observatory. By combining data from a multitude of Chandra observations of G21.5-0.9, performed in past years during mission calibrations, Safi-Harb and Matheson detected a shell scattered around parts of this supernova remnant.

"This is sort of a supernova remnant served on the half-shell," says Matheson. "Why we don't see a full shell as we do around other supernova remnants is the next question we'd like to answer. Why there isn't one around the famous Crab nebula is another mystery."

The astronomers say the thickness and visibility of the shell perhaps depends on the amount of material available in the interstellar medium or the amount of ejecta from the star. It is also possible that some star explosions are somehow weaker than a typical supernova.

Safi-Harb and Matheson say they also have evidence of a pulsar in the center of SNR G21.5-0.9, which would be the core remains of the exploded star. With an additional 68 hours of observation time using a different Chandra instrument, the scientists uncovered faint filaments, or wisps, close to the center of the SNR. These filaments are from shocked winds moving nearly at the speed of light, generated by a hidden, fast-spinning pulsar. In theory, the pulsar should be there, but it has not yet been identified.

For an image of G21.5-0.9, the subject of a NASA Chandra image press release, go to: http://chandra.harvard.edu