Researchers at the University of Manitoba have solved a decades-old
mystery of why some star explosions create shells around them from the
ashes that fly off into space, while other explosions appear not to.

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