STAR EXPLOSIONS ON THE HALF-
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
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 wed like to answer. Why
there isnt 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
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 more information, contact:
Dr. Samar Safi-Harb