Credit: NASA/CXC/U.Manitoba/H.Matheson & S.Safi-Harb Finding a
Crab's Shell Sometimes it's what's on the outside that counts.
This seems to be the case with G21.5-0.9, a supernova remnant
identified 30 years ago by radio astronomers. G21.5-0.9 is (or
was) a mysterious member of the class of so-called "Crab-like"
(or plerionic) supernova remnants, one (like the class namesake,
the Crab Nebula itself) lacking an outer "shell" where the
supernova blast wave shocks the interstellar medium. The lack of
a shell is a surprise and not well understood. But a deep image
of G21.5-0.9 (shown above) by the Chandra X-ray Observatory has
helped resolve the mystery, at least in this one particular
case. The Chandra observations show that G21.5-0.9 is indeed
surrounded by a shell of X-ray emitting plasma. The Chandra data
helps pinpoint the total amount of energy in the original
explosion, as well as helping to determine when the star blew
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