This artist's concept illustrates a massive star before and after it blew
up in a cataclysmic "supernova" explosion. NASA's Spitzer Space Telescope
found evidence that this star -- the remains of which are named Cassiopeia
A -- exploded with some degree of order, preserving chunks of its
onion-like layers as it blasted apart.
Cassiopeia A is located 10,000 light-years away in the constellation
Cassiopeia. It was once a massive star 15 to 20 times larger than our sun.
Its fiery death would have been viewable from Earth about 340 years ago.
The top figure shows the star before it died, when its layers of elements
were stacked neatly, with the heaviest at the core and the lightest at the
top. Spitzer found evidence that these layers were preserved when the star
exploded, flinging outward in all directions, but not at the same speeds.
As a result, some chunks of the layered material traveled farther out than
others, as illustrated in the bottom drawing.
The infrared observatory was able to see the tossed-out layers, because
they light up upon ramming into a "reverse" shock wave created in the
aftermath of the explosion. When a massive star explodes, it creates two
types of shock waves. The forward shock wave darts out quickest, and, in
the case of Cassiopeia A, is now traveling at supersonic speeds up to
7,500 kilometers per second (4,600 miles/second). The reverse shock wave
is produced when the forward shock wave slams into a shell of surrounding
material expelled before the star died. It tags along behind the forward
shock wave at slightly slower speeds.
Chunks of the star that were thrown out fastest hit the shock wave sooner
and have had more time to heat up to scorching temperatures previously
detected by X-ray and visible-light telescopes. Chunks of the star that
lagged behind hit the shock wave later, so they are cooler and radiate
infrared light that was not seen until Spitzer came along. These lagging
chunks are made up of gas and dust containing neon, oxygen and aluminum --
elements from the middle layers of the original star.