Fragments of Celestial Fireworks
Cassiopeia A, or Cas A for short, is what astronomers
call a supernova remnant, the glowing remains
of a massive star that died in a supernova explosion.
With an estimated age of 300 years, Cas A is currently
the youngest known supernova remnant identified
in our Milky Way galaxy. The star that blew up
was a relative heavyweight, and probably had an
initial mass between 15 and 25 times the mass
of our Sun.
Such so-called ``high-mass'' stars have relatively
short lives, on the order of a few tens of millions
of years, and end up dying catastrophically when
their central cores run out of nuclear fuel for
the fusion reactions that provide them with their
energy. When this happens, their cores rapidly
collapse in on themselves and release enormous
quantities of gravitational energy. This sudden
burst of energy reverses the collapse and throws
most of the mass of the star off into space with
velocities as high as 20 thousand kilometers per
second (45 million miles per hour).
Cas A was first discovered through its intense
radio emission in the 1950's, and has been extensively
studied with ground-based optical telescopes.
But the remnant is simply too far away (about
10,000 light years away) to get a clear view of
the stellar debris through Earth's distorting
atmosphere. The Hubble images presented here have
finally shown us just what the aftermath of a
high mass supernova explosion really looks like.
With nearly ten times better resolution than ground-based
data, they show that the supernova debris is clumped
into filaments and chains composed of thousands
of small, cooling knots of gas.
This month's release image of the Cas A supernova
remnant displays a section of the northern rim
of the remnant's expanding shell. The star that
exploded was located in the seemingly empty region
near the bottom center of the picture. Near the
top of the image one can make out dozens of chains
of tiny knots. Even though they look small, each
of these clumps is actually tens of times larger
than the diameter of our solar system! Although
initially starting out as relatively small blobs
of gas, small variations in their 20 million mile
per hour velocities have, over a span of 300 years,
expanded these fragments to their current immense
sizes.
Supernova explosions enrich the cosmos with heavy
elements forged in the centers of stars over their
long lifetimes. Images taken through three different
filters have been combined to create a false-color
image of Cas A that highlights variations in the
chemical compositions of various components of
the supernova debris. Dark blue knots and filaments
are those richest in oxygen, red ones turn out
to be rich in sulfur, while white, pink and orange
filaments contain varying mixtures of both oxygen-
and sulfur-rich gas. A few lime-green looking
knots are actually bits of material gently blown
off by the star long before the supernova explosion.
These leftovers from the star's old age have been
subsequently run over and lit up by the supernova
blast wave. The arch of delicate light blue-green
filaments seen in the upper portion of the image
indicate is oxygen and sulfur rich material very
recently heated by the supernova's shock wave.
What can these images tell us about the supernova
explosions in general? The exact process by which
the explosion generated at the center of a high
mass star rips upwards through the outer layers
is not well-understood. Some theories suggest
stars blow up rather spherically, leading to a
fairly-uniform, bubble-shaped remnant. Others
propose that the core collapse greatly magnifies
the star's initial magnetic field and rotation,
channeling the explosive energy into two jets
that first rip out through the poles of the rotating
star. This leads to a supernova remnant with two
rapidly-expanding jets of debris, with material
from the equatorial and middle latitudes of the
star thrown off less violently into a broad, doughnut-shaped
ring of debris.
The upper-left portion of Cas A's debris shell
does indeed shows evidence for a breakout maybe
caused by a plume of ejected debris. This lends
some support to the jet-explosion model. But a
thorough study of these images both in terms of
the chemical distribution of the ejected material
and the motions of the debris structure is needed
before reaching any clear conclusion.
