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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.

This image of the supernova remnant Cassiopeia A was created with the National Science Foundation's Very Large Array telescope in New Mexico. Cassiopeia A is the remnant of a supernova explosion that occurred over 300 years ago in our Galaxy. It is one of the brightest radio sources in the sky. The massive star that exploded is not visible in this image, but the material that was ejected from the explosion can be seen as bright filaments. This material is still moving outward at speeds of over ten million miles an hour. Cassiopeia A is about 11,000 light years away.

Image credit: Lawrence Rudnick, Tracey Delaney, Jonathan Keohane & Barron Koralesky (University of Minnesota) and NRAO

Visit a Related Site of Cas A at the NRAO Website!

Chandra X-ray image of the supernova remnant Cassiopeia A (Cas A). The red, green, and blue regions in this Chandra X-ray image of the supernova remnant Cassiopeia A show where the intensity of low, medium, and high energy X rays, respectively, is greatest. The red material on the left outer edge is enriched in iron, whereas the bright greenish white region on the lower left is enriched in silicon and sulfur. In the blue region on the right edge, low and medium energy X rays have been filtered out by a cloud of dust and gas in the remnant.
Image credit: NASA/CXC/SAO/Rutgers/J.Hughes

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