Cassiopeia A
 It looks like a brightly colored Christmas ornament, but this is really the expanding remains of an exploded star known as Cassiopeia A. This supernova remnant is about 10 light-years in diameter. It formed when a supergiant star exploded about 400 years ago (as seen from Earth). Its outer layers, which are shown in red and green in this X-ray image from the orbiting Chandra X-Ray Telescope, are expanding at almost one-tenth of lightspeed. The outer blue layer is a shockwave produced as material from the exploding star plows into surrounding gas. Cassiopeia A is an important object in the effort to understand how massive stars explode. [NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.]
Because it was the first radio source discovered in the constellation Cassiopeia, it's known as Cassiopeia A -- Cas A for short.
It's a shell of gas and dust about 10 light-years in diameter. The shell is expanding at 10 million miles an hour. As the material moves through space, it produces radio waves.
From the speed of this expanding shell, astronomers calculated that the star that created Cas A must have exploded around 1680. But there are only sketchy hints that it was bright enough to see from Earth, even though it should have outshined most of the other stars in the galaxy combined.
As Cas A blew itself apart, it "fused" lighter-weight elements together to form heavier elements, like oxygen, silicon, and iron. These elements may provide some of the raw materials for new stars and planets. As Cas A expands, it may even help new stars take shape by squeezing nearby clouds of gas.
Cassiopeia wheels high across the north tonight. It looks like a flattened letter M or W. Cas A isn't visible to the unaided eye, but astronomers continue to study the remnants of its violent death.
More about Cas A tomorrow.
Script by Damond Benningfield, Copyright 2002, 2006
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