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PHOTO RELEASE NO.: STScI-PRC95-11
FOR RELEASE: February 20, 1995


HUBBLE'S CLOSE-UP VIEW OF
A SHOCKWAVE FROM A STELLAR EXPLOSION

This image shows a small portion of a nebula called the "Cygnus Loop."
Covering a region on the sky six times the diameter of the full Moon,
the Cygnus Loop is actually the expanding blastwave from a stellar
cataclysm - a supernova explosion - which occurred about 15,000 years
ago.

In this image the supernova blast wave, which is moving from left to
right across the field of view, has recently hit a cloud of denser
than average interstellar gas. This collision drives shock waves into
the cloud that heats interstellar gas, causing it to glow.

Just as the microscope revolutionized the study of the human body by
revealing the workings of cells, the Hubble Space Telescope is offering
astronomers an unprecedented look at fine structure within these shock
fronts. Astronomers have been performing calculations of what should
go on behind shock fronts for about the last 20 years, but detailed
observations have not been possible until Hubble.

This image was taken with Hubble's Wide Field and Planetary Camera 2
(WFPC2). The color is produced by composite of three different
images. Blue shows emission from "doubly ionized" oxygen atoms (atoms
that have had two electrons stripped away) produced by the heat behind
the shock front. Red shows light given off by "singly ionized" sulfur
atoms (sulfur atoms that are missing a single electron). This sulfur
emission arises well behind the shock front, in gas that has had a
chance to cool since the passage of the shock. Green shows light
emitted by hydrogen atoms. Much of the hydrogen emission comes from an
extremely thin zone (only several times the distance between the Sun
and Earth) immediately behind the shock front itself. These thin
regions appear as sharp, green, filaments in the image.

This supernova remnant lies 2,500 light-years away in the constellation
Cygnus the Swan.

Credit: Jeff Hester (Arizona State University) and NASA