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Lars Lindberg Christensen
Hubble European Information Centre
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A gauzy remnant of an ancient supernova flies through space and is
captured by cameras aboard the NASA/ESA Hubble Space Telescope, showing
part of the famous Cygnus Loop in unprecedented detail.
The fascinating smoke-like wisps of gas in this Hubble image are a
record of the enormous amounts of energy released as the fast-moving
supernova explosion ploughs into its surroundings and creates a shock
front. The rapid release of energy in the shock of the collision, when
the supernova material smashes into the gas of the interstellar medium
at a speed of more than 600,000 kilometres per hour, makes the gas glow.
In this image, the motion of the shock front is upwards.
The Cygnus loop, also known as the Veil Nebula, is well-known to amateur
astronomers as a challenging target for larger telescopes. It has a
diameter of about 3 degrees (corresponding to 6 full moons). This
spectacular nebula was created when a massive star ended its days in an
immense supernova explosion. A bubble of dust and gas was expelled into
space and has continued to expand outwards ever since.
The Cygnus Loop consists of two main arcs, designated NGC 6992/95 for
the Eastern arc (to the left in the background image) and NGC 6960 for
the Western arc (also called the Witch's Broom Nebula, to the right,
close to the bright, magnitude 4, star 52 Cygni). The tiny area of the
Hubble image is seen in the upper left-hand corner of the nebula, at the
outer edge of one of the large filaments, just where the blast wave rams
into the surrounding interstellar gas. We are seeing the shock front
almost exactly edge-on, thus explaining its wispy, sheet-like
appearance.
The image is a striking example of how processes that take place
hundreds of light years away can sometimes resemble effects we see
around us in our daily life. The image has similarities with the pattern
formed by the interplay of light and shadow on the bottom of a swimming
pool (known as a caustic network), rising smoke or a ragged cirrus cloud
seen in a summer sky.
By comparing this Hubble image with an old ground-based photograph from
1953, scientists have measured how far the shock front has actually
moved. The result shows that the supernova remnant lies much closer to
Earth than previously thought, at a distance of merely 1500 light years.
This also implies that the explosion may have occurred only 5,000 years
ago and must then have been an awesome sight for early civilisations,
even visible at daytime (with an estimated brightness of at least
magnitude -8, corresponding to the crescent Moon).
Facts about the image
This image comes from the large archive of scientific observations
performed with the Hubble Space Telescope. It was obtained with the
WFPC2 instrument in November 1997 with an exposure time of 7400 seconds.
The image shows light emitted by neutral hydrogen (656.3 nm) and
measures 150 x 70 arc-seconds.
The ground-based image of the Cygnus Loop (shown in the background)
measures 3 x 2 degrees and was taken with the Oschin Schmidt Telescope
and scanned as part of the Digitized Sky Survey.
Images and captions are available on: