The Cygnus Loop (also called the Veil Nebula), is a dim ring of glowing gas invisible to the unaided eye, which extends over a region six times the apparent diameter of the Full Moon in the northern constellation Cygnus. It lies about 2,500 light years away. Although listed as a supernova remnant, the remains of an exploded star, the Cygnus Loop actually consists overwhelmingly of interstellar clouds that have been swept up by the blast wave from the supernova explosion that occurred an estimated 15,000 years ago. The blast wave has heated the interstellar clouds to temperatures of tens of thousands of degrees. In this view, the blast wave is moving from left to right and has recently hit a cloud of interstellar gas that is denser than average, causing it to glow.
The image shown here is a false color composite of three images, taken through filters that isolate the light from three kinds of atoms, each representing a different temperature range in the Cygnus Loop. The light from oxygen ions (with temperatures of 30,000 to 60,000 kelvins, or about 50,000 to 100,000 degrees Fahrenheit) is shown in blue. The light from hydrogen atoms that occur throughout the Loop is represented in green. Much of the hydrogen emission comes from an extremely thin zone immediately behind the shock front itself. These thin regions appear as sharp green filaments in the image. The light from sulfur atoms, shown in red, is produced in gas that has cooled to about 10,000 kelvins (18,000 degrees Fahrenheit).
Camera: WFPC2
Credit: J. Hester (Arizona State University), and NASA
