This spectacular image shows a small part of the Cygnus Loop (also called the Veil Nebula), as recorded on April 24, 1991. The Loop, a dim ring of glowing gas invisible to the unaided eye, extends over a region six times the apparent diameter of the Full Moon in the northern constellation Cygnus, about 2,600 light years from the Earth.
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. It is roughly analagous to the snow that accumulates ahead of the blade of a snowplow. But the Loop is far hotter than snow! The blast wave has heated the interstellar clouds to temperatures of tens of thousands of degrees.
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, including the striking ribbon of light that extends from left to right. A study led by Dr. Jeff Hester (Arizona State University, Tempe, AZ) suggests that this may be an "interstellar bullet," a clump of gas that was ejected from the supernova long ago and that is just now catching up with the blast wave at a speed of about 5 million kilometers per hour (about 3 million miles per hour). The blast itself has been slowed by running into the interstellar clouds.
The light from hydrogen atoms that occur throughout the Loop is represented in green, but is hardly discernible in the image because it coincides with the regions shown in other colors. 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: WFPC-1
Credit: J. Hester (Arizona State Universty), and NASA
