Supernova Remnants

When a supernova explodes, it blasts a rapidly expanding shell of material from the outer layers of the star into the surrounding space. We call this expanding shell a supernova remnant. The shock wave associated with the expanding shell of hot gas and dust modifies the interstellar medium as it passes through it, both by shock heating and compression, and by enriching it in heavy elements produced in the star that went supernova (see the right panel).

Expanding Nebulae

When the remnant grows large enough, it may be visible from Earth as an expanding nebula. A famous example is the Crab Nebula in Taurus, which is the remnant of the supernova of A.D. 1054. In many cases, such supernova remnants can be identified that, because of their size and expansion velocities, must be associated with supernovae that exploded long ago. By extrapolating the motion of the expanding remnant backwards, we can infer approximately when and where the supernova that produced it exploded. The images on this page show examples of such supernova remnants.

The Cygnus Loop

The adjacent image shows a portion of a beautiful supernova remnant called the Cygnus Loop or NGC 6960/95. The Cygnus Loop is a nebula in the constellation Cygnus that is about 2500 light years away in the plane of the galaxy and covers about 3 degrees of the sky (six times the diameter of the full Moon). Analysis of its motion indicates that it is the remains of a supernova that exploded in Cygnus about 15,000 years ago.

As the shockwave moves through the interstellar medium it excites the atoms that it encounters and they emit light in the visible and other regions of the spectrum. Thus, a supernova remnant serves as a probe of the thin gases in the interstellar medium. Although the Cygnus Loop has been slowed in its expansion by the encounter with the interstellar medium, its faster parts are still moving at several million kilometers per hour.

Interpreting the Colors
The image shown above left was taken with the Wide Field and Planetary Camera (WFPC2) on the Hubble Space Telescope. It is a superposition of three images. In the color coding used here, the green regions represent hydrogen emission, the blue regions represent emission from doubly ionized oxygen, and the red regions indicate emission from singly ionized sulfur atoms.
Generally, the oxygen emission (blue) results from heating of gas behind the shock front in the temperature range 30,000-60,000 K and the sulfur emission (red) occurs in gas well behind the shock front that has had time to cool to around 10,000 K since passage of the shock. The hydrogen emission (green) occurs only in a thin region a few AU in width immediately behind the shock front and defines sharp green filaments in the image.