When a supernova explodes it blasts a rapidly expanding shell of material from the outer layers of the star called a supernova remnant into the surrounding space. 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. It is likely that most of the heavier elements that we presently find in the Universe (and in our bodies) were distributed in this way: parts of you such as the carbon atoms in your cells or the iron atoms in your blood were likely flung out of a supernova explosion in the distant past of our galaxy!

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 1054 A. D.

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 above right image shows a portion of a beautiful supernova remnant called the Cygnus Loop or NGC 6960/95 (Ref). 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 (6 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 as it passes through them. 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.

The image shown above 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 (Ref). Generally, the oxygen emission (blue) results from heating of gas behind the shock front in the temperature range 30,000-60,000 degrees Celsius and the sulfur emission (red) occurs in gas well behind the shock front that has had time to cool to around 10,000 degrees C since passage of the shock. The hydrogen emission (green) occurs only in a thin region a few A. U. in width immediately behind the shock front and defines sharp green filaments in the image.

The Tarantula Nebula

The image adjacent left shows a portion of the Tarantula Nebula in the Large Magellanic Cloud. The region at the lower right is a group of brilliant massive stars called Hodge 301. Many massive stars in Hodge 301 have already become supernovae. The material that they have blasted into the surrounding nebula is shocking and compressing the gas and producing the sheets and filaments seen in the rest of the image. There are at least 3 red supergiants in Hodge 301 that are likely to become supernovae soon, so the modification of this portion of the Tarantula Nebula by supernova remnants will continue (Ref).

X-Ray Emission from Supernova Remnants

The Cygnus Loop also emits at other wavelengths. The image below left shows an overview of the entire Loop at X-ray wavelengths. We see that the shock heating of the interstellar medium by the supernova leads to a diffuse X-ray glow associated with this entire region of space. Below right is a ROSAT X-ray image of the remnant of the supernova of 1572, also known as Tycho's supernova, since it was observed by Tycho Brahe (Ref).

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