Supernova remnants are the final remains of a star which has exploded as a supernova. The supernova remnant (SNR) contains most of the material which once comprised the star. This ejected material then expands out into the interstellar medium (IM) spreading the products of nucleosynthesis.

The expansion of a young SNR can actually be observed over a somewhat short timescale, for instance: In 1960 the Crab Nebula was observed and then again in 1974. When superimposed the two images showe an expansion had taken place.

The ejected material sweeps up any surrounding gas and dust as it expands. This then produces a shockwave which excites and ionizes the gas of the remnant causing it to become visible as an emission nebula. The X-rays emited by the supernova itself also plays a large role in the ionization of nearby gasses.

SNR's generally have magnetic fields that are many times stronger than those existing in the IM as well as a large abundance of high-energy electrons. These high energy electrons travelling along the magnetic field lines are accelerated in a direction perpendicular to the field. This force cause the electron to trace out a helix as it circles around the field lines. The velocity changes taking place (in direction) show the electron must be accelerating. Classical electromagnetic theory states that an accelerating charge must give off radiation, which in this form is called synchotron radiation. This causes SNR's to be emiters in the radio band of the electromagnetic spectrum, which we can then use to study them.

The enormous shockwave that is the SNR plows through the interstellar gas and heat it to temperatures on the order of a few million kelvin (our sun is ~5800 kelvin) just behind the shockwave. This heated gas emits X-rays by virtue of its temperature. These X-rays can then be observed and analyzed. One good exapmle is Kepler's SNR, seen to the left. This image combines images from three of NASA's Great Observatories. Blue and green are high and low-energy X-rays, respectively, as imaged by Chandra. Yellow is optical emission from dense knots seen by Hubble. Red is emission from warm dust grains imaged by Spitzer.

As can be seen, SNR's are typically symetrical shells that have varying degrees of brightness around their rims. This is generally due to the patchy structure of the surrounding IM. For a more detailed introduction to supernova remnants, see Brian Williams' SNR page.