A supernova remnant (SNR) is the structure resulting from the gigantic explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.
There are two possible routes to a supernova: either a massive star may run out of fuel, ceasing to generate fusion energy in its core, and collapsing inward under the force of its own gravity to form a neutron star or a black hole; or a white dwarf star may accumulate (accrete) material from a companion star until it reaches a critical mass and undergoes a thermonuclear explosion.
In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 1% the speed of light, some 3,000 km/s. When this material collides with the surrounding circumstellar or interstellar gas, it forms a shock wave that can heat the gas up to temperatures as high as 10 million K, forming a plasma.
Perhaps the most famous and best-observed young SNR was formed by SN 1987A, a supernova in the Large Magellanic Cloud that appeared in 1987 (and exploded approximately 168,000 years ago). Other well-known, older, supernova remnants include Tycho (SN 1572), a remnant named after Tycho Brahe, who recorded the brightness of its original explosion (AD 1572) and Kepler (SN 1604), named after Johannes Kepler. The most recent remnant in our galaxy is G1.9+0.3, discovered in the galactic center.
Summary of stages
An SNR passes through the following stages as it expands:
Types of supernova remnant
There are three types of supernova remnant:
- Shell-like, such as Cassiopeia A
- Composite, in which a shell contains a central pulsar wind nebula, such as G11.2-0.3 or G21.5-0.9.
- Mixed-morphology (also called "thermal composite") remnants, in which central thermal X-ray emission is seen, enclosed by a radio shell. The thermal X-rays are primarily from swept-up interstellar material, rather than supernova ejecta. Examples of this class include the SNRs W28 and W44. (Confusingly, W44 additionally contains a pulsar and pulsar wind nebula; so it is simultaneously both a "classic" composite and a thermal composite.)
Origin of cosmic rays
Supernova remnants are considered the major source of galactic cosmic rays. The connection between cosmic rays and supernovas was first suggested by Walter Baade and Fritz Zwicky in 1934. Vitaly Ginzburg and Sergei Syrovatskii in 1964 remarked that if the efficiency of cosmic ray acceleration in supernova remnants is about 10 percent, the cosmic ray losses of the Milky Way are compensated. This hypothesis is supported by a specific mechanism called "shock wave acceleration" based on Enrico Fermi's ideas, which is still under development.
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