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-1. 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 was discovered in 1987. A few other well-known, older, supernova remnants are the Crab Nebula, a remnant of a relatively recent explosion (AD 1054); 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.
Summary of Stages
A SNR passses through the following stages as it expands:
- Free expansion of the ejecta, until they sweep up their own weight in circumstellar or interstellar medium. This can last tens to a few hundred years depending on the density of the surrounding gas.
- Sweeping up of a shell of shocked circumstellar and interstellar gas. This begins the Sedov-Taylor phase, which can be well modeled by a self-similar analytic solution. Strong X-ray emission traces the strong shock waves and hot shocked gas.
- Cooling of the shell, to form a thin (< 1 pc), dense (1-100 million atoms m-3) shell surrounding the hot (few million K) interior. This is the pressure-driven snowplow phase. The shell can be clearly seen in optical emission from recombining ionized hydrogen and ionized oxygen atoms.
- Cooling of the interior. The dense shell continues to expand from its own momentum, in a momentum-driven snowplow. This stage is best seen in the radio emission from neutral hydrogen atoms.
- Merging with the surrounding interstellar medium. When the supernova remnant slows to the speed of the random velocities in the surrounding medium, after roughly a million years, it will merge into the general turbulent flow, contributing its remaining kinetic energy to the turbulence.