Asupernova remnant (SNR) is the structure resulting from the explosion of astar in asupernova. The supernova remnant is bounded by an expandingshock 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 common 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 aneutron star or ablack hole; or awhite dwarf star mayaccrete material from a companion star until it reaches a critical mass and undergoes athermonuclear explosion.
In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 10% the speed of light (or approximately 30,000 km/s) and a strongshock wave forms ahead of the ejecta. That heats the upstreamplasma up to temperatures well above millions of K. The shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds or thousands of years and over tens ofparsecs before its speed falls below the local sound speed.
One of the best observed young supernova remnants was formed bySN 1987A, a supernova in theLarge Magellanic Cloud that was observed in February 1987. Other well-known supernova remnants include theCrab Nebula; Tycho, the remnant ofSN 1572, named afterTycho Brahe who recorded the brightness of its original explosion; and Kepler, the remnant ofSN 1604, named afterJohannes Kepler. The youngest known remnant in theMilky Way isG1.9+0.3, discovered in theGalactic Center.[1]
An SNR passes through the following stages as it expands:[2]
There are three types of supernova remnant:
Remnants which could only be created by significantly higher ejection energies than a standard supernova are calledhypernova remnants, after the high-energyhypernova explosion that is assumed to have created them.[3]
Supernova remnants are considered the major source ofgalactic cosmic rays.[4][5][6] The connection between cosmic rays and supernovas was first suggested byWalter Baade andFritz 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 onEnrico Fermi's ideas, which is still under development.[7]
In 1949, Fermi proposed a model for the acceleration of cosmic rays through particle collisions with magnetic clouds in theinterstellar medium.[8] This process, known as the "Second OrderFermi Mechanism", increases particle energy during head-on collisions, resulting in a steady gain in energy. A later model to produce Fermi Acceleration was generated by a powerful shock front moving through space. Particles that repeatedly cross the front of the shock can gain significant increases in energy. This became known as the "First Order Fermi Mechanism".[9]
Supernova remnants can provide the energetic shock fronts required to generate ultra-high energy cosmic rays. Observation of theSN 1006 remnant in the X-ray has shownsynchrotron emission consistent with it being a source of cosmic rays.[4] However, for energies higher than about 1018 eV a different mechanism is required as supernova remnants cannot provide sufficient energy.[9]
It is still unclear whether supernova remnants accelerate cosmic rays up to PeV energies. The future telescopeCTA will help to answer this question.
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| Pre-stellar nebulae | |
| Stellar nebula | |
| Post-stellar nebulae | |
| Clouds | |
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| Intergalactic blobs | |
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