Supernova remnant

A supernova remnant, or SNR, is the complex structure left behind after a star explodes. These remnants are vital to our understanding of the life cycles of stars and the evolution of galaxies. An SNR is composed of two main parts: the ejected material from the star and the interstellar material it sweeps up. As the star explodes, it creates an expanding shock wave. This wave moves through the surrounding space, collecting gas and dust along its path.

There are two primary ways a supernova occurs. In the first route, a massive star runs out of its nuclear fuel. The star stops generating fusion energy in its core. This causes the star to collapse inward under the force of its own gravity. This collapse can result in the formation of a neutron star or a black hole. In the second route, a white dwarf star pulls material from a companion star. Once the white dwarf reaches a critical mass, it undergoes a carbon detonation. Both processes result in a massive explosion that expels stellar material at incredible speeds. These velocities can reach 30,000 km/s, which is about 10% of the speed of light.

As the explosion happens, a strong shock wave forms ahead of the ejected material. This shock wave heats the upstream plasma to temperatures well above millions of Kelvin. The remnant moves through several distinct stages as it expands. First, it undergoes free expansion of the ejecta. This lasts until the ejecta sweeps up an amount of interstellar medium equal to its own weight. This stage can last from tens to a few hundred years. Next, the remnant enters the Sedov-Taylor phase. During this phase, the remnant sweeps up a shell of shocked gas. This stage is often modeled using a self-similar analytic solution known as the Taylor–von Neumann–Sedov blast wave.

As the remnant ages further, it enters the pressure-driven snowplow phase. In this stage, the shell of gas cools down. This creates a thin, dense shell surrounding a hot interior. The shell can be less than one parsec thick. It can contain between 1 and 100 million atoms per cubic meter. This shell is often visible through optical emission from ionized hydrogen and oxygen. Eventually, the interior of the remnant cools as well. The dense shell continues to expand due to its own momentum. This stage is best observed through radio emission from neutral hydrogen atoms. Finally, after roughly 30,000 years, the remnant merges with the surrounding interstellar medium. It does this when its speed falls below the local sound speed. At this point, it contributes its remaining kinetic energy to the general turbulent flow of space.

Astronomers classify supernova remnants into three main types. Shell-like remnants, such as Cassiopeia A, consist primarily of a shell of material. Composite remnants feature a shell that contains a central pulsar wind nebula. Examples include G11.2-0.3 and G21.5-0.9. Mixed-morphology remnants, also called thermal composites, show central thermal X-ray emission enclosed by a radio shell. In these cases, the X-rays come from swept-up interstellar material rather than the star's own ejecta. The remnants W28 and W44 are examples of this class. Additionally, hypernova remnants are a special type. These are created by explosions with much higher ejection energies than a standard supernova.

History has provided us with many famous examples of these events. The Crab Nebula is one of the most well-observed young remnants. The Tycho remnant is named after Tycho Brahe, who recorded its original explosion in 1572. Similarly, the Kepler remnant is named after Johannes Kepler, who observed an explosion in 1604. In February 1987, astronomers observed the supernova SN 1987A in the Large Magellanic Cloud. This provided a very clear look at a young remnant. Within our own Milky Way, the youngest known remnant is G1.9+0.3. This was discovered near the Galactic Center.

Supernova remnants are also considered the major source of galactic cosmic rays. This connection was first suggested by Walter Baade and Fritz Zwicky in 1934. Scientists Vitaly Ginzburg and Sergei Syrovatskii noted in 1964 that if the efficiency of cosmic ray acceleration is 10 percent, it could compensate for cosmic ray losses in the Milky Way. This is explained by shock wave acceleration. Enrico Fermi proposed a model in 1949 where particles collide with magnetic clouds. This is known as the Second Order Fermi Mechanism. A later model, the First Order Fermi Mechanism, suggests particles gain energy by repeatedly crossing a powerful shock front. While remnants produce very high energy particles, it is still unclear if they can accelerate particles up to PeV energies. The future CTA telescope may help answer this question.