Supernova remnants (SNR) are one of the four categories of nebulae that may exist in a Galaxy (the other three are, diffuse, dark, and planetary nebulae). Everything in the Universe evolves and changes with time, and SNRs are no exception. So, let’s see how a supernova explosion happens, and see the evolution of the resulting supernova remnant.
What is a Supernova Remnant?
A supernova remnant (SNR) is a nebulous structure that results from the explosion of a star in a supernova. The remnant is bounded by an expanding shock wave and comprises from expanding material that was ejected from the eruption of a star, and the interstellar material, it sweeps up and shocks along the way.
Supernova explosion mechanism
There are two distinct paths for a supernova explosion. The first evolves a white dwarf (i.e. a stellar corpse of a star with initial mass < 8 times that of the Sun), which accretes material from a companion star until it reaches an upper mass limit known as the Chandrasekhar limit (this is mass is equal to 1.44 times the mass of Sun) and then undergoes as a thermonuclear explosion. The other evolves a star that with an initial mass > 8 times that of the Sun that runs out of fuel. Since there is no energy production, there is nothing that can stop the star from collapsing. Thus, the star ends up its life through an explosion, leaving behind it a neutron star or black hole.
Both explosion types eject material with a velocity that may reach up to 30,000 km/s, forming a strong shock-wave that is ahead of the ejecta. The shock-wave heats the gas to temperatures that exceed one million K. As the shock propagates in the interstellar medium it progressively slows down, but it continues to sweep up material for a timescale that may reach hundreds thousands of years until its speed falls below the local speed of sound.
Famous supernova remnants
Some of the most famous SNRs in our Galaxy are the Crab Nebula, Tycho’s supernova (also known as SN 1972), and Kepler’s supernova (SN 1604). The youngest SNR in our Galaxy is G.19+0.3. This is located at a distance of 8,500 light-years in the constellation of Sagittarius. The explosion mostly likely happened between 1890 and 1908. Back then we could not observe it since the explosion was obscured from the thick layers of gas and dust that are present near the Galactic center.
On this list, we should also include SN 1987A, a very young SNR in the Large Magellanic Cloud that was formed from the explosion of a massive star, that its light reached to us for the first time in February 1987. SN 1987A due to its proximity offered a unique opportunity to study the evolution of an SNR. Together with the Crab Nebula, they are two of the most studied celestial bodies.
Types of Supernova Remnants
Based on an ideal scenario, a stellar explosion would result in a uniform spherical expanding remnant. The reality is completely different since supernovae have different progenitors. To this, we need to consider density variations in the local interstellar medium, which makes sure that the ejecta will not expand uniformly. Thus, the morphological types of supernova remnants are classified into three categories. These are:
- Shell type remnants: As the term indicates, shell-type SNR consist and emit most of their energy from a shell of shocked material. Thus, the remnant appears as a bright ring. This is a projection effect, and it happens since we will look at the edges of the three-dimensional shell, so there is more shocked material along the line of sight than elsewhere. This effect is known as limb brightening. Typical examples of shell-type remnants are E102-72 in the Small Magellanic Cloud and G1.9+0.3 in our Milky Way Galaxy.
- Crab-like remnants or plerions: This category of supernova remnants got its name from the prototype example, the Crab nebula. These remnants are powered by a pulsar located at their center. The emission on plerions comes from within their shell, thus they appear as a filled region of emission instead of a ring. Typical examples of plerions are SNR 0540–69.3 (this is known as the twin of the Crab Nebula), in the Large Magellanic Cloud, and G21.5-0.9 in our Galaxy.
- Composite or mixed morphology remnants: These remnants are a crossover between the two remnant types we saw above. So, these remnants appear either shell-like or plerion-like depending on the wavelength of the observations. Thus, some remnants appear shell-like at radio wavelengths and Crab-like in X-rays. Additionally, there are Crab-like remnants that are Crab-like at both radio and X-rays, but they also display shell structures.
Evolution stages of a supernova remnant
A supernova remnant expands into its local interstellar medium, and during this process, its evolution can be divided into distinct phases. These are:
- Free expansion: After the supernova explosion, a shock wave is launched from the stellar core that starts to propagate through the star. The shock wave passes through the star, and it starts expanding into the interstellar medium in a forward direction, but there is also a reverse shock that goes back to the ejecta. The free expansion phase lasts until the shock-wave sweeps mass equal to the ejecta mass. This can last from tens to a few hundreds of years. This depends obviously on the ejecta mass and the density of the local medium.
- Sedov-Taylor phase: When the amount of swept mass exceeds that of the original ejecta, the remnant enters what is known as the Sedov-Taylor phase. The equations that are used to describe this phase are taken from those that were used to describe the expanding fireball of a thermonuclear bomb. At this point, the emission of the remnant comes from a thin shell behind the blast wave. As the shock expands, the pressure drops between the blast wave and the ejected material. This phase lasts between 10,000 and 20,000 years.
- Radiative phase and merge with the ISM: As the remnant continues to expand, the shock wave will keep on cooling. Eventually, the temperature will drop below 20,000 K. This will allow free electrons to recombine with atoms. This is a much more efficient emission mechanism than the thermal emission (i.e., radiation emission generated by the motion of particles) during the Sedov-Taylor phase. This phase can last up to 500,000 years. After this point, the shock-wave slows down to the point that the remnant merges with the local interstellar medium, and it is not distinguishable from it.
Importance of supernova remnants
Supernova remnants affect significantly the evolution of a galaxy. First of all, they enrich the local interstellar medium with heavy elements that are important for planet formation (carbon, calcium, oxygen, etc), and in the case of our planet, life formation. Shockwaves from supernovae can also trigger star formation in their local medium. Studies of SNRs help us to understand the process of stellar nucleosynthesis (i.e., formation of elements) and the evolution of stars by measuring element abundances. The compact remnants that leave behind them (i.e., neutron stars and black holes), provide crucial elements in the physics of matter under extreme conditions. Finally, it is believed that SNRs are responsible for the acceleration of Galactic cosmic rays.