Supernova Remnants

Well, this post will hopefully be smaller, but just as explosive (or implosive?) as the last one!  Today, we are dealing with the actually large topic of supernova remnants, or SNRs.  These things are technically nebula, but not the kind that we start a star off with. No, these are actually for the end of a star!  There are many types, stages, effects, and definitely much significance from yet another one of our magnificent acronyms.

As for types, there are effectively two ways to categorize them.  One is by the original SN that created them.  Let’s start off with that (but technically this isn’t an exact type like the actual SNR types).  Essentially, a SN happens and we are left with the star expanding and releasing heat at extremely high rates.  For all SNs, the SNR that is produced essentially follows the properties of that SN.  For example, Type Ia SNRs would all generally be around the same mass, and the most massive element in them would be iron.  SNRs are also used to find the progenitor event or SN.  Some Type Ia SNRs have been used to create models of white dwarf explosions, leading to the double degenerate progenitor model.  Type Ib, Ic, and II SNRs, on the other hand, give us even higher mass elements, such as gold and even up to uranium. They can also have interesting objects (neutron stars and black holes) left over, and produce neutrinos – which sadly cannot travel faster than light.  I guess you could say they just make quite the big bang.

You’d better thank SNRs for this! Credit: Thomas Jefferson National Accelerator Facility

Next we have the actual way they are classified.  For these remnants of the past, there are many types based on the shape of the SNR and the radiation detected.  Shell types are characterized by being…a shell.  That is, the outer edges are brighter compared to the inner edges.  Next, there are crab remnants, plerions, or pulsar wind nebulae (PWN).  They have less exact shapes than their shelled-up cousins.  They also notably have a pulsar in the center and emit X-rays.  But yes, it basically is based after the Crab Nebula in case you haven’t guessed it.  Then we also have composite remnants, mixtures of both depending on what wavelength is being viewed.  This creates two subtypes of composites.  Thermal composites stay cooped up in shells in the radio, but in X-rays they are crab-like (though, not exactly…), while plerionic composites somehow only show spectral lines in certain parts of the x-ray spectrum, and also have shells.

Cas A, a shell SNR. Credit: University of Minnesota and HST

The Crab Nebula, a crab-type SNR (we figured we might as well go with the obvious example). Credit: European Southern Observatory

W44, a composite SNR (radio = orange, infrared = red, x-ray = blue, gamma-ray = magenta). Credit: NASA/DOE/Fermi LAT Collaboration, ROSAT, JPL-Caltech, and NRAO/AUI

As for stages, there are a few different opinions.  Since NASA uses three, we will too (just for fun).  You may be asking, how are there stages for an SNR?  All the stages of stellar evolution are already over!  But wait, these stages generally just explain what happens after the SN goes bang.  So, we start off with free expansion, where the a shock wave is formed at constant temperature and expansion velocity.  That lasts a few hundred years.  Then we have the Sedov or adiabatic phase (adiabatic means no heat flow occurs).  During this phase the SNR starts to slow down and cool, and parts of the SNR start to mix.  This develops a magnetic field inside, and it lasts from 10,000 to 20,000 years.  The reason we say no heat transfer occurs is because energy is not actually released by radiation from the gas.  Pretty much, so far the SNR has gotten bigger, and it’s starting to have the original insane amounts of heat dissipate throughout it a little bit.  But then we come to our third phase, the snow-plow phase (sadly, no snow will be used in this explanation).  After the SNR has cooled down, electrons actually recombine with elements that were ionized, radiating energy to the point where the SNR actually starts to cool down Remember however, that this is astronomy, so our “cooling down” means it is still pretty hot.  But the snowball effect of recombining causes the expansion speed continues to slow down, and eventually, the material goes into the interstellar medium (ISM), forever…until it is used for another star! Got to love the circle of (stellar) life.

There are a few confusing things that maybe should be cleared up.  So let’s switch wavelengths (or topics)!  For an SNR, there is certainly a forward shock wave or bow shock.  What this means is that the magnetic fields that are being created meet space, and end up forming a curved shape, like a bow.  The word “shock” itself mainly refers to the transfer of energy carried by the expansion.  Sometimes a reverse shock also drives inward, slowing down the expansion.  The next topic of discussion are cosmic rays.  Pretty much, they are high energy radiation of charged particles, but they are puzzling because their energies are so high that we don’t know what they could have come from.  Theoretically, they may come from SNRs, which have a mechanism that accelerates charged particles like electrons to extremely high speeds.  Another aspect that we haven’t mentioned yet is Rayleigh-Taylor instabilities, which are basically differences in density that create mixing and loss of energy within an SNR.  For example, if you’ve ever seen two fluids with different densities (such as oil and water) in a cup, you’d know the more dense substance would sink.  But space isn’t precisely a cup…meaning we get a sort of twisted finger-like structure.

Rayleigh-Taylor instabilities in a glass of water. Credit: American Physical Society


TL;DR — After all this talking, if you haven’t noticed, SNRs are actually important to astronomy.  They are classified based on the type of SN that produced them, or by their shape and radiation.  We aren’t always able to see SNs, but we can find their SNRs to see their properties.  They show fantastic examples of how elements spread around the universe, which means you better thank astronomy for your gold watch or jewelry!  SNRs can show interesting properties such as bow shocks and Rayleigh-Taylor instabilities. To add more fuel to the fire (or star going supernova), they may even produce the mysterious cosmic rays.  Also, they just look pretty awesome.

Sources and links for further reading:

Carroll and Ostlie, An Introduction to Modern Astrophysics 2nd edition, p. 633 to 646


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