# 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

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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.

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

# Black Holes

Apologies for the lengthiness between posts all!  We had a combination of immense amounts of work and a slightly confusing topic (though, mainly the former).  But wait, that’s a perfect progenitor to create the title object!

Black holes—one of the great mysteries of the universe. They made Stephen Hawking’s head hurt, helped spawn relativistic theory…they are, indeed, wonderful objects that may or may not confuse the heck out of you.

In general terms, black holes are warped regions in spacetime where singularity points (singularities) with infinite density have enough gravity to pull in anything—including light, which means that we can’t see them. It is infinitely dense because density = mass/volume, and the volume of a singularity is a point or zero. There are a lot of technicalities in black holes—we’ll cover them one by one.

Firstly, let’s look at how black holes are formed. They are giant—maybe even supergiant—stars that explode into a supernova and then collapse in on themselves. Normally, they would turn into a neutron star and stay there, but if the neutron star exceeds the Tolman-Oppenheimer-Volkoff limit, then it collapses and turns into—drumroooooll…a black hole.

An illustration of Cygnus X-1, widely thought to be a black hole. Credit: CXO

Now we’ll go over general black hole anatomy. The fabric of spacetime is usually smooth and even, with paths going out equally in all directions that can be traveled on. A black hole warps that—it makes a hole, essentially, that pulls everything in. The paths don’t go equally in all directions anymore, but instead all lead towards the center of the black hole. The centre of a black is a singularity of no mass, which warps spacetime to an infinite degree. Around it, there is this thing called an event horizon, which is the “point of no return”. Once an object goes past the event horizon, it cannot escape the black hole’s gravitational pull. A black hole may grow in size by accreting other objects of mass (contrary to popular belief, black holes do not “suck”), slowly growing larger and larger.

A black hole has three independent physical properties (which means properties that can be observed from outside the black hole): mass, charge, and angular momentum. Any two black holes found to have the same values for all of these are said to be indistinguishable and therefore black hole “twins”, if you will. This is what we call the no-hair theorem. There are two main kinds of black holes—the kind that rotate, and the kind that don’t.

The ones that don’t rotate are called Schwarzschild black holes. These black holes have no electric charge or angular momentum, but only mass. They are as simple as you get—unmoving, gravitational vortexes in the spacetime continuum. These are also just about the only objects in the universe that are perfect—and I mean perfect—spheres. They’re also special in another way—once you get past the event horizon, there is absolutely no way you will be able to avoid getting sucked into the singularity. None. You don’t stand a chance against the forces of a black hole.

Kerr black holes, on the other hand, do rotate. There is a theory—just a theory, mind—that states that it is possible, under perfect conditions, for matter to travel through its rotating ring of a singularity. The theory says that when you exit the black hole, you will most likely end up in another universe, with the black hole acting as an Einstein-Rosen bridge, or wormhole.  This is because the spinning produces what’s called frame dragging, producing an ergosphere.  This actually spins the fabric of space time, making objects able to travel around the black hole, and sometimes around the singularity as we said.

Theoretically, charged black holes exist as well, but in reality, the charge tends to attract particles of the opposite charge, which quickly cancel it out. Charged black holes that don’t rotate are called Reissner-Nordström black holes, and the ones that do rotate are called Kerr-Newman black holes.

The event horizon (Schwarzschild Radius) of a black hole. Credit: University of Colorado, Center for Astrophysics and Space Astronomy

The Schwarzschild radius (derived from escape velocity for those who are curious) is given by the equation ${ R }_{ s }=\frac { 2GM }{ { c }^{ 2 } }$ , where G is the gravitational constant, M is the mass of the black hole, and c is the speed of light.

Another way to change up black holes is by mass, which is shown with that there Schwarzschild radius.  Black holes are normally created by stars with masses between lower and upper mass limits of about 3 to 100 solar masses.  In real life…weird results have come up with impossibly massive stars or stars very close to the lower limit).  But even then, there are all sorts, theoretical or not. The types include:

• Micro black holes—Hawking theorized that low-mass black holes could be formed by high pressures in the early universe, if these exist, they might emit bursts of Hawking radiation in the (relatively) near future as they evaporate.  They also may, for an instant, be created here on Earth in the LHC!  Alternatively, we hear of work being done with lasers to make them (oh, how science is awesome).
• Stellar-mass black holes—3 to 15 solar masses, the major example used is usually Cygnus X-1, and they can form high energy X-ray binaries.
• Intermediate mass black holes—100 to 1000 solar masses, theoretically the center of globular clusters, may form from mergers of stellar-mass black holes, and may eventually merge into a supermassive black hole.
• Supermassive black holes—10^5 to 10^9 solar masses, the theoretical centers of spiral galaxies, thought to explain AGNs and quasars.
• Primordial black holes—very large and small ones that formed in the early universe, 10^-8 to 10^5 solar masses.  They aren’t fully understood.

Black holes also have another effect called gravitational lensing, which is when gravity causes light to bend so much that we can see multiple images of the same object. We follow the light rays in a straight path (which is normally what’s done with lenses), so we can see their apparent locations. Cool, huh? And if the black hole is perfectly symmetrical with respect to the line from Earth to whatever object we’re viewing, well, we can see an entire ring of the object. It looks something like this:

Credit: NASA Goddard Space Flight Center

Unfortunately, however, the black hole is rarely ever perfectly in line with Earth and the object we’re trying to view. So, instead, we see two images, like this:

Credit: NASA Goddard Space Flight Center

So what happens if you fall into a black hole, you ask? Well, it’s really not pretty. Once you get past the event horizon, you’re basically dead meat. There are ways to slow your descent into the singularity, but really…it’s hopeless. Once you get to a certain point, you just get sucked into the singularity, where your body is crushed into infinite density and added to the mass of the black hole. Right before that, however, you get torn apart by tidal forces of gravity and turn into a long, noodle-ish looking thing. That’s called spaghettification. It is really not pleasant. I would advise you to never try that.

But wait! There’s more! How do we know black holes exist if we can’t see them? The answer is this: Even though we can’t see them, they have effects on the surrounding regions that lets us know that they are indeed there. Their gravity is still great enough that they have effects on surrounding objects, though really, it’s not obvious enough that we’d be able to find it. But the most obvious thing they do? Because they accrete so much stuff, they radiate X-rays and energy like you wouldn’t believe. Just tons and tons of X-rays and energy—and best of all? Usually, they’re accompanied by these giant relativistic jets of gas, spouting from either end of the black hole. We don’t know why, or how, but there you go. Huge jets of glowing gas blowing out of black holes. And that is how we know they’re there.

Active galaxy Cen A, with jets powered by the supermassive black hole residing at its center. Credit: European Southern Observatory

There’s also something else called a naked singularity, which technically isn’t a black hole because it lacks an event horizon, but for all other purposes and intents, is the same thing. It is a massless singularity of infinite density that sucks in just about everything around it—it just doesn’t have an event horizon. However, no naked singularities have been discovered yet, so it seems that black holes like to stay clothed (as stated by the Cosmic Censorship Theory).

We mentioned Hawking radiation earlier in the post, but what exactly is it? Well, the real explanation is beyond the comprehension of any of your resident astro geeks, but we’ll just say that sometimes particle-antiparticle pairs are created at the event horizon, and one escapes and one does not. These particles result in evaporation, which takes away energy and therefore mass from the black hole.  Hawking radiation increases as the black hole loses mass, so at the very end of a black hole’s life, it releases a powerful burst of high-energy radiation.  In fact, some primordial black holes are thought to be going through this right about now.

Another point to make is what happens when “stuff” falls into a black hole. This “stuff” has various properties that are dubbed information. From there, there is a debate.  Perhaps not the Great Debate, but a fairly important one that has resulted in the black hole information loss paradox. This pretty much debates whether information is lost by a black hole. Some say it is, some say it isn’t (cue the Thorne-Hawking-Preskill debate). It is a paradox because two scientists, Hawking and Bekenstein, found that with Hawking radiation, a black hole evaporating information thrown into it would make that information permanently disappear. Forever. But then this violates some more quantum mechanics that are somewhat difficult to comprehend. Some say that the information should also exist just in a slightly…changed form. Again, we don’t even pretend to really understand these theories, but it just goes to show how complicated these strange astronomical objects are.

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TL;DR: Black holes are the condensed remnants of extremely massive stars. Their density is so high that all sorts of pressures that would normally oppose gravity can’t fight anymore, and the battle for equilibrium is over.  So the mass basically condenses down to a point with many properties that aren’t fully understood. First of all, black holes technically can’t directly be seen, as light would warp around it.  But we can still manage to infer properties with this major disadvantage!  For example, properties of a black hole can be focused to mass, charge, and spin—these factors result in different types of black holes, but that’s beside the point (see what we did there?).  Black holes also have tons of anatomy to them, even if this isn’t biology.  Most importantly, their “size” is defined by a Schwarzschild radius, which defines the event horizon, the outer limit where the black hole’s effects exists. Hawking radiation is where black holes lose mass through some fancy quantum physics that we don’t pretend to begin to fully comprehend, but it does show that black holes don’t actually last forever.  Interestingly, they also end with a bang.