There’s a whole bunch of different types of supernovae: Type Ia, Type Ib, Type Ic, and Type II. I am not going to talk about Type Ib and Type Ic because they’re variants of Type II and thus really boring. Type Ia are kind of interesting but the ones I really want to talk about today are Type II supernovae, which are what everyone thinks of as a “classical” supernovae. To make a Type II supernova you take a really, really big star – eight or nine solar masses is the minimum limit – let it chew through its supply of hydrogen through nuclear fusion over a few hundred million years (the bigger a star, the shorter its lifespan) and then wait for the fireworks to start.
Once the star has finished fusing hydrogen into helium, the nuclear fusion process will stop and there will no longer be any outwards radiation pressure resisting the inwards pressure from gravitational collapse. The star will therefore start to gravitationally collapse (duh). This compresses the helium at the heart of the star, heating it up and kickstarting helium fusion into carbon while incidentally restoring some sort of hydrostatic equilibrium to the star, halting the collapse. As helium burns at a higher temperature than hydrogen the core of the star expands and then cools, resulting in a layer of hydrogen surrounding a layer of helium inside which helium is fusing into carbon.
Eventually all the helium runs out and the process repeats; the star begins to collapse, the core compresses, the carbon ignites and the star then expands into a four-layer structure: hydrogen, helium, carbon, and then the inner core where carbon is fusing into neon. This cycle continues again and again – neon goes to oxygen, oxygen goes to silicon, and silicon goes to iron, with a new element layer being added to the star each time giving it this onion-like structure. As the elements being fused get heavier the fusion reaction gets hotter and quicker; it takes a star millions of years to burn its supply of hydrogen, but only a few days to fuse silicon into iron. As a result the star is becoming increasingly unstable, and once the star has fused all of its silicon that instability cumulates as iron fuses into nickel-56, which cannot be further fused as the next step up in the fusion chain – zinc-60 – would actually consume energy to fuse rather than releasing it. The fusion process therefore hits a brick wall. Literally.
Fusion stops. The outer layers of the star now resume their gravitational collapse towards the core unimpeded by any further fusion pressure. What happens next depends on the mass of the core of the star.
Less than 1.38 solar masses: The outer layers of the star accelerate in their collapse until they’re travelling at a sizeable fraction of the speed of light (0.23c). As the star gets smaller the temperature and density of the core increases and the star begins spitting out all sorts of harmful cosmic radiation. Eventually the core has been crushed up so small that what’s propping it up against further collapse is something called the electron degeneracy pressure; this is a quantum thing involving the Pauli exclusion principle1 which says that no two electrons may occupy the same quantum state. If you think back to your secondary school chemistry you may recall that electrons orbit the atomic nuclei in strictly ordered shells of 2-8-8-8-etc. You can’t fit more than 8 electrons into a shell; you have to start a new one if you want to add more electrons to the atom. The Pauli exclusion principle is roughly analogous to this; it sets a limit on the number of electrons you can cram into a single space by forcing new electrons to occupy the lowest empty energy level rather than sharing one that’s already occupied by another electron. Jam enough electrons together and the “lowest empty” energy level will still require a staggering amount of energy for a new electron to occupy, and if the gravitational force of the star cannot provide this amount of energy the star will halt its collapse. In other words, the Pauli exclusion principle provides a quantum pressure that counteracts the collapse of the star – the electron degeneracy pressure.
Once the collapsing stellar matter hits the electron degeneracy pressure limit (and remember, it’s a crapload of mass travelling at 0.23c) it “bounces” off the core and releases a staggering amount of energy in a supernova explosion. This ejects most of the star’s mass at speeds of up to 0.1c as well as an awful lot of radiation, which is bad news for anyone living within a couple of hundred light years. Supernova release so much energy that they briefly outshine all other objects in the sky; a supernova in a another galaxy will outshine and obscure all the other stars in that galaxy combined. This is where the term nova comes from (new star), as the first we see of a supernova is usually a bright new point of light in the sky where there was none before.
There’s still a remnant of the star left after the supernova explosion: the sub-1.38 solar mass core. This is now compressed up very small into a white dwarf; a dead star with the mass of the Sun but with the diameter of the Earth. White dwarves no longer produce heat or light through fusion – all they emit is leftover thermal radiation from the collapse process.
Between 1.38 and 2 solar masses. As above, except the gravitational force of the collapsing star is now so great that the electron degeneracy pressure can no longer prop it up. The star’s gravity doesn’t exactly break the Pauli exclusion principle but it does sidestep it; while the energy requirements to smoosh up electrons together into ever higher energy levels may still be excessive, there is an out provided in that the energy of the electrons will now be high enough that it becomes possible for them to directly bond with protons, creating a whole mess of neutrons. Instead of a white dwarf, the core remnant is now a neutron star propped up by the neutron degeneracy pressure. Since we’ve gotten rid of all those pesky electrons and protons and are just left with neutrons we can compress the core remnant up much more than we would be able to otherwise; neutron stars are thought to be less than 12km in diameter whilst retaining a mass of up to two Suns, making them incredibly dense and giving them the same comedy tidal properties mentioned in the spaghettification part of Tuesday’s post.
A quirk of neutron stars is that they retain all of the parent core’s angular momentum as they collapse. In other words, you now have the total rotational momentum of a 100,000 kilometre diameter core packed down into a 12 kilometre diameter remnant, making them rotate ridiculously fast; neutron stars spin at anywhere from a few rotations per second to hundreds of rotations per second. Since neutron stars blast out intense beams of electromagnetic radiation from their magnetic poles (which are not necessarily aligned with their rotational poles) this can produce a “lighthouse” effect if you happen to live on a planet which is swept by the beam where your radio telescopes will pick up very strong, very regular radio signals that can be mistaken for alien radio transmissions. In actuality it’s just a specific type of neutron star called a pulsar.
Between 2 and 3 solar masses. The core might create a quark star but nobody has any idea how these work or even if they exist so they’re boring. BORING.
Greater than 4 solar masses. The core’s gravitational force is so great that even the neutron degeneracy pressure can’t prop it up against collapse. At this point conventional physics breaks down, the remnant continues to collapse until it disappears up its own fundament, and a singularity is created, with all the associated hijinks detailed in Tuesday’s post.
What’s so important about a stellar core having 1.38 solar masses? This is something called the Chandrasekhar limit, which is the maximum mass a white dwarf can have before it collapses further into a neutron star. It’s possible for white dwarves to exceed the Chandrasekhar limit after they’ve gone supernova by absorbing mass from another nearby source like a companion star; however, if a specific type of white dwarf (a carbon-oxygen white dwarf) does this it instead heats up and undergoes a runaway reaction as it approaches the limit, resulting in a second supernova. Since the trigger mass for this supernova is the same every time (about 1.38 solar masses) the luminosity of the supernova is the same every time; this is the Type Ia supernova I talked about a couple of months ago that can be used as standard candles.
So now you know how black holes are created. Be sure to bore all your friends with this information the next time you’re at a party!
1. It’s quantum in that we’re trying to jam a bunch of electrons together to the point where we can no longer distinguish between them based on their position, therefore we must be able to distinguish between them based on their energy level.
The pun, it burns!
I’m very proud of it.