When a star dies, that can be a violent affair. I wrote about the birth of stars and planets before, and about black holes , but not their violent deaths. So today, I’ll discuss supernovae.
A supernova is a violent and very bright explosion of a star.
There are actually several types of supernovae. In all cases, a huge amount of energy and matter is expelled by the star, resulting in an explosion that can sometimes be visible from Earth with the naked eye. In fact, there’s a theory that a rock carving in India from 3600 BC is in fact a drawing of a supernova. A more certain observation is from China in 185 AD.
These days, we have equipment sophisticated enough to observe supernovae that might occur in different galaxies altogether.
Supernovae are bright, and thus easy to spot. Unfortunately, they are also rare and don’t last very long — relatively speaking. The chances of witnessing one as it happens are extremely small. However, luck would have it that that is exactly what happened in Argentina two years ago.
A star is really just a giant fusion reactor. It takes atoms and the combination of heat and pressure to start up nuclear fusion. This means that the atoms are fused together to form new heavier atoms. This leads to the conversion of mass to energy, causing new heat, fueling the process. In this way stars can burn through elements, starting with hydrogen, then helium, and onward, burning lithium, carbon, etc. until they reach iron.
Iron is the tipping point on the period table. Beyond iron, fusing atoms doesn’t actually produce energy, it costs energy — and vice versa, starting at Iron, nuclear fission starts to yield energy instead of costing it. So, when this happens in a star, the nuclear reaction in the star becoming weaker, and gravity can start to take over. This happens when the mass exceeds the so-called Chandrasekhar limit, which is around 1,4 times the mass of our sun.
When exceeding the limit, the iron core of the star will collapse in on itself, creating a neutron star or black hole. This process jettisons the outer layers of the star violently. And voila, a supernova.
Well, okay, the iron collapsing is one mechanism. There are variations, with supermassive stars resulting in something called photodisintegration, and electron capture in a degenerate O+Ne+Mg core, but aside from the cool names, they basically have the same end result: star go boom.
Thermal runaway supernovae
Core collapse is one process, but it turns out there is a second mechanism. Normally, when a white dwarf star is lighter than the Chandrasekhar limit, it should not go nova. However, if a white dwarf star gets hot enough, carbon fusion can start to occur, which causes a positive feedback loop of energy causing more fusion. This chain reaction then blows up the stars, completely destroying it.
This process can not occur without ‘help’. A white dwarf is too small, and an external source of mass is needed. This can happen when two white dwarfs orbit each other in a binary system and one is bigger than the other. The bigger will start to accrete mass from the smaller, leading to a big boom — and ejecting the smaller in the process.
Another possibility is for the two white dwarfs to merge into one big star which is larger than the Chandrasekhar limit.
Finally, there is a hypothesis that a lower-luminosity variation of this type of supernova can lead to a so-called zombie star instead of blowing up completely, consisting mostly of helium.
What about the sun?
By now you might be wondering: will our sun go supernova? The answer is: no, it almost certainly won’t. Our sun is a type of yellow dwarf. It’s mass is not sufficient to go supernova. Instead, at some point in the future, it will turn into a red giant.
When a star like our sun burns through most of its hydrogen, it will switch to burning helium. When that happens, it will become roughly two hundred times larger than it is today and envelop Mercury, Venus, and possibly Earth. Even before that, though, we’ll have been cooked by its increased brightness.
Don’t worry, though, this is roughly 5 billion years in the future. By that time we’ll probably have a solution to the problem, if we survive that long. At the rate we’re going, I’ll be surprised if we last to the end of this millennium, let alone five billion years.
More worrying are supernovae happening in our galactic backyard. If Alpha Centauri went supernova tomorrow, a massive wave of radiation would reach us in four years, ripping away our ozone layer and boiling us alive.
But even farther away, the danger would be real. The biggest supernova we know of is as bright as all the hundred millions of stars in the entire milky way. Now that one is rare, but others could occur. If that happened within a few hundred parsecs, we’d be f*cked.
It would be visible in the sky, brighter than most stars, bombarding us with x-rays and gamma radiation. Our ozone layer would suffer, and we’d have higher temperatures for centuries. It could lead to an extinction level event.
Lucky for us, there don’t appear to be stars on the verge of going supernova in our neighborhood. Still, like asteroids crashing to earth, it could happen at any time. Ask the dinosaurs how fun that is.
Supernovae are the biggest firework shows in the universe. They can be dangerous, but mostly, they’re just interesting.
Let’s just hope one doesn’t happen too close by.