Everyone has heard of supernovae, but it seems like there is some unwritten rule that they must never be depicted correctly in popular sci-fi. Starcraft 2 is, sadly, no exception. I don’t think it’s too much of a spoiler to say that there is a mission in the campaign where you have to battle your enemies to gain control of a relic on a planet that is about to be consumed by the fires of a huge blue star that is “going nova”. Or supernova. The words are used interchangeably.
As an astronomer, I always cringe when this happens, so I want to clear up what exactly supernovae and novae are.
The confusion is understandable: both are stellar explosions, and their names imply that a supernova is just a big nova. But in most cases this isn’t true. Let’s consider supernovae first, since by understanding them we will already be well on our way to understanding novae.
A supernova is when a very large star explodes. But that’s the end of a long process, so let’s rewind to the beginning: a star starts off as a ball of (mostly) hydrogen gas that is compacted into a dense sphere by its own gravity. At some point, the immense pressure of all that hydrogen is enough to force the atoms in the star’s core to collide with one another and merge into helium, releasing a huge amount of energy. This process is called fusion, and for most of a star’s life, it sits there turning hydrogen into helium and producing lots of energy. That energy makes the star so hot that it glows, and it also provides the pressure that keeps gravity from crushing the star any more. Most stars are poised in perfect balance with gravity trying to compress them and the nuclear reactions in their cores trying to expand them.
The problem with this lovely equilibrium is that helium is more dense than hydrogen, so it sinks to the core of the star. Eventually, there isn’t enough hydrogen in the core to maintain fusion and the internal energy source fizzles. Without fusion supporting the star, it begins to collapse. The pressure and temperature at the core increases until the helium atoms are colliding often enough and hard enough to form carbon. This produces energy again and prevents the star from collapsing, but the core has shrunk and is much hotter. Surrounding the helium-burning core is a shell of hydrogen burning. The intense radiation from the core puffs up the outer layers of the star, giving it much more surface area. It’s a bit unintuitive, but the surface of the star actually cools down when this happens. It has expanded so much that even though the star is producing more energy than before, that energy is spread out over its much larger surface, and so instead of being a bright white or blue star, the star becomes a red giant.
In really large stars, this cycle repeats many times. When the carbon in the core runs out, it compresses even more until carbon fuses into neon. Neon breaks down to form oxygen, and oxygen fuses with hydrogen, helium and neon to form heavier elements. This process continues, forming layers of fusion in the star’s core. Like an atomic assembly line, each layer uses the products from the layer above it to produce new elements and more energy. Until it reaches iron.
Iron is a special element because its nucleus is extremely stable. So stable, that unlike lighter elements, it consumes energy to fuse two iron nuclei together, rather than releasing energy. When a star’s core fills up with iron, it suddenly loses its internal energy source. Gravity, a weak but infinitely patient force, finally wins out over the star’s internal pressure. The star implodes.
As the billions of tons of matter come rushing together at relativistic speeds, all hell breaks loose. Atoms are disintegrated and recombined into exotic isotopes and giant atoms like lead and plutonium and uranium. Every atom in the universe heavier than iron was formed in the chaotic forge of a supernova. Faster than the blink of an eye, the collapse rebounds off of itself and becomes a tremendous explosion that shatters the star and sends all of its newly formed elements streaming into space.
That is a supernova. Left behind where the core of the star was is a tiny remnant. If the star was less than about 20 times the mass of the sun, the remnant will be a neutron star: a ball of neutrons the size of a city but with the mass of a star. Neutron stars are among the most awesome objects in the universe. Instead of the pressure due to fusion that supports normal stars, neutron stars are supported by “degeneracy pressure”: the fact that you can’t put two neutrons in the same place. Neutron stars spin thousands of times per second, they have tremendous magnetic fields, and they are so dense that a teaspoon of neutron star-stuff would weigh about ten million tons. If you dropped that teaspoon on earth, it would punch holes in our planet like it wasn’t even there.
If the star was really big, then even degeneracy pressure isn’t enough to hold up the material left over in the remnant. In this case gravity really does win. The stuff collapses and just continues to collapse forever. It becomes so dense that you would have to go faster than the speed of light to escape its gravity. The core of the star forms a black hole.
Ok, so that’s a supernova. So what’s a nova? A nova is a special case that happens in binary star systems. See, not all stars explode as supernovae and form neutron stars or black holes. Most stars like the sun die a much gentler death, shrugging off their outer shells of gas in a gently expanding nebula and leaving behind a white-hot ball of carbon and oxygen called a white dwarf. In a binary system, one star can evolve to become a white dwarf while the other is still happily burning hydrogen, so you end up with a white dwarf and a normal star orbiting each other. When the second star begins to expand to form a red giant, something weird happens. It can’t expand! Instead, once it grows past a certain size, the white dwarf begins to suck up the outer layers of its neighboring star. Eventually, enough hydrogen accumulates on the surface of the white dwarf for fusion to begin again: boom! The white dwarf lights up as it burns up the fuel it borrowed from its neighbor. But that fuel doesn’t last long, so the star cools down after a little while and waits until it has enough fuel again. That’s a nova. It’s a brief, brilliant flash of light from a dead star that is sucking fuel from its neighbor like a vampire.
Just to complicate things, it is possible for a nova to become a supernova. Eventually, the white dwarf will accumulate so much mass from its neighbor that it can no longer hold itself up against gravity. In a fraction of a second, the white dwarf will collapse and rebound in a supernova explosion. This type of supernova (type Ia) leaves no remnant, but it always produces the same amount of light, since it always comes from a white dwarf that has the same mass. This makes Type Ia supernova very useful for measuring distance in the universe. If you know how bright something is, and you can see how dim it looks due to distance, you can figure out how far away it is.
So what does all of this have to do with starcraft? Well, in the mission in question, the star is “about to go nova”. But it’s clearly not a white dwarf. If anything, based on the picture it is a blue giant. If we’re meant to understand that it is about to go supernova, then we run into another problem: the planet. Stars that are about to go supernova have expanded to hundreds or thousands of times their original size, and in doing so they typically consume their planets. When the sun becomes a red giant, its surface will be about where Earth is now. Even if the planet was far enough away that it was spared, the solar wind from a giant star would be powerful enough to rip the atmosphere away from a rocky planet easily. In fact, since the star depicted is blue, it probably has such a powerful stellar wind that its cool outer layers have been blown away. The planet would be a radiation-baked wasteland long before the star exploded. And if you were on a planet when its star exploded, you wouldn’t be faced with a slowly advancing wall of fire. The planet would be scorched like a moth in a bug zapper. Poof. A rocky, highly radioactive lump might be left behind.
And finally, you can’t look at a giant star and say “we have four hours before that thing goes supernova!”. It doesn’t work like that. You’d be lucky if you could pinpoint the explosion to within a few million years. Stars evolve on a timescale that is so much longer than anything we’re used to, it’s hard to comprehend.