Science fiction games are littered with dead and dying stars. Mass Effect has its supernova-blasted Mu Relay, eezo mines near neutron stars, and Adepts slinging miniature black holes around. In the Halo universe, artificially triggered supernovae are the ultimate weapon for exterminating Flood-infested systems. In Spore, you can use black holes as entrances to the galactic subway system of wormholes, and in Starcraft 2, there is a mission where a star "goes nova", enveloping the map in fire.
All of these dead and dying stars are undeniably awesome, but how accurate are they? To find out, we need to take a look at how stars live -- and how they die.
The life and death of a star
At the core of a star, the ridiculously high pressure and temperature forces atoms of hydrogen to fuse into helium, producing a huge amount of energy in the process. The sun produces 180 million times more energy every second than all of the nuclear weapons ever exploded.
For most of a star's life, it sits poised in perfect balance, with gravity trying to crush it while the nuclear reactions in the core are trying to blow it apart.
That balance can't last forever.
As hydrogen gets fused into heavier elements, those heavy elements need higher temperatures and pressures for fusion to continue. The core shrinks and heats up, while the outer layers of the star expand like a balloon. This inflated stage in a star's life is called the "red giant" phase. Even though the star is producing more energy than before, that energy is spread out over a much larger surface, so the surface of the star cools down until it is glowing an angry red.
You won't like the sun when it's angry: it will grow until its surface is about where the Earth's orbit is now, burning our planet to a crisp.
Most stars are small, like the sun, and they die once the core fills up with carbon. In their last gasps, the radiation from the core blows the rest of the star into space, forming solar-system-sized bubbles called "planetary nebulae". (They have nothing to do with planets. The name is a holdover from when telescopes weren't very good and a circle of gas in the sky looked like the disk of a faint planet.) At the centre of each planetary nebula is a white dwarf: a glowing-hot ember from the atomic fires that once powered the star. White dwarfs are about the size of the earth and are made mostly of super-dense carbon. As a white dwarf cools, the carbon can actually crystallize to form planet-sized diamonds.
The great thing about astrophysics is that there are real things out in the universe that are stranger than any science fiction. Some of the impressive discoveries in astrophysics like black holes do make their way into popular culture and video games, but other awesome objects and phenomena slip through the cracks. For instance, white dwarfs just aren't very well known. That's a shame because: hey, planet sized diamonds! Ancient white dwarfs that have cooled down so that they aren't glowing anymore would be practically invisible, and could make great secret caches of resources or rendezvous points in otherwise featureless interstellar space. Adding this extra level of sophistication would make the game more realistic while appealing to increasingly well-educated gamers.
And as we'll see in the next section, white dwarfs can be destabilized to explode as a special type of supernova, which makes for some very interesting possibilities.
The Helix nebula is a famous planetary nebula, formed when a star like the sun died.
Repeat after me: a nova is not a supernova
A nova is a special type of explosion that can happen in systems where two stars are orbiting each other (think Tatooine). In this sort of system, one star can evolve to become a white dwarf before the other, so you end up with a white dwarf and a normal star orbiting each other. When the second star starts to form a red giant, the white dwarf will start to suck up the outer layers of its neighbour. Eventually, enough hydrogen piles up on the white dwarf for fusion to begin again, causing a bright explosion. The borrowed hydrogen 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 neighbour like a vampire.
A white dwarf steals fuel from its companion until it has enough for fusion to re-start, causing a nova. Eventually it may get big enough to explode as a special "Type 1a" supernova.
Just to confuse things, it is possible for one of these vampiric white dwarfs to eventually explode as a supernova. There is a certain mass where the white dwarf just can't support its own weight any more, and when it reaches that mass it collapses and is obliterated in a "Type 1a" supernova. These explosions are always the same brightness, so astronomers use them to find out how far away distant galaxies are, and to understand the expansion of the universe.
"The star will go nova in a few hours. Meantime the planet's getting bombarded by waves of fire..." — Matt Horner, Starcraft II
When Horner says that above line in StarCraft II, he doesn't actually mean "nova," of course. He means "supernova," the title of the mission. Actual novae aren't often depicted in games because they are overshadowed by their more extreme supernova cousins. Still, for the purposes of the StarCraft II mission it actually would have been better if the star in question was a nova. The idea behind the mission is that the "waves of fire" from the unstable star are advancing across the map, making for a fast-paced race against time. A supernova would just blow the planet to pieces, but a nova is just the right amount of violence: not enough to destroy the whole system, but the sudden blast of energy would certainly cook the sun-facing side of the planet. If the planet rotated very slowly, then the wall of fire in the mission could be the advancing dawn. The intense heating and global fires would cause some pretty extreme windstorms blowing toward the day side, making things dangerous even before the scorching starlight is visible.
And then there's the tantalising possibility of triggering a white dwarf supernova, something that really needs to be exploited in a game. Every white dwarf supernova occurs at the exact same mass, so if you're looking for the ultimate doomsday weapon, a very advanced civilisation (say, Mass Effect's Reapers or Halo's Forerunners) could lob a white dwarf that was just below the critical mass toward a regular star. When they collide, you'd get an instant supernova.
In a struggle to stave off their inevitable demise, really large stars that are 9 to 50 times as big as the sun resort to fusing heavier and heavier elements, ending up with layers of fusion surrounding the core like a thermonuclear onion. The end comes when the star starts producing iron in its core.
Fusing iron consumes energy, so when a star's core fills up with iron, it loses its energy source and implodes. The implosion rebounds off of itself and becomes a supernova explosion that shatters the star. A supernova can be hundreds of billions of times brighter than a normal star, out-shining the entire rest of the galaxy.
In the explosion, atoms are disintegrated and recombined into exotic isotopes and giant atoms like gold and plutonium and uranium. Every atom in the universe heavier than helium was made in the core of a star, and every atom heavier than iron was forged in the crucible of a supernova. As Carl Sagan put it: "The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff."
The onion-like layers (not to scale) of a massive star that is about to explode.
"Threading a needle while accelerating around an exploding star, inside a planet that's falling apart? Sure! Why not?" — Serena, Halo Wars cutscene "One Less Star in the Universe"
In Halo Wars, the slipspace drive of the ship Spirit of Fire is jettisoned into a star, causing the star to explode as a supernova and conveniently destroying the Flood-infested "shield world" that was surrounding the star. But can a supernova really be triggered?
We saw above that a white dwarf supernova can be triggered just by adding enough mass. Regular supernovae are more difficult since all the action happens in the core. Halo slipstream drives function by sending material into otherwise inaccessible dimensions, so if you could get the drive to the core of a star intact, it could pump some core material into other dimensions. The resulting void could cause an implosion leading to a supernova.
If the exploding star was between 9 and 20 times the mass of the sun, the supernova will leave behind a ball of neutrons about the size of a city but with the mass of a star. Neutron stars are mind-bogglingly dense. If you were to take every car in the world -- about 1.1 billion metric tons of scrap metal -- and compact them to the density of a neutron star, they would form a sphere about the size of a gumball.
The Crab Nebula was a star once. It exploded as a supernova in 1054, and was so bright that it was visible during the day for 23 days and at night for two years. At its centre is the Crab Pulsar: a rapidly spinning neutron star.
Supernova aftermath: neutron stars
Neutron stars have magnetic fields so strong that they could rip you apart atom by atom, and they spin hundreds of times per second. If you've ever seen a spinning figure skater pull in their arms to spin faster you'll understand why. All stars spin, and when you crush one down to the size of a city, you end up with some serious RPMs. The intense, spinning magnetic fields produce beams of radio waves that shine out from the neutron star like a hyperactive lighthouse. When the path of those beams lines up with the earth, the neutron star appears as a pulsing beacon to our radio telescopes: a pulsar.
Neutron stars make an appearance in Mass Effect as a source for element zero: "Eezo is generated when solid matter, such as a planet, is affected by the energy of a star going supernova. The material is common in the asteroid debris that orbits neutron stars and pulsars. These are dangerous places to mine, requiring extensive use of robotics, telepresence, and shielding to survive the intense radiation from the dead star." — Mass Effect Wiki
Mass Effect's element zero is fictional, of course, but neutron stars are among the weirdest places in the universe, so it makes sense for them to be the location for such a weird element. You can't actually mine on a neutron star. The gravity would crush you. The magnetic fields would rip you apart. The blistering temperatures would vaporize you. But it is accurate that there is debris around neutron stars. In fact, the first planets ever discovered around another star were around the neutron star with the poetic name PSR B1257+12! With all of the radioactive elements produced in the supernova and the deadly radiation from the neutron star, it's true that a neutron star planet would be an extremely nasty place to work, but it is at least more feasible to mine than the actual surface of the neutron star.
Artist's rendition of planets orbiting a neutron star.
Final stop: Black Holes
If the star was really big, then there is no force in the universe strong enough to stop the collapse, and it just goes on forever forming a singularity: a point of infinite density. To escape from the singularity you would have to go faster than the speed of light, which is not possible. In other words, you have yourself a black hole. Black holes are one of the frontiers of physics: Einstein's theory of general relativity does a great job for most of the universe, but inside a black hole it begins to break down.
Even out where we can explain the physics with relativity, the results can be really weird. The gravity around a black hole is so strong that it distorts space and time. If you were watching someone falling into one, you would see their clock slowing down until it appeared to stop, but you would never actually see them fall in past the event horizon! They would disappear, but only because the light that they are emitting would be shifted to longer and longer wavelengths by the intense gravity. The person falling into the black hole wouldn't notice anything different about their clocks, and they would pass right through the event horizon without noticing their time slowing down. Of course, long before that happened the tides would shred them into a stream of atoms (a process called "spaghettification") so it's sort of a moot point.
Black holes get a bad rap as cosmic vacuum cleaners, relentlessly sucking up anything in their path, but they don't actually increase in gravity compared to the original star. If the sun were to somehow instantly turn into a black hole today, all the planets would continue to orbit normally. The mass would still be the same, so we wouldn't suddenly go spiraling down the drain.
Even though black holes by definition don't give off any light, stuff falling into a black hole certainly does. As a black hole eats a star, the gases form a disk around the black hole, and as the gasses in the disk move past one another, friction heats the disk up to millions of degrees so that it shines as a blindingly bright x-ray source.
Black holes are always popular in games since they are so weird and destructive. In Mass Effect, Adepts can sling singularities that are like miniature black holes. These singularities lift enemies (and anything else) into orbit, causing damage and making them vulnerable to other attacks.
In theory, small black holes can be created by smashing matter together with enough force to cause it to collapse into a singularity. In the real world, this is only possible in particle accelerators and the resulting black hole would be subatomic in size. A weird property of black holes is that they can "evaporate" by giving off particles of Hawking radiation. Smaller black holes evaporate faster, so tiny subatomic black holes would immediately disintegrate in a shower of other particles.
In the Mass Effect universe, the ability to manipulate the mass of objects makes it more plausible to form small but non-microscopic black holes. To have the effect seen in the game, the mass of the black hole would have to be much larger than the mass of a person. The black holes produced by Adepts would still be really nasty sources of radiation, disintegrating really quickly, but it's much more fun to send your enemies into orbit than to give them acute radiation poisoning.
It's also common for games to use black holes as entrances to "worm holes" in space/time, thus allowing faster-than-light travel. For example in Spore, you can hop from place to place in the galaxy using black holes. The problem with this is that, even if black holes were the entrance to some sort of space-time tunnel (there's no evidence that this is true, and for worm holes to be stable you would need exotic matter with negative mass), you still have the minor problem of surviving a trip through a black hole.
There's no doubt that, as gamers get more sophisticated, more and more effort is being poured into games to make them smarter. Often it seems like there must be a tradeoff between good gameplay and good science, but in the right context (and in the right doses), real science can be incorporated into video games, making them more believable while also inspiring innovative gameplay.