Supernova 1987A: The Explosion

Week of February 28, 2000

``... even the brightest star won't last forever.''
	The Alan Parsons Project, Prime Time

Last week we saw how SN87A was discovered. A more interesting question might be ``Why did we see it''? In other words, what makes a star explode? Fair warning: this is not an easy subject, and even with the gross simplifications I have made, this Snack is a bit long. Still, I think you'll enjoy it.

Stars, like people, behave the way they do because of internal struggles. Stars play out a game of tug-of-war between gravity, which makes them want to collapse, and heat and pressure, which make them want to expand. Stars are very massive, which gives them a lot of gravity. This force is considerable, and squeezes the insides of the star. Stars are basically big balls of gas, and the gas responds to this squeezing by getting hotter and increasing their internal pressure. If things work out just right (and they usually do), the inward force of gravity is balanced by the outward force of pressure and heat. A stable star results.

This balance is maintained for most of a star's life. The pressure and heat are so enormous inside the star's core that hydrogen is fused into helium. This fusion gives off a lot of heat, which is used again to help hold the star up against its own gravity. Mind you, this fusion is happening deep in the core of the star. You can think of a star as having two layers: a very tiny core where all the fusion action is, and the outer layers of the star, which actually contain most of the star's mass. This is important later!

But there's a problem: a star doesn't have an infinite supply of hydrogen. Some day, it will run out of fuel. The length of time it takes depends on the star's mass: the more massive it is, the faster it burns its fuel. A star like the Sun has enough hydrogen to keep its fusion fires going for billions of years, but a star like Rigel, a massive star in Orion, burns its hydrogen so fast it may run out in just a few million years. Low mass stars are very miserly with their hydrogen, and may shine for hundreds of billions of years.

When the hydrogen in a star runs out, things get complicated. Depending on the star's mass, it may start to fuse helium into carbon, or it may simply lack the mass to do that and fusion reactions stop. It will become a red giant, shed its outer layers and become a beautiful (if temporary) planetary nebula-- but we'll be talking about that in next week's Snack, I promise.

For now, the more interesting case is when a massive star runs out of fuel. It has enough gravity to compress the core further, starting up a chain of reactions: helium fuses to carbon, and when helium runs out, carbon gets fused into oxygen, then oxygen to magnesium, and magnesium into silicon. This listing is oversimplified, but it gives you an idea of what's going on. Each element in turn is fused into a heavier element. An alchemist's dream! The stars starts to resemble an onion, with layer after layer of different fusion products.

Each successive step happens at a faster pace; a star may spend millions or tens of millions of years fusing hydrogen in to helium, but the last few steps may happen in literally tens or hundreds of years. The problem is, at this point something bad happens: silicon fuses to iron.

Why is that bad? Because up until now, all these reactions have produced energy in the form of heat. That heat holds the star up. However, iron is a special case. It takes energy to fuse iron into heavier elements, energy which comes from the star itself. When enough iron builds up in the core, the pressure becomes great enough that it starts to fuse. This robs energy from the star. Worse, the fusion of iron eats up copious amounts of electrons, and electrons help hold up the star too.

When iron starts to fuse, things go bad fast. The iron core collapses, since the heat and electrons holding it up get used to fuse the iron. The tremendous gravity of the core collapses it down from something thousands of kilometers across to a ball of compressed matter just a few kilometers in diameter in a thousandth of a second. This acts like kicking the legs out from under a table. Like when Wile E. Coyote suddenly realizes he is no longer over solid ground and starts to fall, the outer layers of the star come rushing down. They slam into the compressed core at a goodly fraction of the speed of light. This does two things: it sets up a huge rebound, sending the outer layers of the star back out, and also releases a vast number of neutrinos, subatomic particles that carry away the energy of the collapse. The gas from the outer layers absorbs these neutrinos, which is like lighting a match in a fireworks factory. The outer layers explode upwards, and several solar masses of doomed star tear outwards at speeds of several thousand kilometers per second.

The vast amount of energy released is what we see as a supernova. It may take hours or days for the light to increase to a maximum, but during that time it can produce as much energy in one second as our Sun does over its entire lifetime, and be seen from clear across the Universe. When SN87A went off nearly 180,000 light years away it was easily seen by the naked eye, and it was considered to be an underluminous event! Still, 87A was bright enough to illuminate everything around it for hundreds of light years, and to that I'll aim our Snack for next week.

An interesting end note: in 1987, there were several neutrino detectors in operation here on Earth. They were being used to monitor the particles form the Sun. However, on that evening of February 23, one detector in Japan detected a minor burst of 11 neutrinos, which is actually far more than it usually receives from the Sun at any given moment. Neutrinos, as it happens, are very difficult to stop in their flight, and most of the neutrinos made in the core collapse of a star fly right out into space. That means that the neutrinos detected by the Japanese detector came straight from the death of the star. While it may take the light a few days to reach maximum, the neutrinos represent the actual moment of collapse. From that, we know that Supernova 1987A was born at 7:36 Greenwich time, February 23, 1987. [note: before I get tons of email saying it was actually born 180,000 years before that date, let me say that yes, I know, but I don't want to get into thorny discussions of relativity, simultaneity and the like. Most astronomers give dates like this, meaning that this is when we detected the light, and so I will also use that shorthand.]

• For a great discussion of how gravity balances pressure in a star, take a look at Nick Strobel's Astronomy Notes website. He has a fun animated graphic of it. He also has a far more detailed description of how stars evolve as well. It should help you fill in some of the blanks I have left here.

• My old professor John Hawley at the University of Virginia also has a nice page about making supernovae. It's a little more complicated but has great graphs and images. The onion layer image above is from his site.