Why Does a Star Explode?

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Nothing lights up the cosmos like a supernova. When a star dies in such a spectacular fashion, its demise releases colossal quantities of mass and energy, but why does a star explode to begin with?

Only a few varieties of stars end their lives this way and astronomers sort the explosions into two basic categories: type Ia supernovae and type II supernovae. While the exact self-destruction process varies with each type, all stellar explosions ultimately depend on the star's enormous mass.

When a Star Collapses

Type II supernovae are also known as core-collapse supernovae. These explosions only occur with stars at least eight times the size of our own (eight solar masses).

To understand exactly what takes place, you have to see a thriving star as a balance of inward and outward forces. The star's own mass exerts an inward pull of gravity, while the nuclear fusion reactions in its core apply an outward push of pressure. One force tries to expand the star; the other tries to crush it.

"Stars spend their life fusing various elements by nuclear reactions into heavier elements," says author and astrophysicist Mario Livio of the Space Telescope Science Institute. "For example, our own sun fuses hydrogen atoms into helium atoms, and that's the source of its energy."

Yet a star can only carry on this balancing act to a point. Eventually, the star's core turns to iron and without the outward push of fusion, the star collapses in on itself. Core temperature skyrockets under this intense pressure, breaking down the iron nuclei and causing the core itself to break down.

"The core collapses down to stupendous density," says Livio, "at which point there is something that is called a bounce. It's like when something hits a brick wall. The core collapses up to a point where it becomes extraordinarily hard to squeeze it anymore."

The bounce takes the form of a powerful shock wave, which blasts the entire steller envelope (several solar masses worth of material surrounding the core) away at a speed of 10,000 miles (16,093 kilometers) per second.

What's left of the core then either forms a black hole or becomes a super dense object called a neutron star. While neutrons stars boast a mass roughly equal to Earth's sun, its radius barely spans six miles (almost 10 kilometers).

Critical Mass

At this point you're probably wondering about type Ia supernovae. These stellar explosions only occur with white dwarfs.

"These are stars that have a mass roughly the mass of the sun, but a radius just like that of the Earth," Livio says. "So they are very compact stars. And it turns out that such stars cannot have a mass that is higher than 1.4 times the mass of the sun. At a higher mass than this, the star cannot hold itself against gravity."

If a white dwarf happens to accrete material, either from a companion star or by merging with another white dwarf, it can surpass the critical number of 1.4 solar masses — and you can guess what happens next.

"A thermonuclear reaction starts, and the whole star explodes to smithereens," Livio says. "The white dwarf is completely blown away. There's no remnant."

Predicting Supernovae

Astonomers can't forecast supernovae with a tremendous degree of accuracy, but they can tell which stars are most likely to go out with a bang. Take a type Ia supernova, for instance. Any white dwarf found gaining mass is a likely candidate. The closer it is to achieving critical mass, the more imminent the blast.

Type II supernovae also hinge on mass.

"The more massive the star, the shorter it lives," Livio said. "So if you have a star that is 20 solar masses, then you can be pretty sure that it will explode at some point in the next million years or so. Now you cannot tell if it will explode tomorrow, but you can tell to within maybe a million years, which is not long time as far as cosmological times go."

Astrophysicist Peter Tuthill of the University of Sidney points out that a star's mass, luminosity and surface temperature tell us a great deal about its life cycle, but even this information can't pinpoint the moment everything blows. 

"Once a massive star approaches the precipice of the supernova tipping point, it is very hard to predict the exact moment of doom," Tuthill says. "There are several in the night sky — stars like Betelgeuse and Eta Carinae — which could explode tomorrow or in another hundred thousand years. Stellar lives are long, and millions of years are but the blink of an eye to most stars."

So keep watching the skies. One of the most impressive sights in the universe might play out before your eyes.

photo credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al