Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

How does a supernova completely destroy a star?

Category: Space      Published: December 11, 2012

animation of falling balls transferring momentum
Falling rubber balls demonstrate the gravitational-rebound principle that leads to a supernova explosion. They also demonstrate why a core is always left behind in this kind of explosion. Public domain image, source: Christopher S. Baird.

A supernova does not completely destroy a star. Supernovae are the most violent explosions in the universe. But they do not explode like a bomb explodes, blowing away every bit of the original bomb. Rather, when a star explodes into a supernova, its core survives. The reason for this is that the explosion is caused by a gravitational rebound effect and not by a chemical reaction, as explained by NASA. It is true that within most stars there are violent hydrogen fusion reactions churning away, but these do not cause the supernova. Stars are so large that the gravitational forces holding them together are strong enough to keep the nuclear reactions from blowing them apart. It is the gravitational rebound that blows apart a star in a supernova.

Consider the typical momentum transfer exhibit found in many science museums, as depicted in the animation on the right. Rubber balls of different sizes are held at different heights. The balls are then let go at the same moment. Gravity pulls them all down and they all fall towards the ground. In the next few moments, the bottom ball hits the ground and bounces back, and then the balls start colliding. Momentum equals mass times velocity. This means that a heavy object going slow has as much momentum as a light object going fast. When two objects collide, they transfer some momentum. When a heavy slow object collides with a light object, it can give it a very high velocity because of the conservation of momentum. As this animation shows, by arranging the rubber balls from heaviest on the bottom to lightest on the top, momentum is transferred to ever lighter objects, meaning ever higher speeds. As a result, even though gravity is pulling all the balls downwards, the upper balls rebound at incredible speeds. This is all in keeping with the law of conservation of momentum. The lower balls are too heavy and too slow to fly off. They remain behind as the surviving core of the original system. On the other hand, the upper balls are blown away (in a science museum exhibit, they are captured at the top of the apparatus so that the demonstration can be rerun). This explosion of rubber balls occurs without any significant chemical or nuclear reactions taking place. This explosion is simply due to gravity and momentum transfer, i.e. a gravitational rebound. If you look closely at the animation, you see that the rebound takes the form of an outward shock wave that gains in intensity as it spreads.

crab nebula
What we see after a supernova explosion is an expanding shell of star dust thrown out into the universe, like in this Crab Nebula image. What we don't see is the part of the star that survives – its core. In the Crab Nebula, the force of the supernova has collapsed the star's core down to a neutron star. Public Domain Image, source: NASA.

A supernova is the same kind of explosion as this rubber-balls demonstration. An aging star is composed of denser layers down towards the center, and thinner layers near the surface. The star's nuclear reactions typically balance out the force of gravity. But when the star runs out of fuel, the nuclear reactions slow down. This means that gravity is no longer balanced. Gravity begins collapsing the star. After the core of a collapsing star reaches a critical density, its pressure becomes strong enough to hold back the collapse. But, like the rubber balls, the star has been falling inwards and now bounces back. The outer layers are blown off into space in a giant explosion, spreading fertile dust clouds through-out the universe . But because of the momentum transfer, the star's core survives. The collapsing event has so intensely squeezed the star's core, that it transforms into something exotic. If the star started out with between 5 and 12 times the mass of our sun, the core becomes a big ball of neutrons called a neutron star. If the star started out with more than 12 times the mass of our sun, the core becomes a black hole. You may be tempted to argue that when a star explodes so that all that remains is a black hole, there is nothing left and the star has therefore been completely destroyed. But a black hole is not nothing. Black holes have mass, charge, angular momentum, and exert gravity. A black hole is just a star that is dense enough, and therefore has strong enough gravity, to keep light from escaping. The black hole created by a supernova is the leftover core of the star that exploded.

Not all stars experience a supernova. Stars that have less than 5 times the mass of our sun are too light to experience this violent transformation. They simply don't have enough gravity to collapse and rebound so violently. Instead, when lighter stars run out of nuclear fuel, they go through a series of stages and then settle down as long-lived white dwarfs. Whether stars end up as neutron stars, black holes, or white dwarfs, they never go completely away.

Topics: black hole, collapse, gravitational collapse, gravitational rebound, gravity, mass, momentum, neutron star, rebound, supernova, white dwarf