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How does a supernova completely destroy a star?
Category: Space
Published: December 11, 2012
Updated: November 28, 2023
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
A supernova usually does not completely destroy a star. Supernovas are one of the most violent explosions in the universe. For a brief period of time, a single supernova can be brighter than an entire galaxy, emitting as much energy in a short time period as our sun will emit in its whole lifetime. However, supernovas do not explode like bombs, blowing away every bit of the original bomb. Rather, when a star explodes as a supernova, its core usually survives. The reason for this is that the explosion is caused by a gravitational rebound effect which is given a boost by a flash of energy released from nuclear reactions. A supernova has nothing to do with a chemical reaction. It is true that within most stable stars there are violent hydrogen nuclear fusion reactions churning away, but these do not cause the supernova. Stable 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 and the flash of energy released from nuclear reactions that blow apart a star in a supernova.
Consider the typical momentum transfer exhibit found in many science museums, as depicted in the animation above. 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 toward 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 downward, 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, and are also pushed down from the collisions. They remain behind as the surviving core of the original system. 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.
A supernova is the same kind of explosion as this rubber-balls demonstration, but with some added effects. An aging star is composed of denser layers in the center and thinner layers near the surface. The star's outward pressure from its nuclear reactions typically balances out the inward force of gravity. But when the star runs low on nuclear fuel near the end of its life, the nuclear reactions slow down. This means that gravity is no longer balanced. Gravity begins collapsing the star inward. 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 low-density outer layers of the star collide and bounce off of the high-density core of the star. As a result, the outer layers are blown off into space in a giant explosion, spreading fertile dust clouds through-out the universe. Additionally, at the moment of the collision, the core is squeezed so tightly that it undergoes a drastic nuclear reaction involving protons turning into neutrons which suddenly releases a huge amount of energy. This flash of energy amplifies the outward explosion of the outer layers. While the outer layers bounce outward because of the collision and because of the flash of energy, the core of the star bounces inward, crushing it even more.
The collapsing event has so intensely squeezed the star's core that it transforms into something exotic. If the star started out with a mass of between approximately 8 to 40+ times the mass of our sun, the core becomes a superdense ball of mostly neutrons called a neutron star. The exact values of the upper and lower limits of this range of mass depends on the metallicity of the original star. If the star started out with a mass of more than 25 times the mass of our sun for low-metallicity stars or a mass of 40+ times the mass of our sun for high metallicity stars, 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. To some extent, a black hole is just a star that is dense enough that light cannot escape. The black hole created by a supernova is the leftover core of the star that exploded.
Not all stars experience a supernova. Stars that start out with less than 8 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.
In summary, the most massive stars eventually go supernova and become black holes, the medium-mass stars eventually go supernova and become neutron stars, and the least massive stars never go supernova but eventually end up as white dwarfs. Whether stars end up as black holes, neutron stars, or white dwarfs, they almost never go completely away.
In a way, black holes, neutron stars, and white dwarfs are dead stars because none of them are internally active, i.e. none of them experiences nuclear fusion. Therefore, none of them generates additional heat internally. Neutron stars and white dwarfs continue to glow for a while because of the residual heat still present. However, with no internal heat source, neutron stars and white dwarfs steadily cool off and dim over time (assuming that there are no external effects that would complicate the situation - for instance, two neutron stars can merge to make a bigger neutron star or even a black hole). Eventually, white dwarf stars will cool and fade to become black dwarf stars. However, the universe has not existed long enough yet for this to actually happen to any white dwarf. The ultimate fate of isolated neutron stars is less understood. Perhaps neutron stars simply cool enough to eventually become black neutron stars. If any black neutron stars currently exist, they would be difficult to detect because they would be so small and so dark. None have ever been detected. Or, neutron stars may eventually become white dwarfs which become black dwarfs. This is an area of ongoing research. Supernovas continue to be searched for, identified, and studied by scientists. Fortunately, a new artificial intelligence tool is speeding up this process.