Supernovae

Throughout history, astronomers have on several occasions discovered a "new star" in the sky, or a "stella nova" as Tycho Brahe called it in 1572. Today we know that these are not actually new stars, but rather very old stars that lay so far away we had never been able to see them, and have now ended their lives in a gigantic explosion that sends its powerful light all the way down to us. We call these explosions supernovae, and they are home to some of the wildest physics we encounter in the universe!
 

Massive stars

Not all stars will end their lives in supernovae, but if stars are large enough (more than eight times the mass of the Sun), or under exactly the right conditions, enormous explosions will occur. You can read more about the different types of stars in the section Stars.

Energy production in all stars occurs through what is called fusion, which is when lighter elements are fused together into heavier ones. Very small stars can only convert hydrogen into helium, while larger stars like the Sun can also convert helium into carbon. The most massive stars can form even heavier elements such as nickel and iron.

The fusion processes take place in the cores of stars or in shells surrounding the core. In the image below you can see how the heaviest elements are formed in the core of a massive star and the lighter elements in the shells around it. The process of fusion releases energy which ultimately keeps the star alive and makes it shine.

As a massive star ages, it burns through its fuel layer by layer, starting with hydrogen in the core, then helium, then carbon, and so on, producing progressively heavier elements. Each time a fuel source is exhausted in the core, the star contracts and heats up enough to burn the next heavier element, while the outer layers expand, causing the star to swell into a red supergiant. Once massive stars have produced iron in their core, they cannot continue the fusion processes. This is because iron is the most tightly bound element, meaning it costs more energy to fuse iron into heavier elements than is released in the process. The stars can therefore no longer generate energy, and the countdown begins.

What happens once there is no more iron?

When a massive star has produced enough iron in its core, energy production stops, and the star enters the ending phase of its life. The energy in the star's core has until now maintained a balance with gravity, with energy pushing outward and gravity pushing inward. When energy is no longer being produced in the core, there is no longer an outward pressure to counteract gravity, and so the star collapses inward, it implodes.

The material collapses rapidly and strikes the hard iron core with tremendous force, bouncing outward again and creating an outward pressure wave. The shockwave tears the star apart in a gigantic explosion, a supernova. The energy released is utterly extreme. In just a few seconds, more energy is released than the Sun will produce over its entire lifetime. For several weeks afterwards, a supernova can shine more brightly than an entire galaxy of billions of stars.
 

Stellar remnants
After a supernova explosion, the outer layers of the star are completely torn apart, and the innermost core is compressed with tremendous force. What the core becomes depends on how massive the star was when it exploded: it can either become a neutron star a black hole, two of the most extreme objects in the universe.

Stars between 10 and 25 solar masses leave behind a neutron star. The material falling inward heats and compresses the iron core so intensely that the iron atoms cannot withstand it. The electrons and protons in the iron atoms are all squeezed together into neutrons, packed together with unimaginable density.

Neutron stars are the smallest and densest stars in the universe. A stellar core weighing twice as much as the Sun can be compressed into a neutron star just 20 km across. That is smaller than the island of Bornholm! A matchbox filled with neutron star material would weigh 13 million tonnes, roughly the same as nearly a hundred thousand blue whales.

If the original star is more massive than 25 solar masses, its death takes a slightly different form. There is so much material that the iron core cannot withstand the pressure when the star collapses. It is therefore compressed to an infinitely small point of extreme density. The point is so small and so massive that its gravity will not let anything escape, not even light! That is when a black hole is born.

So far we have learned that the innermost cores of massive stars become either neutron stars or black holes, but what happens to the outer layers of the star that were torn apart in the supernova? They are hurled out into space, forming a spectacular bubble around the neutron star or black hole. This supernova remnant contains both the many heavier elements produced during the star's lifetime, and even heavier elements formed during the violent explosion itself. 

Over time, the material in the supernova remnant will mix with all the other gas and dust in the galaxy. And perhaps one day, after billions of years drifting through space, it will gather together again. Maybe into a new star. Maybe into a planet. Maybe even into life on that planet. The iron in your blood, the calcium in your bones, the oxygen in every breath you take, all of it was forged in the heart of a dying star. We are not just made of atoms. We are made of stardust.

Kilonova

Not even supernovae are powerful enough to create the very heaviest elements, such as platinum, gold, and lead. These are created by even more violent explosions: kilonovae.

Kilonovae occur when two neutron stars collide. First, the neutron stars will orbit each other, and because they are so enormously dense and massive, this cosmic dance causes all of space to ripple back and forth. This creates the gravitational waves that we have begun to detect in recent years (see more in the Gravitational Waves section).

Finally, the two neutron stars collide and explode in an enormous burst of heavy elements that spreads out as a cloud around the stars. These elements rush outward from the kilonova at speeds of up to one fifth of the speed of light, and like the other elements from dying stars, they become part of the galaxy's great nebulae.