Stellar Evolution: How Stars Die

Stellar Evolution: How Stars Die

Depending on their mass, stars follow one of three evolutionary routes. When they end their lives they may become white dwarves, neutron stars, or black holes.

All stars fuse hydrogen into helium at their core, releasing energy that balances the force of gravity that pulls inwards. Helium is denser than hydrogen, so as it is created, it makes the star’s core hotter and more pressurised. Eventually, the temperature is high enough to fuse the hydrogen around the core, and as fusion reactions happen faster and faster, the force released pushes away the outer layers of the star. Since the star’s radius increases, its energy radiates away over a larger surface area and it emits cooler red light. The star is now a red giant. When it reaches temperatures of 100 million Kelvin and begins to fuse helium into carbon and oxygen, it may take three paths depending on its initial mass.

In light stars like our sun, all their fuel will have run out before temperatures get hot enough to fuse carbon and oxygen into heavier elements. All the excess gas and dust is blown off by stellar wind, forming a planetary nebula, and eventually the star simply becomes a hot, dense carbon and oxygen core: a white dwarf. To take this evolutionary route, a star must be less than 1.4 times the mass of our own Sun, otherwise known as the “Chandrasekhar Limit”. When a star stops producing energy, the only thing stopping gravity from collapsing the star is electron degeneracy pressure: when electrons are squeezed together, they repel each other, thus resisting gravitational collapse.

A star more massive than the Chandrasekhar Limit will collapse even further. As a red giant, its temperatures will become high enough to fuse oxygen and carbon into heavier elements. Fusion continues on and on until iron is created—and since fusing iron uses up energy instead of creating it, fusion stops. Electron degeneracy pressure is the only thing stopping the star from collapsing, and then that gives in too. Electrons are squeezed into the nucleus of the atom, protons turn into neutrons, and the core of the star is soon just a super dense ball of neutrons. An enormous amount of energy is released in the collapse—material hurtles down towards the core and bounces off, creating an immense shockwave that blasts away the rest of the star in a supernova. It’s so bright and energetic that it can temporarily outshine an entire galaxy, and can fuse even heavier elements, like uranium and gold. After the explosion, all that’s left is a neutron star: a stellar corpse 1.5 to 3 times the mass of the sun, but compressed to the size of a city in Earth (30 km in diameter).

But if the original star is more than 20 times the mass of our sun, something even wackier will form at the end of its life: a black hole. It goes from red giant to supernova to neutron star—but it doesn’t stop collapsing. Instead of forming a ball of neutrons, the star’s mass is so great that it collapses down and down to a point so small it must have infinite density. This is called a singularity. To quote Sir Roger Penrose, ‘Thou shalt not have naked singularities’, meaning that they cannot exist with an accompanying event horizon. Anything that crosses beyond this horizon can’t escape the immense gravitational pull of the singularity, not even light. Together, a singularity and its event horizon form a stellar mass black hole.

Denser and denser and denser, the corpse of a star and the grave in which is rests depends largely on its mass. These are the endpoints of stellar evolution, and will be the eventual fate of every point of light we see when we look up.

(Image Credit: Wikimedia Commons and Universe Today)