Why mass changes the outcome

1. A star with far more mass than the Sun has a far stronger inward pull of gravity acting on its core.
2. Balancing that stronger pull needs a much greater outward push, which the star can only supply by running its core at a higher temperature and pressure.
3. A hotter core fuses hydrogen far more rapidly, so a massive star gets through its fuel much faster even though it started with more of it.
4. The main sequence stage of a massive star therefore lasts a few million years rather than the billions of years a star like the Sun manages.
5. The high core temperature also lets fusion continue on to heavier elements that a smaller star can never reach.
6. Mass is the single quantity that decides the whole later life of a star, which is why two stars formed in the same nebula can end so differently.

Life of a massive star

1. A massive star begins in exactly the same way as any other, with gravity pulling a nebula together until the core is hot enough for fusion.
2. It then spends a short main sequence stage as a very hot, very bright star, burning blue or blue white rather than yellow.
3. When the core hydrogen is gone the outward push drops, the core contracts under gravity, and the outer layers swell into a red supergiant.
4. This is the same process that makes a red giant, but on a far larger scale, so a red supergiant can be hundreds of times wider than the Sun.
5. Inside it the contracting core keeps reaching new temperatures, so helium fuses into carbon and then heavier nuclei form in a series of stages.
6. Fusion stops once the core is made of iron, because fusing iron nuclei takes energy in rather than giving energy out.
7. With no energy released the outward push collapses, and gravity is left completely unopposed.

The supernova explosion

1. The unsupported core collapses in a fraction of a second, and the outer layers fall inwards behind it.
2. The infalling material rebounds off the collapsed core and is blasted back out in an enormous explosion called a supernova.
3. For a few weeks a single supernova can shine more brightly than all of the other stars in its galaxy put together.
4. The explosion supplies enough energy to build nuclei heavier than iron, including gold, lead and uranium.
5. All of those elements are thrown out into space, where they enrich the gas and dust that later collapses into new nebulae.
6. Every atom heavier than helium on Earth was made inside an earlier star or in a supernova, which is why the Solar System contains rocky planets at all.

Neutron star or black hole

1. What is left behind after the supernova depends on how much mass the collapsed core still holds.
2. A core of moderate mass is squeezed until its material is packed as tightly as the inside of a nucleus, forming a neutron star.
3. A neutron star packs more mass than the Sun into a ball only about $20km$ across, so a teaspoon of it would have a mass of billions of tonnes.
4. If the remaining core is more massive still, nothing can resist gravity and the collapse continues without limit.
5. The result is a black hole, where the gravitational field is so strong that not even light travels fast enough to escape.
6. A black hole emits no light of its own, so it is detected by its gravitational pull on nearby stars and gas rather than by being seen directly.
7. The full order for a massive star is therefore nebula, main sequence star, red supergiant, supernova, and then either a neutron star or a black hole.