How It Works

How do black holes form?

When a giant star exhausts the hydrogen fuel for fusion in its core, the fusion process moves out into a spherical ‘shell’ while the core begins to fuse helium into heavier elements. As each fuel source in the core is spent a new shell is created, while the core moves on to fusion of ever heavier elements. Stars with eight times the Sun’s mass continue the process until their cores start to fill with iron. The star cannot generate energy by fusion of iron, so when it tries, its energy supply cuts out and it collapses. The core is squashed to an incredible density, while a shockwave rebounds through the rest of the star, ripping it apart. In most cases, the star’s core stabilises as a neutron star, but if the core weighs more than three to four Suns, the pressure between neutrons can’t halt the collapse. The neutrons are torn apart and the core collapses to a single superdense point: a singularity. The singularity’s gravity is so powerful that anything that comes too close – even light – can’t escape from it. As it takes material in from its vicinity, it may briefly release a burst of highly energetic gamma rays along its axis of rotation.