Nib asked for
How black holes form
When a very massive star runs out of fuel, its core collapses under its own gravity into something so dense that the escape speed there passes the speed of light.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the tug-of-war
A star is a constant tug-of-war. Gravity pulls all its gas inward, while the heat from nuclear fusion in the core pushes outward. For millions of years the two balance, and the star holds a steady size.
A star stays puffed up because fusion's outward push balances gravity's inward pull.
a common mix-upStars do not burn like a fire using oxygen; they shine by fusing atoms in the core, and that fusion is what holds them up.
If fusion suddenly switched off, which side of the tug-of-war would win?
Show the answer
Gravity, because nothing would be pushing back against the inward pull.
2iron core
Fusion eats fuel. A very massive star, many times heavier than our Sun, fuses lighter atoms into heavier ones until its core fills with iron. Iron is a dead end: fusing it does not release energy, so the core's outward push fades.
Fusion runs out of useful fuel when the core turns to iron.
Why is iron the end of the line for the star's energy supply?
Show the answer
Fusing iron does not release energy, so it cannot keep up the push against gravity.
3the collapse
With the push gone, nothing holds the core up. Gravity wins and the core falls inward on itself in a fraction of a second, crushing matter far beyond anything on Earth. The more mass in the core, the more violent the squeeze.
Once fusion stops pushing back, the core collapses in a fraction of a second.
a common mix-upThe collapse is not a slow shrinking over ages; the core's final fall takes a fraction of a second.
4escape speed
As the core shrinks, the same mass is packed into less space, so gravity at its surface grows enormous. Recall the escape speed from page 2: the speed needed to break free. For a small enough, heavy enough core, that speed climbs past the speed of light, and a black hole exists.
Squeeze enough mass small enough and the escape speed passes light speed.
Does the core need more mass or less size to reach a black hole, or both?
Show the answer
Either helps: what matters is packing enough mass into a small enough space that escape speed exceeds light speed.
5supernova
The collapse also sets off the star's outer layers. As the core falls in and bounces, it blasts the rest of the star outward in a supernova, a brilliant explosion. The supernova is triggered by the collapse; it is not what makes the black hole. The black hole is the core that stays behind.
The collapse triggers the supernova blast, but the black hole is the core left behind.
a common mix-upThe supernova explosion does not create the black hole; it is a consequence of the collapse, and the black hole is the leftover core.
So a black hole is what is left when a giant star loses its fight with its own gravity: nothing new is added, just a core squeezed until light itself cannot leave.