1A black hole forms when a massive star collapses under the weight of its own gravity.
2The collapse squeezes the star's entire remaining mass into a region smaller than a city.
3The gravity in that region becomes so overwhelmingly powerful that not even light itself can ever escape.
4The invisible boundary that surrounds every black hole is known to scientists as the event horizon.
5Any object unfortunate enough to cross this mysterious boundary can never return to the outside universe again.
6Time itself behaves in strange ways near a black hole and appears to slow down dramatically.
7Astronomers cannot observe black holes directly, so instead they carefully study their effects on nearby stars.
8Gas spiralling into a black hole heats up to extreme temperatures and glows brightly in X-rays.
9Supermassive black holes, millions of times heavier than the Sun, sit quietly at the centres of galaxies.
10In twenty nineteen, scientists released to the world the first actual photograph of a black hole's shadow.
11These remarkable discoveries continue to test Einstein's theories in the most extreme conditions found anywhere in nature.