Birth of a Black Hole

A young, massive star creates energy by nuclear fusion, converting hydrogen into helium. That's what makes it shine. When all the hydrogen in the core is used up, the star begins fusing an outer shell of hydrogen that lies beyond what is now a helium core. At this later stage in its life, the star expands into a red supergiant (shown here).

If you were to split the red supergiant open, you'd find a massive core. Here, deep inside, the sheer volume of matter is drawn inward by its own gravity. The temperature of the core eventually rises high enough to begin nuclear fusion of the surrounding helium.

The pressure sends temperatures soaring above a billion degrees, and the rising gravitational force makes lighter elements fuse into heavier ones: first helium to oxygen, then oxygen to silicon, and, finally, silicon to iron. Throughout, the slowly expiring star grows denser and denser.

Heavy and dense, the red supergiant is sunk deep in space-time, the four-dimensional fabric in which all objects in the universe, including us, are embedded and move within. Fusion continues until all elements are gone except iron.

When only iron remains, the energy created by nuclear fusion can no longer support the core of the star against its own gravity. In a millisecond, the core collapses in on itself, triggering the astronomically monstrous explosion known as a supernova.

What remains after the supernova depends on how massive the surviving remnant is (which itself depends on how massive the star was to begin with). If the collapsed core is between about 1.4 and two times as massive as our sun, it will become a neutron star. Composed primarily of neutrons—its atoms have all succumbed to the extreme pressure—a neutron star is so dense that a teaspoonful of its matter would weigh about a billion tons.

If the collapsed star is greater than about three times the solar mass, then nothing that we know of in nature can withstand the force of its gravity, and it crumples inexorably into a black hole. It's a mysterious place where gravity has become so powerful that the velocity that an object would need to escape its grip is greater than the 670,000,000-mile-per-hour speed of light, which means that not even light can escape. Hence its simple but deeply evocative name, black hole.