A potter shapes a vessel: if the lump of clay stays flat, it spins on the wheel for a long time, but if you squeeze it into a dense ball, the wheel stops sooner. Galaxies are much the same: some stay flat disks and keep forming stars, while others collapse into balls and fall silent. Why does a galaxy change shape and stop forming stars? Scientists traced the history of more than two thousand galaxies in a computer model of the Universe to find the answer.

For a long time, researchers thought collisions between galaxies were the only culprit. When two galaxies collide, their disks are torn apart, and the gas in the center flares up and feeds the black hole. The black hole releases energy, the gas is swept away, and the galaxy goes dormant. But there is a puzzle: collisions make a galaxy bigger, whereas quieted galaxies are, on the contrary, smaller. So something else is at work. That "something" is the galaxy's shape.

Scientists found four pathways by which a galaxy's shape leads to a halt in star formation. The first is collisions: they increase size, suppress rotation, and grow the black hole. The second is the growth of the black hole itself: in compact galaxies, the black hole grows faster. The third is size: in small galaxies, the black hole's energy sweeps out gas more effectively. The fourth is the disk's internal instability: a flat disk warps on its own and turns into an elongated system, barely gaining any mass. This pathway accounts for about half of all elongated galaxies at a mass of about 40 billion solar masses. So a galaxy's shape, like the shape of a lump of clay, determines how quickly it will go quiet.