At the center of a very dense stellar system, a vacuum cleaner is at work. Only it doesn't suck up dust but gas shed by aging stars. This gas spirals into a funnel and gradually falls onto the black hole, making it heavier and heavier.
Previously, it was unclear whether a black hole could grow this way. Scientists built a computer model in which hundreds of aging stars eject gas. It turned out that the black hole's gravity captures this gas and swirls it into a disk. Over 0.188 million years, 481.7 solar masses of gas accumulated near the black hole.
For 12 of the 17 systems studied, the calculations showed that the black hole could have started with a small mass and grown to a huge one. So the vacuum is a real growth mechanism. Observers can test this by looking for young stars with high velocities and unusual compositions in such systems.
In some models, the disk around a $10^4$ M⊙ black hole contained nearly 900 solar masses of gas, which is like having a cloud weighing several hundred Jupiters swirling around an object the size of a city.