Narrator (April Hobart, CXC): On Earth, precipitation happens when water is heated by the Sun and forms steam (like the steam you see rises from a kettle as the water boils). The steam rises up into the air where it cools down, reforming into tiny droplets of water. These water droplets group together and create the clouds we see in the sky.
Sometimes, something similar happens in galaxies. Clouds of hot cosmic gas cool down, becoming clouds of cold cosmic gas instead. This is also called precipitation. Can you see how the two processes are similar?
However, while precipitation on Earth allows planets and animals to grow, precipitation actually stops the growth of galaxies. At least, it does if the galaxy has a giant black hole at its center.
This is because stars are born from cold clouds of cosmic gas. But in galaxies with central black holes, when a cloud cools down, it is an easier target for a black hole to capture and feed on.
As the black hole feeds it releases a hot jet of energy. The jet then re-heats any nearby clouds of cold gas before they have chance to form into stars.
Revised: May 19, 2022