The same process can happen when galaxies smash together. Gas from both galaxies gets compressed, producing more stars and light. After travelling through clouds of dust, this light can then excite hydroxyl ions, which consist of hydrogen and oxygen atoms, into higher energy states. When these excited ions are blasted with radio waves, such as from a supermassive black hole, they can suddenly relax and produce a beam of extremely bright and focused microwave radiation, known as a maser.