For all practical purposes, stars traveling across space never smash into each other. The ratio between a starlike sun's diameter and its distance to the next neighboring star is about 1:10 million. Galaxies do collide though. The separation between our Milky Way and neighboring Andromeda galaxy is a staggering 2.2 million light-years. That's a ratio of only 1:20. And there will eventually be a collision and merger between the two giants.

When galaxies collide their black holes merge into one giant black hole. Nearly all galaxies contain supermassive black holes in their centers. The Hubble Space Telescope and the Chandra X-ray Observatory looked in the heart of a pair of colliding galaxies and uncovered twin supermassive black holes waltzing around each other. The black hole duo, engorged with infalling gas, blazes brightly as active galactic nuclei (AGN). They are approximately 300 light-years apart – the closest AGN pair seen in visible-light and X-ray wavelengths.