Monsters lurk in the cores of all galaxies: supermassive black holes millions or billions of times the mass of our Sun. And as galaxies merge, so, too, do these black holes, which gradually approach each other over millions of years.
During that time, the gravitational effects of merging galaxies and black holes funnel stars, planets, gas, and dust to the center. That material begins to swirl into the black holes, forming a hot, bright accretion disk around each one that can be seen from anywhere in the universe.
Such a brightly shining disk around a supermassive black hole is called a quasar.
Normally, quasars hang out by themselves, absorbing material from the young galaxies in which they form. When the galaxy settles, the quasar shuts down as the black hole consumes all nearby matter and runs out of food. However, chaotic events such as galaxy mergers can reignite quasars, resulting in a one-of-a-kind double quasar pair that eventually merges into a single, brighter, and even more massive black hole. Astronomers estimate that one in every 1,000 quasars is a double quasar, though some are too close to one another to be visibly separated at such great distances.
Double quasars, such as the two pairs recently imaged by the Hubble Space Telescope (shown directly above and depicted in the artist’s concept at the top of this page), are common in the distant universe. However, future residents of our galaxy will have a front-row seat to such a spectacular event in about 4 billion years. When the nearby Andromeda Galaxy gets close enough to the Milky Way, the black holes in both galaxies will light up as quasars, shining at least as bright as the Full Moon and blocking out many of the stars. When these quasars eventually merge, they will emit intense radiation that could cause problems for any inhabited planets in the resulting Milkomeda galaxy.
Fortunately, we have plenty of time to get out of town before our galaxy’s date with destruction.