Astronomers used historical sky images dating back to 1900 to find evidence of a binary supermassive #blackhole system in galaxy PG 1553+153, with a 20-year pattern of light emissions https://theconversation.com/some-black-holes-at-the-centers-of-galaxies-have-a-buddy-but-detecting-these-binary-pairs-isnt-easy-233393 #space #astronomy