In 2003, [astronomers] discovered the longest, oldest, lowest note in the universe — a black hole’s song... Although it is too low and deep for humans to hear, the B flat note, 57 octaves below middle C, appeared as sound waves that moved out from explosive events at the edge of a supermassive black hole in the galaxy NGC 1275.
The notes stayed in the galaxy and never reached us, but we couldn’t have heard them anyway. The lowest note the human ear can detect has an oscillation period of one-twentieth of a second. This B flat’s period was 10 million years.Joanna Klein An Earthling's guide to black holes
Supermassive black holes — a million to a billion times more massive than our sun — exist only in the center of a galaxy. At the center of the Milky Way, 26,000 light-years from Earth, scientists are hoping to make an image of Sagittarius A*, which is believed to be our own supermassive black hole, with the mass of four million suns.
Image from Utriusque Cosmi by Robert Fludd (1622)