When the black holes get down to a separation of about a parsec — about 3.26 light-years — they get stuck, circling each other eternally. Related: Black hole singularities defy physics.

Parsecs close parsecA unit of distance used by astronomers. Parsec is short for 'parallax second'. One parsec is approximately 3.26 light years.: One parsec is approximately 3.26 light years.