H1821+643
H1821+643 is a quasar in the constellation of Draco. It is situated in a massive, strong cooling flow cluster. Astronomers in 2014 identified H1821+643 as the most massive black hole with a precisely measured mass, at 30 billion solar masses. Several other black holes are possibly more massive, but they have less accurate mass estimates. The Schwarzschild diameter of this black hole is about, which is about 14.5 times the diameter of Pluto's orbit. The average density of the hole is 22 g/m³, less than air on Earth.
