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SDSS J182157.2+642036
Observation data (Epoch J2000.0)
Right ascension 18h 21m 57.2365s
Declination+64° 20′ 36.226″
Distance3.4 gigalight-years (1.0 Gpc)
Apparent magnitude (V)14.24
See also: Quasar, List of quasars

H1821+643 is a quasar in the constellation of Draco. It is situated in a massive, strong cooling flow cluster.[1] Astronomers in 2014 identified H1821+643 as the most massive black hole with a precisely measured mass, at 30 billion solar masses.[1] Several other black holes are possibly more massive, but they have less accurate mass estimates. The Schwarzschild diameter of this black hole is about 172 terametres (1,150 AU), 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.[2]


  1. ^ a b Walker, S. A.; Fabian, A. C.; Russell, H. R.; Sanders, J. S. (2014). "The effect of the quasar H1821+643 on the surrounding intracluster medium: Revealing the underlying cooling flow". Monthly Notices of the Royal Astronomical Society. 442 (3): 2809. arXiv:1405.7522. Bibcode:2014MNRAS.442.2809W. doi:10.1093/mnras/stu1067.
  2. ^ Mass 3.0×1010 * 2.0×1030=6.0×1040 kg. Volume at radius 8.6×1013 m is 2.66×1042 m³.