H1821+643 is an extraordinarily luminous, radio-quiet quasar in the constellation of Draco.

[1] The associated Active Galactic Nucleus (AGN) is situated in the Brightest Central Galaxy (BCG) of a massive (

), strong cooling flow cluster, CL 1821+64.

[2] Russel et al (2010) spatially isolated its X-ray signal from the surrounding cluster in Chandra X-ray observatory observations and computed

from the observed X-ray luminosity.

[2] The SMBH centred in CL 1821+64 is believed to be among the most massive in the known Universe.

[2] A variety of techniques have found different values for the mass.

5 studies found values

Kim et al (2004) and Floyd et al (2008) used galactic bulge luminosity fits derived from Hubble data to find

Russell et al (2010) provided a rough estimate of

Kolman et al (1991) and Shapovalova (2016) independently modelled the quasar UV spectrum to find

Capellupo et al (2017) found

2 independent X-ray studies found significantly higher values.

by modelling reflection from the accretion disc and Walker et al found

by modelling the interaction of the black hole with the Intracluster medium (ICM) as a Compton-cooled feeding cycle.

[2] The Schwarzschild diameter of this black hole is between 9.4 terametres (63 AU) and 188 terametres (1,260 AU), which is about 16 times the diameter of Pluto's orbit.

If the hole were a Euclidean sphere, the average density would be 18 g/m3,

the density of air at sea level on Earth.

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