Several dozen optical echelle spectra demonstrate that HR 6819 is a hierarchical triple .
A classical Be star is in a wide orbit with an unconstrained period around an inner 40 d binary consisting of a B3 III star and an unseen companion in a circular orbit .
The radial-velocity semi-amplitude of 61.3 km/s of the inner star and its \textcolor blackminimum ( probable ) mass of 5.0 M _ { \odot } ( 6.3 \pm 0.7 M _ { \odot } ) imply a mass of the unseen object of \geq 4.2 M _ { \odot } ( \geq 5.0 \pm 0.4 M _ { \odot } ) , that is , a black hole ( BH ) .
The spectroscopic time series is stunningly similar to observations of LB-1 .
A similar triple-star architecture of LB-1 would reduce the mass of the BH in LB-1 from \sim 70 M _ { \odot } to a level more typical of Galactic stellar remnant BHs .
The BH in HR 6819 probably is the closest known BH to the Sun , and together with LB-1 , suggests a population of quiet BHs .
Its embedment in a hierarchical triple structure may be of interest for models of merging double BHs or BH + neutron star binaries .
Other triple stars with an outer Be star but without BH are identified ; through stripping , such systems may become a source of single Be stars .