  We report spectroscopic and photometric follow-up of a dormant black hole
(BH) candidate from Gaia DR3. The system, which we call Gaia BH2, contains a
$\sim 1M_{\odot}$ red giant and a dark companion with mass $M_2 = 8.9\pm
0.3\,M_{\odot}$ that is very likely a BH. The orbital period, $P_{\rm orb} =
1277$ days, is much longer than that of any previously studied BH binary. Our
radial velocity (RV) follow-up over a 7-month period spans more than 90% of the
orbit's dynamic range in RV and is in excellent agreement with predictions of
the Gaia solution. UV imaging and high-resolution optical spectra rule out all
plausible luminous companions that could explain the orbit. The star is a
bright ($G=12.3$), slightly metal-poor ($\rm [Fe/H]=-0.22$) low-luminosity
giant ($T_{\rm eff}=4600\,\rm K$; $R = 7.8\,R_{\odot}$; $\log\left[g/\left({\rm
cm\,s^{-2}}\right)\right] = 2.6$). The binary's orbit is moderately eccentric
($e=0.52$). The giant is strongly enhanced in $\alpha-$elements, with $\rm
[\alpha/Fe] = +0.26$, but the system's Galactocentric orbit is typical of the
thin disk. We obtained X-ray and radio nondetections of the source near
periastron, which support BH accretion models in which the net accretion rate
at the horizon is much lower than the Bondi-Hoyle-Lyttleton rate. At a distance
of 1.16 kpc, Gaia BH2 is the second-nearest known BH, after Gaia BH1. Its orbit
-- like that of Gaia BH1 -- seems too wide to have formed through common
envelope evolution. Gaia BH1 and BH2 have orbital periods at opposite edges of
the Gaia DR3 sensitivity curve, perhaps hinting at a bimodal intrinsic period
distribution for wide BH binaries. Dormant BH binaries like Gaia BH1 and Gaia
BH2 likely significantly outnumber their close, X-ray bright cousins, but their
formation pathways remain uncertain.
