  Afterglow, or long-lived emission, has now been detected from about a dozen
well-positioned gamma-ray bursts. Distance determinations made by measuring
optical emission lines from the host galaxy, or absorption lines in the
afterglow spectrum, place the burst sources at significant cosmological
distances, with redshifts ranging from ~1--3. The energy required to produce
the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of
the rest mass energy of a neutron star, if the emission is isotropic. Here we
present the discovery of the optical afterglow and the redshift of GRB 990123,
the brightest well-localized GRB to date. With our measured redshift of >1.6,
the inferred isotropic energy release exceeds the rest mass of a neutron star
thereby challenging current theoretical models for the origin of GRBs. We argue
that the optical and IR afterglow measurements reported here may provide the
first observational evidence of beaming in a GRB, thereby reducing the required
energetics to a level where stellar death models are still tenable.
