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X-ray observations of hot gas are ideal for measuring the total gravitating mass and the hot gas mass of a spherical system provided the gas is in hydrostatic equilibrium and has well-defined temperature and density profiles. These conditions are satisfied for Abell 1991, a richness class 1 cluster with a central cD galaxy at $z$ = 0.0589. ROSAT PSPC observations reveal that the system is well-modeled with a hydrostatic beta model where $\beta$ = 0.55. The temperature has a minimum in the central region, increases with radius and then decreases at large radii; the mean emission-weighted mean cluster temperature is 1.3 keV. The ratio of the gaseous to total gravitating mass is 0.2-0.25 within a radius of 0.5 Mpc. If this baryonic fraction is representative of the mean value in the universe, it is impossible to accommodate an $\Omega$ =1 universe simultaneously with Big Bang nucleosynthesis.
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