Orion's belt. Optical image of the line of three bright stars making up the 'belt' in the constellation of Orion. The stars are, from lower left to upper right: Alnitak (Zeta Orionis), Alnilam (Epsilon Orionis) and Mintaka (Delta Orionis). The reddish area at lower left is a region of emission nebulosity, in which radiation from the nearby stars ionises hydrogen gas, causing it to glow.