Beehive Cepheid variable Hertzsprung-Russell diagram Hyades Messier catalog NGC Pleiades Seven Sisters absolute magnitude binary star black hole double star dwarf star fixed star giant star globular cluster gravity star magnitude main sequence star mass-luminosity law neutron star nova open cluster populations pulsar quasar quasi-stellar radio source radio star red giant star relative magnitude sky atlas spectrum-luminosity diagram star star catalog star chart star cloud star cluster stellar magnitude variable star white dwarf star