Atlantides Beehive Cepheid variable Hertzsprung-Russell diagram Messier catalog NGC Napaea Pleiades Seven Sisters absolute magnitude binary star black hole double star dryad dwarf star fixed star flower nymph giant star glen nymph globular cluster gravity star hamadryad limoniad magnitude main sequence star mass-luminosity law mountain nymph neutron star nova nymph nymphet nymphlin open cluster oread 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 supernova tree nymph variable star vila white dwarf star willi wood nymph