What is an "emission nebula"?

   While interstellar gas is generally cold, the gas near very hot stars becomes heated and ionized (electrically charged) by ultraviolet radiation given off by those stars. The glowing areas of ionized gas in the interstellar medium are called "emission nebulae." Two well-known examples of emission nebulae are the Lagoon nebula in the constellation Sagittarius, and the Orion nebula, visible through binoculars just south of the hunter's belt in the constellation of the same name. The Orion nebula is punctuated by dark patches of cosmic dust.