Originally the Andromeda galaxy was classified as one of these objects before Hubble discovered the ex- istence of galaxies other than our own.
When backlit, some of these objects appear dark, and all are strong sources of infrared radiation. Originally the Andromeda galaxy was classified as one of these objects before Hubble discovered the ex- istence of galaxies other than our own. Some that are lit internally or from the side are easily observable, such as the one below Orion's belt. Some are remnants of supernovas, while others are the earliest stages of star and planetary formation. These objects do not have defined boundaries, as they are loose collec- tions of dust and gas held together by gravity. Name this type of astronomical object whose name comes from the Latin for cloud, the best well known of which are the Horsehead and Crab.
When backlit, some of these objects appear dark, and all are strong sources of infrared radiation.
nebula (accept nebulas or nebulae)