<P> A nebula (Latin for "cloud" or "fog"; pl . nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases . Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way . The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others . </P> <P> Most nebulae are of vast size, some are hundreds of light years in diameter . A nebula that is barely visible to the human eye from Earth would appear larger, but no brighter, from close by . The Orion Nebula, the brightest nebula in the sky that occupies a region twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers . Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth--a nebular cloud the size of the Earth would have a total mass of only a few kilograms . Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffuse they can only be detected with long exposures and special filters . Some nebulae are variably illuminated by T Tauri variable stars . Nebulae are often star - forming regions, such as in the "Pillars of Creation" in the Eagle Nebula . In these regions the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter, and eventually will become dense enough to form stars . The remaining material is then believed to form planets and other planetary system objects . </P> <P> Around 150 AD, Claudius Ptolemaeus (Ptolemy) recorded, in books VII--VIII of his Almagest, five stars that appeared nebulous . He also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star . The first true nebula, as distinct from a star cluster, was mentioned by the Persian astronomer Abd al - Rahman al - Sufi, in his Book of Fixed Stars (964). He noted "a little cloud" where the Andromeda Galaxy is located . He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster . The supernova that created the Crab Nebula, the SN 1054, was observed by Arabic and Chinese astronomers in 1054 . </P>

Gas and dust in interstellar nebulae can form-