Nebula

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, the term was used to describe 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.Around 150 AD, 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.There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution.Herbig–Haro object HH 161 and HH 164.The Omega Nebula, an example of an emission nebulaThe Horsehead Nebula, an example of a dark nebula.The Cat's Eye Nebula, an example of a planetary nebula.The Red Rectangle Nebula, an example of a protoplanetary nebula.The delicate shell of SNR B0509-67.5Tycho Supernova remnant in X-ray lightObjects named nebulae belong to 4 major groups. Before their nature was understood, galaxies ('spiral nebulae') and star clusters too distant to be resolved as stars were also classified as nebulae, but no longer are.