A nebula (Latin for "cloud";〔(Nebula ), Online Etymology Dictionary〕 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 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.
Most nebulae are of vast size, even hundreds of light years in diameter.〔('Formey, Johann Heinrich Samuel. "Nebula." The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project. Translated by Amanda Oberski. Ann Arbor: Michigan Publishing, University of Michigan Library, 2011. Web. 1 April 2015.
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 larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets and other planetary system objects.
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/Muslim 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.〔Lundmark K. (1921), ''(Suspected New Stars Recorded in the Old Chronicles and Among Recent Meridian Observations' )', Publications of the Astronomical Society of the Pacific, v. 33, p.225〕〔Mayall N.U. (1939), ''(The Crab Nebula, a Probable Supernova )'', Astronomical Society of the Pacific Leaflets, v. 3, p.145〕
In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula wouldn't be performed until 1659 by Christiaan Huygens, who also believed himself to be the first person to discover this nebulosity.〔
In 1715, Edmund Halley published a list of six nebulae. This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. From 1751–53, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, with most of them being previously unknown. Charles Messier then compiled a catalog of 103 "nebulae" (now called Messier objects, which included what are now known to be galaxies) by 1781; his interest was detecting comets, and these were objects that might be mistaken for them.
The number of nebulae was then greatly expanded by the efforts of William Herschel and his sister Caroline Herschel. Their ''(Catalogue of One Thousand New Nebulae and Clusters of Stars )'' was published in 1786. A second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster.〔
Beginning in 1864, William Huggins examined the spectra of about 70 nebulae. He found that roughly a third of them had the emission spectrum of a gas. The rest showed a continuous spectrum and thus were thought to consist of a mass of stars. A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster. Thus the nebula radiates by reflected star light.
In about 1922, following the Great Debate, it had become clear that many "nebulae" were in fact galaxies far from our own.
Slipher and Edwin Hubble continued to collect the spectra from many diffuse nebulae, finding 29 that showed emission spectra and 33 had the continuous spectra of star light.〔 In 1922, Hubble announced that nearly all nebulae are associated with stars, and their illumination comes from star light. He also discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B1 or hotter (including all O-type main sequence stars), while nebulae with continuous spectra appear with cooler stars. Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner.〔
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