Editing Orion Nebula (section)

==Evolution==
[[File:Orion.nebula.arp.750pix.jpg|thumb|right|Panoramic image of the center of the nebula, taken by the Hubble Telescope. This view is about 2.5 light years across. The Trapezium is at center left.]]
[[Interstellar cloud]]s like the Orion Nebula are found throughout [[galaxy|galaxies]] such as the [[Milky Way]]. They begin as gravitationally bound blobs of cold, neutral hydrogen, intermixed with traces of other elements. The cloud can contain hundreds of thousands of [[solar mass]]es and extend for hundreds of light years. The tiny force of gravity that could compel the cloud to collapse is counterbalanced by the very faint pressure of the gas in the cloud.

Whether due to collisions with a [[spiral arm]], or through the shock wave emitted from [[supernova]]e, the atoms are precipitated into heavier molecules and the result is a molecular cloud. This presages the formation of stars within the cloud, usually thought to be within a period of 10–30&nbsp;million years, as regions pass the [[Jeans mass]] and the destabilized volumes collapse into disks. The disk concentrates at the core to form a star, which may be surrounded by a protoplanetary disk. This is the current stage of evolution of the nebula, with additional stars still forming from the collapsing molecular cloud. The youngest and brightest stars we now see in the Orion Nebula are thought to be less than 300,000 years old,<ref>"[http://archive.seds.org/hst/OrionFull.html Detail of the Orion Nebula]", HST image and text.</ref> and the brightest may be only 10,000 years in age. Some of these collapsing stars can be particularly massive and can emit large quantities of ionizing [[ultraviolet]] radiation. An example of this is seen with the Trapezium Cluster. Over time the ultraviolet light from the massive stars at the center of the nebula will push away the surrounding gas and dust in a process called [[photoevaporation]]. This process is responsible for creating the interior cavity of the nebula, allowing the stars at the core to be viewed from Earth.<ref name="successor" /> The largest of these stars have short life spans and will evolve to become supernovae.

Within about 100,000 years, most of the gas and dust will be ejected. The remains will form a young open cluster: a cluster of bright, young stars surrounded by wispy filaments from the former cloud.<ref>Kroupa, P., Aarseth, S.J., Hurley, J. 2001, MNRAS, 321, 699, [http://adsabs.harvard.edu/abs/2001MNRAS.321..699K "The formation of a bound star cluster: from the Orion nebula cluster to the Pleiades"]</ref>
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