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Star formation

The Orion Nebula is an example of a stellar nursery where new stars are being born. Observations of the nebula have revealed approximately 700 stars in various stages of formation within the nebula.

In 1979 observations with the Lallemand electronic camera at the Pic-du-Midi Observatory showed six unresolved high-ionization sources near the Trapezium Cluster. These sources were interpreted as partly ionized globules (PIGs). The idea was that these objects are being ionized from the outside by M42. Later observations with the Very Large Array showed solar-system-sized condensations associated with these sources. Here the idea appeared that these objects might be low-mass stars surrounded by an evaporating protostellar accretion disk. In 1993 observations with the Hubble Space Telescope have yielded the major confirmation of protoplanetary disks within the Orion Nebula, which have been dubbed proplyds. HST has revealed more than 150 of these within the nebula, and they are considered to be systems in the earliest stages of solar system formation. The sheer numbers of them have been used as evidence that the formation of planetary systems is fairly common in the universe.

Stars form when clumps of hydrogen and other gases in an H II region contract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extreme temperatures by converting gravitational potential energy to thermal energy. If the temperature gets high enough, nuclear fusion will ignite and form a protostar. The protostar is 'born' when it begins to emit enough radiative energy to balance out its gravity and halt gravitational collapse.

Typically, a cloud of material remains a substantial distance from the star before the fusion reaction ignites. This remnant cloud is the protostar's protoplanetary disk, where planets may form. Recent infrared observations show that dust grains in these protoplanetary disks are growing, beginning on the path towards forming planetesimals.

Once the protostar enters into its main sequence phase, it is classified as a star. Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster. Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.