The Orion Nebula is the nearest H+ region to the Earth and the one that has received the greatest attention from astronomers. It is visible as a 4th magnitude fuzzy patch of light in the Sword region of the Orion constellation. On long exposures the nebula appears to be about half a degree across and to consist of a beautiful series of loops and filaments which emit the characteristic red light of hydrogen gas. Most of the light, however, comes from a more compact region a few minutes of arc across which appears white and over-exposed .

The Orion Nebula is ionized by a small group of 0 and B stars known as the Trapezium Cluster. The brightest of these stars is of spectral type 06. The cluster lies very close to the brightest part of the nebula and is surrounded by a rather uneven distribution of ionized gas. To the immediate east of the Trapezium stars is a thick cloud of neutral gas, known as the Dark Bay, which has not yet been ionized by the stars. To the west and south the density of the gas appears to decrease more gradually.

Behind the nebula, but invisible to optical astronomers is a cloud of molecular hydrogen. The contours in approximate extent of this cloud, and they are based on radio measurements of the strength of carbon monoxide and formaldehyde emission. The cloud has two prominent peaks, the southern of which, OMC1, lies fairly close to the Trapezium Cluster and contains some 500 MG of hydrogen. Each cloud contains at its centre a cluster of infrared sources, at least one of which, the Becklin-Neugebauer object is almost certainly a protostar. The temperature and density of the molecular clouds is highest close to the infrared sources, and it therefore seems very likely that these are the parts of the clouds where gravitational collapse and star formation have been happening most recently. The presence of a group of OH and H2O masers close to the infrared sources in OMCl provides additional evidence for this conclusion.

The relationship between the H+ region and the molecular cloud behind it is not completely clear, but it seems likely that the two regions are in physical contact and that the H+ region is eating its way into the molecular cloud. At the present time, the southern infrared cluster generates about the same total amount of power (at infrared wavelengths) as does the Trapezium cluster at optical wavelengths. The stars being born there are therefore potentially 0 stars. We may thus anticipate that some tens of thousands of years in the future, as the H+ region fades away, a new H+ region will be formed inside what is now the molecular cloud, ionized by the stars that we currently see in the process of formation.

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