The Orion Nebula

The Orion Nebula is a blister-type HII region, a cavity formed in the surface of the molecular cloud by ultraviolet radiation from the Trapezium stars (especially  C)--stars < 1 Myr old--which lie about 0.2 - 0.3 pc in front of the nebula (Baldwin, et al 1991). The HII region as whole has a radius of 0.3 - 0.5 pc (Hillenbrand 1997). Behind the HII region is an ionization front and photodissociation region which is the interface between the HII region and the molecular cloud; this has a thickness of about 10^-4 pc (O'Dell 1993). The far edge of the molecular cloud itself is about 500 pc away (Brown, de Geus, & de Zeeuw 1994), and so is perhaps as much as 50 pc thick in the vicinity of the Orion Nebula. Figure 1 shows a diagram displaying the relationship between the various layers of the nebula and molecular cloud.

The optically visible nebula emits lines typical of HII regions, such as [NII], [OIII], and [SII], and hydrogen recombination lines. These various layers are partially revealed in the Orion Bar, a bright linear structure about 2 arcminutes southeast of the Trapezium. The Bar is formed by the curvature of the HII region; at this point the cavity curves steeply toward earth, allowing us to see the PDR edge-on (Hogerheijde, Jansen, and van Dishoeck 1995).

The surface of the nebula is not smooth, but composed of a variety of wisps, bubbles, and ridges, whose brightness and spectra are dependent mainly on their distance from the Trapezium (Wen & O'Dell 1995). The nebula is also the site of at least two systems of Herbig-Haro (HH) objects. HH objects are by-products of star formation: emission nebulae formed when an outflow from a young star impacts surrounding material, shocking both outflowing and ambient material. The southern system (HH 202 - 204) probably has its source(s) in the vicinity of the Trapezium, while the northern system includes the fast, optically-emitting ``fingers'', detailed below (O'Dell 1997).

© Copyright Astronomical Society of Australia 1997