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Right Ascension Offset Aa (arcmin)

6h0m

5h48m

5h36m

5h24m

Right Ascension a

Figure 1.2 Map of the Orion Molecular Cloud in the 2.6 mm line of 12CiaO. See Appendix A for an explanation of the coordinates used here and elsewhere in the book. The insert shows a detail of Orion B in the 3.1 mm line of CS. Shading within the insert marks those regions which emit strongly at 2.2 jam. The (0,0) point coincides with the position of the reflection nebula NGC 2024. This nebula and others are indicated by crosses.

Figure 1.3 Dark clouds in Orion. The large swath is Barnard's loop, a diffuse region of enhanced optical emission. The labeled dark patches are reflection nebulae. Major clouds are also labeled by their Lynds catalogue numbers.

Right Ascension a

Figure 1.3 Dark clouds in Orion. The large swath is Barnard's loop, a diffuse region of enhanced optical emission. The labeled dark patches are reflection nebulae. Major clouds are also labeled by their Lynds catalogue numbers.

Figure 1.4 Infrared view of the Orion Molecular Cloud. This is a composite of three monochromatic images at 12, 60 and 100 pm.

embedded stellar clusters, compact groups containing tens to hundreds of members. Each cluster is associated with one of the more massive bodies of molecular gas, and virtually all cluster members are still nested within their parent dense cores. Thus, we see that stars form in giant molecular clouds at localized, massive peaks in the gas, and predominantly in a cluster mode, rather than in isolation.

1.1.2 Orion Nebula and BN-KL Region

To the south of Orion B, the Orion A cloud consists of a similar clumpy distribution of molecular gas. Within one elongated, high-density region is the famous Orion Nebula, also designated NGC 1976 or M42, the latter nomenclature referring to the 18th century Messier catalogue. As shown in the optical photograph of Figure 1.5 (left panel), the nebula is a turbulent expanse of gas, lit up by an embedded stellar cluster. The conspicuous ridge at the bottom of the photograph is the Orion Bar, whose emission also stems from gas heated and ionized by the cluster stars. This ionization front is seen edge on and is especially well defined because of the cool, dusty region just beyond it. The cluster responsible for such energetic activity is the Ori Id OB association, one of several small groups of massive stars in the giant complex. Near the center of the figure are the four stars of the Trapezium, whose most prominent member, the O star 01 Ori C, has a luminosity of 4 x 105 L& and a surface temperature of 4 x 104 K.

Stars this hot emit most of their energy in the ultraviolet and are thus capable of ionizing hydrogen gas out to considerable distances. The Orion Nebula, which is about 0.5 pc in diameter, is the best studied example of such an HII region. Within the ionized plasma, the gas temperature is comparable to that at the surface of the exciting star. As electrons and nuclei recombine, the atoms emit a plethora of spectral lines, including optically visible radiation from m:

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