2.4. HII REGIONS AND GIANT MOLECULAR CLOUDS 43
Figure 2.12: Left: e Orion Nebula, in the “sword” of the constellation Orion, is a glowing
cloud of ionized hydrogen. Photo by the author. Right: e constellation of Orion lies in the
direction of a giant molecular cloud, of which the Orion Nebula is only a tiny part. (Image made
with the Aladin sky atlas. DSS2, alasky.u-strasbg.fr/DSS/DSSColor [Bonnarel et al., 2000,
Lasker et al., 1996].)
than M 11. But when we consider that this globular cluster contains 300,000 stars—one hundred
times as many as M 11—it is clear that inside a globular cluster the stars are much closer together.
2.4 HII REGIONS AND GIANT MOLECULAR CLOUDS
e left side of Figure 2.12 shows a small part of the constellation Orion—the faint “sword” just
below the three stars of the prominent “belt”—as imaged with a long exposure through a small
telescope. e glowing red gas is the Orion Nebula, a region of ionized hydrogen, also called an
HII region.
e right-hand image in Figure 2.12 is a mosaic of images made with a large telescope,
and it shows that nearly the entire constellation of Orion is swathed in a complex of both dark
and glowing gas and dust. is giant molecular cloud (GMC) is about 240 ly across. At a distance
of less than 1500 ly, it covers over 10
˝
of the sky. e Orion nebula can be seen as the small bright
patch just below the three stars that make up the belt of Orion. A typical GMC is about 150 ly
across [Carroll and Ostlie, 2017, p 407], and so at about 240 ly the Orion GMC is a big one.
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.