44 2. LOOKING OUTWARD
Figure 2.13: A mosaic of 12 photographs showing the Milky Way (as seen from the Southern
Hemisphere) stretching across the sky from horizon to horizon. Photograph by the author.
e HII region we see as the Orion Nebula is much smaller than the GMC it is part
of. e light comes from ionized hydrogen gas, and the energy for the ionization comes from
ultraviolet light produced by hot type O and B stars. And so the size of the glowing region is
limited by the ultraviolet light emitted by such stars; an HII region may be less than a light year
across. But since multiple stars can contribute to the ionization, the largest HII regions are a few
hundred light years across [Carroll and Ostlie, 2017]. e Orion Nebula is about 20 ly across.
Most of the Orion GMC appears dark at visible wavelengths. e gas is relatively dense,
and very cool—typically only about 15 K. e low temperatures mean that molecules can easily
form. e visible HII regions scattered throughout the Orion GMC are places where newly
formed O and B stars are ionizing the gas, and thus raising it to temperatures above 10,000 K.
e red color comes primarily from emission of hydrogen.
e Orion Nebula will eventually become an open star cluster. Indeed, there already is a
newly-formed open star cluster inside it. But the presence of dust obscures our view at visible
wavelengths. e gas and dust in between the stars, including HII regions and GMC’s but also
less-dense material, is called the interstellar medium (ISM).
2.5 GALAXIES
e word galaxy comes from the Greek word galaxias, which means “milky.” is refers to the
dim, milky glow of light that forms a band stretching across the nighttime sky—the Milky Way.
We now know this is our own galaxy as seen from the inside, and it is only one of billions of
galaxies in the universe. See Figure 2.13 for my own photograph of the Southern Hemisphere
Milky Way, made from a mosaic of 12 photographs, stretching clear across the sky from horizon
to horizon.
2.5.1 THE MILKY WAY AND THE ANDROMEDA GALAXY
“e Milky Way” is the name for both the galaxy we are part of, and also its visual appearance
in the nighttime sky. It is shaped like a flat disk with a central bulge, made of several hundred
2.5. GALAXIES 45
Figure 2.14: A panorama of the Milky Way. We are seeing it from the inside, and we have a
view that sees the disk of the galaxy edge-on. (Image by ESO/S. Brunier, CC BY 4.0.)
billion stars. e Milky Way stretches more than 100,000 light years across, and is about 1000
light years thick (thicker at the bulge). at is, the Galaxy is so big that it takes light 100,000
years to travel from one side to the other.
e disk is where most of the stars and nearly all the gas and dust reside, but the Milky
Way also has a halo—a spherical distribution of stars that surrounds the disk and extends to
distances beyond its edge. e globular clusters are part of the halo of the Milky Way, while the
open clusters are part of the disk.
Figure 2.14 shows a photo-montage of the Milky Way, made by the European Southern
Observatory from many individual images stretching nearly around the entire sky. e glow
is made from the combined light of millions of stars too faint to show individually. e dark
patches are dust, concentrated in the disk of the Galaxy, that obscures the view of stars beyond.
We see other galaxies besides our own, very far away, and some look just like we would
expect the Milky Way to appear if seen from afar. e galaxy NGC 891 (see Figure 2.15) in the
constellation Andromeda is a good example.
e constellation Andromeda holds one of the most famous of galaxies—the Andromeda
Galaxy, M 31. It is much closer than NGC 891, and appears somewhat as we expect the Milky
Way would appear if seen with the disk slightly inclined. M 31 is easily visible from the Northern
hemisphere on a dark night with the naked eye. It is probably about 40% larger than our Milky
Way, and it is the nearest large galaxy. With a diameter of 70 kpc and a distance of 0.78 Mpc,
we can easily calculate its size.
46 2. LOOKING OUTWARD
Figure 2.15: We see other galaxies that look like our Milky Way. is is NGC 891, and it is a
spiral galaxy like our own, seen edge-on. (Image by Hewholooks, CC BY-SA 3.0.)
Here I have referred to the common units of measure used by astronomers to describe
the sizes and distances of galaxies. We have already described the parsec (pc) to measure the
distances between stars within the Milky Way. But the size of a typical large galaxy like our
own suggests a more convenient unit, the kiloparsec (kpc). One kiloparsec is equivalent to 1000
parsecs.
For the vast distances between galaxies, the megaparsec (Mpc) is more convenient. A mega-
parsec is equivalent to a million parsecs or 1000 kpc. And so let us look again at the fact that the
Andromeda galaxy is 70 kpc across and 0.78 Mpc distant. If we rewrite 0.78 Mpc as 780 kpc, it is
clear that the angular diameter of the Andromeda galaxy—its diameter divided by its distance—
is 70=780 D 0:009 radians. is is equivalent to about 5
˝
, which is about 10 times larger than the
apparent size of the full Moon. But when observed directly, the Andromeda galaxy appears as a
dim cloud of low surface brightness; good eyes and extremely dark and clear skies are required
to visually trace its outer environs. Long exposure photography, on the other hand, can easily
reveal its beauty; see Figure 2.16.
2.5. GALAXIES 47
Figure 2.16: e Andromeda Galaxy, M 31, is the closest large spiral galaxy to our own Milky
Way. (Image by Adam Evans, CC BY 2.0.)
2.5.2 DWARF GALAXIES
Notice that there are two small blobs of light next to the Andromeda Galaxy, in Figure 2.16.
ese are the dwarf elliptical galaxies M 32 and NGC 205. Dwarf galaxies are sometimes irregu-
lar in shape (dwarf irregulars) or more elliptical or spheroidal (dwarf ellipticals), and they make
up the vast majority of galaxies by number. A good example of a dwarf irregular is the Small
Magellanic Cloud (SMC), easily visible to the naked eye from the Southern hemisphere. It is
about 61 kpc distant and roughly 2 kpc across. Compare this to the Milky Way, which is over
30 kpc across.
Next to the SMC in the sky is the Large Magellanic Cloud (see Figure 2.17). It is a bit
closer than the SMC, and about twice its size. ere is evidence that it is not really irregular;
rather it is the remains of a barred spiral galaxy, distorted by the gravity of the SMC. And so it
is often considered a peculiar and very small barred spiral galaxy, rather than a dwarf irregular
galaxy. Both the SMC and LMC contain on the order of 10 billion stars—far fewer than the
roughly 250 billion that make up the Milky Way.
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