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# 15.1 The Milky Way Galaxy

For millennia, people all over the world have traced out the hazy band of light in the night sky that we call the Milky Way. It was only when Galileo turned a telescope toward the sky that it became clear that the glow of the Milky Way is the light of myriads of stars to faint too see individually. For several hundred years after Galileo, astronomers mapped out the stars in the Milky Way in the belief that they were tracing the shape of the entire universe. As they turned their early telescopes toward this path of visible light, they discovered a collection of hundreds of billions of stars embedded in vast clouds of gas and dust.

We hundreds of years, it was believed that our Milky Way was the entirety of the Universe. In the early 1900s, Edwin Hubble showed that many of the nebulae in the night sky are actually other galaxies of stars separated from us by vast gulfs of space. We now see the Milky Way as just one of a multitude of galaxies in the universe; the Milky Way is not unique. Nonetheless, our study of the Milky Way is important in helping us understand the other galaxies, just as our study of the Sun is important in helping us understand other stars. The properties of our own system of stars can be used to infer the properties of other galaxies. The discovery of galaxies beyond our own is a continuation of the Copernican revolution — the idea that our location in the Milky Way is not a special place in the universe (a concept often referred to as the Mediocrity Principle). It also sets a challenge to astronomers to show that the same physical laws that govern the Milky Way also govern galaxies remote from our own.

Different sets of stars in the Milky Way have particular properties. Astronomers refer to these as the stellar components of the Galaxy. The Sun is located in the disk - the flattened distribution of stars that appears as a band of light in the night sky that gives the Milky Way its name. Many of the stars in the disk are young and blue and gathered into groups (e.g. star forming regions and open clusters). The disk is surrounded by a spherical swarm of stars and groups of stars collectively called the halo. Halo stars are generally old and red, and the groups in this region are referred to as Globular clusters. Toward the center of our galaxy is a concentration of generally old and red stars called the galactic bulge. We can learn about the shape and size of the Milky Way Galaxy by measuring the distances to stars and groups of stars and by plotting the distribution of their positions on the sky, but our ability to probe into the Galaxy varies dramatically with what kind of light we use within the electromagnetic spectrum.

The most familiar view of the Milky Way is the one you can see with the naked eye when you are outside far from city lights. You could make a nice panorama of the Milky Way from a mosaic of pictures taken with a normal 35-mm camera that allows time exposures. Sadly, most people live in suburbs or urban areas and are unfamiliar with this view. Photographs clearly show a difference in the shape of the concentration of stars toward the galactic center. In that direction the Milky Way widens where our own galactic nucleus adds its spheroid of light into the mix of the disk. They also show, wrapping the entire sky, an uneven distribution of stars and a band of obscuring dust that marks the plane of the Milky Way. Dust dims and reddens the light from stars in all directions. Looking directly up out of the disk, we can see past all the stars and through all the interstellar gas and dust, seeing clearly beyond the disk to the other galaxies far beyond. The galaxy is transparent in this direction. However, the dimming is particularly severe in the disk of the galaxy, because that is where the gas and dust associated with young stars are concentrated.

The effects of the dust can be severe; for several centuries our lack of understanding of this dust led astronomers to make a fundamental mistake about our place in space. Were you to, like early astronomers, plot the number of stars per degree on the sky across the entirety of both the Northern and Southern hemispheres, you might think that we lived in the center of disk surrounded by a scattering of stars. This is because the disk is so thick with gas and dust that we can only see a small ways through the disk in any given direction. Just like a person in a bank of fog might actually be able to see their hand before their face but not tell if they are 2 meters or 2 kilometers from the edge of the fog, we in the Milky Way can see the nearest stars, but can't tell using our eyes just how far we are from the edge of the disk. It took some very creative thinking and telescopes to realize we aren't near the center. By mapping the positions of the spherically distributed globular clusters that surround our galaxy, astronomers (beginning especially with Harlow Shapley and his colleagues) were able to build an increasingly accurate picture of our true off-center position.

After a distance of 500 pc through the plane of the disk, the galaxy fairly opaque. At 500 pc, the optical depth τ = 1, indicating that a fraction of 1/e, or about one third, of the light reaches us. Therefore we cannot see much farther than 500 pc. Infrared waves are less affected by dust, so the way to see deeper into the Milky Way is to use long-wavelength radiation. By looking through the galaxy with infrared light instead of visible light, we can actually look through the dust. Short wavelengths, like ultraviolet light and the visible light we see, are easily absorbed by dust and gas, but longer wavelengths, like infrared, millimeter, and radio, are able to traverse all but the densest and deepest dust clouds. Visible light has a wavelength of about 500 nm. If we increase the wavelength by a factor of four to 2000 nm (or 2 micrometers), we can see ten times farther. In other words, the optical depth does not reach a value of one until a distance of 5000 pc. In recent years, astronomers using these long wavelengths have been able to map the disk of our galaxy and determine details of its structure.

Radio telescopes can penetrate all the way to the galactic center. Astronomers view the cold atomic gas with radio telescopes using the 21-cm line of neutral hydrogen. The thin band of cool gas associated with disk stars is roughly uniform in all directions; this view is not obscured by dust. A wavelength of 21-cm is long enough to penetrate all the way to the galactic center. The thinness of the disk is clearly seen, but there are also complex structures of loops and filaments rising out of the galactic plane. These structures outline the ejected gas of supernova remnants. The filaments rise thousands of parsecs out of the galactic plane — supernovae can affect material over thousands of light years!

Far-infrared emission traces cool dust mixed in with the gas and stars that form the plane of the galaxy. Interplanetary dust particles in the solar system are also seen. Several nearby regions of star formation appear above and below the plane of the disk. Near-infrared emission traces stars in the thin disk and the central bulge. Near infrared waves are not long enough to penetrate all the way to the galactic center. The peanut-shaped bulge is crossed by a band of obscuring dust.

Astronomers use images made at optical and radio wavelengths to estimate the galaxy's size. They deduce that we live in a truly enormous system of stars. It is 30,000 pc (or 30 kpc) across, which translates into 30,000 × 3.26 = 100,000 light years, or 100,000 × 9 × 1012 = 1018 km. A size of a billion billion kilometers is very difficult to comprehend!
Together, images of the Milky Way made at different wavelengths reveal structures traced out by gas, dust, and stars. The halo is extremely diffuse and does not show up in any of these pictures. Likewise, globular clusters are too small to show up clearly in the wide-angle views, and individual halo stars are too faint and too thinly distributed to be seen. Modern observations with the Sloan Digital Sky Survey have added further refinements to our understanding of the Milky Way's shape. Today, we know our galaxy has a central bar, and two dominant arms with spurs off the dominant arms.