13-3 Observations of star-forming regions reveal that our Galaxy has spiral arms

The galaxies shown in Figure 13-9 and Figure 13-11 both have spiral arms, spiral-shaped concentrations of gas and dust that extend outward from the center in a shape reminiscent of a pinwheel. Although we cannot see the precise shape of the Milky Way because we are inside it, we believe that the Milky Way Galaxy also has spiral arms. Because interstellar dust obscures our visible-light view in the plane of our Galaxy, a detailed understanding of the structure of the galactic disk did not occur until the development of radio astronomy. Thanks to their long wavelengths, radio waves can penetrate the interstellar medium even more easily than infrared light and can travel without being scattered or absorbed. As we will see in this section, both radio and optical observations reveal that our Galaxy does indeed have spiral arms.

Mapping Hydrogen in the Milky Way with Radio Waves

Hydrogen is by far the most abundant element in the universe. Hence, by looking for concentrations of hydrogen gas, we should be able to detect important clues about the distribution of matter in our Galaxy. Unfortunately, ordinary visible-light telescopes are of little use in this quest. This is so because hydrogen atoms can only emit visible light if their electrons are first excited to high energy levels. This is quite unlikely to occur in the cold depths of interstellar space. Furthermore, even if there are some hydrogen atoms that glow strongly at visible wavelengths, interstellar dust would make it impossible to see this glow from distant parts of the Galaxy.

What makes it possible to map out the distribution of hydrogen in our Galaxy is that even cold hydrogen clouds emit radio waves. As we mentioned earlier, radio waves can easily penetrate the interstellar medium, so we can detect the radio emission from such cold clouds no matter where they lie in the Galaxy. To understand how hydrogen in these clouds can emit radio waves, we must probe a bit more deeply into the structure of protons and electrons, the particles of which hydrogen atoms are made.

In addition to having mass and charge, particles such as protons and electrons possess a tiny amount of spin (more formally described as angular momentum). Very roughly, you can visualize each of the electrons around protons in an atom as a tiny, electrically charged sphere that spins on its axis. (This idea also works for protons.) Because electric charges in motion generate magnetic fields, an electron behaves like a tiny magnet with a north pole and a south pole (Figure 13-12).

Figure 13-12: Magnetic Interactions in the Hydrogen Atom (a) The energy of a pair of magnets is high when their north poles or their south poles are near each other, and low when they have opposite poles near each other. (b) Thanks to their spin, electrons and protons are both tiny magnets. When the electron flips from the higher-energy configuration (with its spin in the same direction as the proton’s spin) to the lower-energy configuration (with its spin opposite to the proton’s spin), the atom loses a tiny amount of energy and emits a radio wavelength photon at 21 cm.

If you have ever played with magnets, you know that two magnets attract when the north pole of one magnet is next to the south pole of the other and repel when two like poles (both north or both south) are next to each other (Figure 13-12a). In other words, the energy of the two magnets is least when opposite poles are together and highest when like poles are together. Hence, as shown in Figure 13-12b, the energy of a hydrogen atom is slightly different, depending on whether the spins of the proton and electron are in the same direction or opposite directions. According to the laws of physics, these are the only two possibilities; the spins cannot be at random angles.

If the spin of the electron changes its orientation from the higher-energy configuration to the lower-energy one—called a spin-flip transition—a photon is emitted. The energy difference between the two spin configurations is very small, only about 10⁻⁶ times as great as those between different electron orbits. Therefore, the photon emitted in a spin-flip transition between these configurations has only a small amount of energy, and thus its wavelength is a relatively long 21 cm. What is important here is that the photon generated from hydrogen in this scenario has a wavelength of 21 cm and is in the radio wavelengths portion of the electromagnetic spectrum.

The distribution of hydrogen gas in the Milky Way is not uniform but is actually quite frothy. In fact, our Sun lies near the edge of an irregularly shaped region within which the interstellar medium is very thin but at very high temperatures (about 10⁶ K). This curious region may have been carved out by a supernova that exploded nearby some 300,000 years ago.

Detecting Our Galaxy’s Spiral Arms

The detection of 21-cm radio emission was a major breakthrough that permitted astronomers to reveal the presence of spiral arms in the galactic disk. Figure 13-13 illustrates how this was done. Suppose that you aim a radio telescope along a particular line of sight across the Galaxy. Your radio receiver, located at S (the position of our solar system), picks up 21-cm emission from hydrogen clouds at points 1, 2, 3, and 4. However, the radio waves from these various clouds are Doppler shifted by slightly different amounts because the clouds are moving at different speeds as they travel with our rotating Galaxy.

Figure 13-13: A Technique for Mapping Our Galaxy If we look within the plane of our Galaxy from our position at S, hydrogen clouds at different locations (shown as 1, 2, 3, and 4) along our line of sight are moving at slightly different speeds relative to us. As a result, radio waves from these various gas clouds are subjected to slightly different Doppler shifts. This permits radio astronomers to sort out the gas clouds and thus map the Galaxy.

It is important to remember that the Doppler shift reveals only motion along the line of sight. In Figure 13-13, cloud 2 has the highest speed along our line of sight because it is moving directly toward us. Consequently, the radio waves from cloud 2 exhibit a larger Doppler shift than those from the other three clouds along our line of sight. Because clouds 1 and 3 are at the same distance from the galactic center, they have the same orbital speed. The fraction of their velocity parallel to our line of sight is also the same, so their radio waves exhibit the same Doppler shift, which is less than the Doppler shift of cloud 2. Finally, cloud 4 is the same distance from the galactic center as the Sun. This cloud is thus orbiting the Galaxy at the same speed as the Sun, resulting in no net motion along the line of sight. Radio waves from cloud 4, as well as from hydrogen gas near the Sun, are not Doppler shifted at all.

These various Doppler shifts cause radio waves from gases in different parts of the Galaxy to arrive at our radio telescopes with wavelengths slightly different from 21 cm. It is therefore possible to sort out the various gas clouds and thus produce a map of the Galaxy like that shown in Figure 13-14.

Figure 13-14: RIVUXG A Map of 21-cm Light emitted by Hydrogen in Our Galaxy This map, constructed from radio-telescope surveys of 21-cm radiation, shows the distribution of hydrogen gas in a face-on view of our Galaxy. The map suggests a spiral structure. Details in the blank, wedge-shaped region at the bottom of the map are unknown. Gas in this part of the Galaxy is moving perpendicular to our line of sight and thus does not exhibit a detectable Doppler shift.

Figure 13-14 shows that this hydrogen gas is not spread uniformly around the disk of the Galaxy but is concentrated into numerous arched lanes. Similar features are seen in other galaxies beyond the Milky Way. As an example, the galaxy in Figure 13-15 has prominent spiral arms outlined by hot, luminous, blue main-sequence stars and the red emission nebulae found near many such stars. Stars of this sort are very short-lived, so these features indicate that spiral arms are sites of active, ongoing star formation. A radio telescope tuned to receive 21-cm light shows that spiral arms are also regions where hydrogen gas is concentrated, similar to the structures in our own Galaxy visible in Figure 13-14. This is a strong indication that our Galaxy also has spiral arms.

Figure 13-15: A Spiral Galaxy Known as the Southern Pinwheel Galaxy, M83 lies in the southern constellation Hydra about 15 million ly from Earth. (a) This visible-light image clearly shows the spiral arms. The presence of young stars and glowing, energized hydrogen indicates that star formation takes place in spiral arms. (b) This radio view at a wavelength of 21 cm shows the emission from interstellar hydrogen gas (H I). Note that essentially the same pattern of spiral arms is traced out in this image as in the visible-light photograph. (c) M83 has a much smoother appearance in this near-infrared view. This shows that cooler stars, which emit strongly in the infrared, are spread more uniformly across the galaxy’s disk. Note the elongated bar shape of the central bulge.

Mapping the Spiral Arms and the Central Bulge with Visible-light Observations

Figure 13-15a suggests that we can confirm the presence of spiral structure in our own Galaxy by mapping the locations of star-forming regions. Such regions are marked by OB associations, H II regions, and molecular clouds. Unfortunately, the first two of these are best observed using visible light, and interstellar dust limits the range of visual observations in the plane of the Galaxy to less than 10,000 ly from Earth. But there are enough OB associations and H II regions within this range to plot the spiral arms in the vicinity of the Sun.

Molecular clouds are easier to observe at great distances because molecules of carbon monoxide (CO) in these clouds emit radio waves that are relatively unaffected by interstellar dust. Hence, the positions of molecular clouds have been observed even in remote corners of the Galaxy, normally uncharted. Taken together, all of these observations demonstrate that our Galaxy has at least a couple of major spiral arms as well as several shorter arm segments (Figure 13-16). The Sun is located just outside a relatively short arm segment called the Orion Arm, which includes the Orion Nebula and neighboring sites of vigorous star formation in that constellation.

Figure 13-16: Our Galaxy Seen Face-on: Artist’s Impression The Galaxy’s diameter is about 160,000 ly and our solar system is a little more than 25,000 ly from the galactic center. The elongated central bulge is about 27,000 ly long and is oriented at approximately 45° to a line running from the solar system to the galactic center. The Galaxy’s two major arms, Scutum-Centaurus and Perseus, can be seen attached to the ends of a thick central bar, while the two smaller arm segments of Norma and Sagittarius are less distinct and located between the Galaxy’s two major arms. Our Sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

Two major spiral arms border either side of the Sun’s position. The Sagittarius Arm is on the side toward the galactic center. This is the arm you see on June and July nights when you look at the portion of the Milky Way stretching across Scorpius and Sagittarius. In December and January, when our nighttime view is directed away from the galactic center, we see the Perseus Arm. The other major spiral arms cannot be seen at visible wavelengths thanks to the obscuring effects of dust.

Figure 13-16 also shows that the central bulge of the Milky Way is not spherical, but rather is elongated like a bar. This is unlike the galaxy NGC 7331 shown in Figure 13-9, but similar to the galaxy M83 depicted in Figure 13-15. The elongated shape of the central bulge had been suspected since the 1980s; this was confirmed in 2005 using the Spitzer Space Telescope, which was used to survey the infrared emissions from some 3 million stars in the central bulge. Thus, the artist’s impression shown in Figure 13-16 is based on observations using both radio wavelengths (for the spiral arms) and infrared wavelengths (for the central bulge). As it turns out, this elongated shape may play a crucial role in sustaining the Galaxy’s spiral structure.

Why are the young stars, star-forming regions, and clouds of neutral hydrogen in our Galaxy all found predominantly in the spiral arms? To answer this question, we must understand why spiral arms exist at all. Spiral arms are essentially cosmic “traffic jams,” places where matter piles up as it orbits around the center of the Galaxy. This orbital motion, which is essential to grasping the significance of spiral arms, is our next topic as we continue our exploration of the Galaxy.