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Exploring Our Galaxy
CHAPTER LEARNING OBJECTIVES
By reading the sections of this chapter, you will learn:
- 13-1 The Sun is located in the disk of our Galaxy, about 25,000 light-years from the galactic center
- 13-2 Observations of different types of dust, gas, stars, and star clusters reveal the shape of our Galaxy
- 13-3 Observations of star-forming regions reveal that our Galaxy has spiral arms
- 13-4 Measuring the rotation of our Galaxy reveals the presence of dark matter
- 13-5 Spiral arms are caused by density waves that sweep around the Galaxy
- 13-6 Infrared and radio observations are used to probe the galactic nucleus
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On a clear, moonless night, away from the glare of city lights, you can often see a hazy, luminous band stretching across the sky. This band, called the Milky Way, extends all the way around the sky’s celestial sphere. Galileo, the first person to write about viewing the Milky Way with a telescope, discovered that this band is composed of countless dim stars. Today, we realize that the Milky Way is actually a disk thousands of light-years across, containing hundreds of billions of stars—one of which is our own Sun—as well as vast quantities of gas and dust. This vast, swirling assemblage of matter gravitationally bound together is collectively called the Milky Way Galaxy.
Just as Galileo’s telescope revealed aspects of the Milky Way that the naked eye could not, modern astronomers use telescopes at wavelengths of light invisible to our eyes to peer through our Galaxy’s obscuring dust and observe what visible-light telescopes never could. As we will see, radio, infrared, and X-ray observations reveal that the very center of the Galaxy harbors a supermassive black hole with a mass of more than 4 million Suns. Our Sun and planet Earth are not at the galactic center, but rather about 25,000 light-years out along the Galaxy’s flat disk, giving us a wondrous view of its center.
Perhaps most surprisingly, astronomers have recently discovered that most of the Milky Way’s mass is not in its visible stars, gas, or dust, but in a halo of dark matter that emits no measurable light. What the character of this dark matter could be remains one of the greatest open questions in astronomy and physics.
The Milky Way is just one of myriad galaxies, or systems of stars and interstellar matter, that are spread across the observable universe. By studying our home Milky Way Galaxy in this chapter, we begin to explore the universe on a grand scale. Instead of focusing on individual stars as we did in earlier chapters, we will now look carefully at the overall arrangement and history of a huge stellar community of which our Sun is but one member. In this way, we gain insights into galaxies, described in the following chapter, and prepare ourselves to ask fundamental questions about the cosmos in the book’s final chapter.