At these wavelengths, the gas within a spiral galaxy actually emits more light than stars. This gas provides a bright emission spectrum at specific optical wavelengths, as well as bright emission at a particular radio wavelength (21 cm). However, gas is also spread throughout the disk of a spiral galaxy. For very nearby galaxies, we can measure the motions of stars. Stars are bright, and there are many of them throughout a galaxy. One might think astronomers would study the stars in galaxies to learn about the motion within galaxies. The real redshift and blueshift we observe in a rotating galaxy is very small and can only be measured by taking a spectrum. Note that the color shift in this image has been exaggerated. One side of the galaxy will appear redshifted, and the other side will appear blueshifted. (bottom) We can measure rotation in spiral galaxies that are seen edge-on. Figure 8.9: (top) We cannot measure rotation in spiral galaxies that are seen face-on. See Figures 8.9 and 8.10 for more on how we observe rotation in galaxies. We will observe blueshift on the side of the galaxy that is rotating toward us (the light will appear “scrunched” and will appear to have shorter, bluer wavelengths). For an edge-on, rotating spiral galaxy, we will observe redshift on the side of the galaxy that is rotating away from us (the light will appear to be “stretched out,” and will appear to have longer, redder wavelengths). On the other hand, if the galaxy is “edge-on,” we can measure the rotation through redshift and blueshift. If the galaxy is “face-on” from our vantage point, we will not be able to observe its rotation. The stars and gas in the disks of spiral galaxies tend to rotate around the center of the galaxy. How do we measure the orbital speeds of the stars and gas within a spiral galaxy? First note that because the bulges and disks of spiral galaxies are so much brighter than their halos, we mainly concentrate on measuring the orbital speeds of the stars and gas in those components of spiral galaxies. Note the bulge, disk, and halo of the galaxy. Figure 8.8: Illustration of a spiral, or “disk”, galaxy. We tend to look right through them.Īll of this material is held together by the mutual gravitational attraction of all the material within the galaxy. The number of stars is so low, in fact, that galaxy halos are nearly invisible. The halo is much larger in extent than the bulge, but there are not very many stars in it compared to the number of stars in the bulge or disk. Finally, there is a much larger sphere-shaped collection of stars and star clusters extending out at least as far as the disk, called the halo. It is called the bulge, or sometimes the central bulge. The second component is a sphere-shaped collection of stars near the galaxy center. You will not be surprised to learn that this is called the disk component. Spiral galaxies (like the Milky Way Galaxy) are large systems that typically have three distinct components (see Figure 8.8): The first is the flat disk that contains stars, gas, and dust, and that is most prominent when we look at these galaxies in visible light. To begin, we will consider spiral galaxies. Observing Rotation Curves of Disk Galaxies
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