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Investigating Other Galaxies
CHAPTER LEARNING OBJECTIVES
By reading the sections of this chapter, you will learn:
- 14-1 When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way until their variable stars were carefully observed
- 14-2 Hubble devised a system for classifying galaxies according to their appearance
- 14-3 Exploding stars release similar amounts of light and their distance can be inferred by measuring their apparent brightness
- 14-4 Galaxies are found in clusters and superclusters
- 14-5 Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena
- 14-6 Dark matter can be inferred by observing the motions of galaxy clusters
- 14-7 Quasars are the ultraluminous centers of the most distant galaxies
- 14-8 Supermassive black holes may be the “central engines” that power active galaxies
- 14-9 Galaxies may have formed from the merger of smaller objects
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A century ago, most astronomers thought that the entire universe was only a few thousand light-years across and that nothing lay beyond our Milky Way Galaxy. Today, we know that picture was utterly wrong. We now understand that the Milky Way is just one of billions of galaxies strewn across billions of light-years. The image opening this chapter shows two of these galaxies, denoted by rather mundane catalog numbers—NGC 1531 and NGC 1532—that give no hint to these galaxies’ magnificence.
Some galaxies are spirals like NGC 1532 or our Milky Way, with arching spiral arms that are active sites of star formation. The bright pink bands we see in NGC 1532 are clouds of excited hydrogen that are set aglow by ultraviolet radiation from freshly formed massive stars. Others, like NGC 1531, are featureless, ellipse-shaped collections of stars, virtually devoid of interstellar gas and dust. Some galaxies are only one-hundredth the size and one ten-thousandth the mass of the Milky Way. Others are giants, with 5 times the size and 50 times the mass of the Milky Way. Only about 10% of a typical galaxy’s mass emits light of any kind; the remainder is made up of the mysterious dark matter.
A single galaxy, vast though it may be, is just a tiny part of the entire observable universe. Just as most stars are found within galaxies, most galaxies are located in groups and clusters. In this chapter, we will explore what astronomers know about galaxies and the clusters of galaxies that stretch in huge, lacy patterns across the universe.