The Cosmos, 4th ed. by Pasachoff & Filippenko Chapter 16 Concept Review » Around the turn of the 20th century, some astronomers believed that the spiral nebulae were clouds of gas within our Milky Way star system, while others thought they might be huge, completely separate systems of stars bound together by gravity (Section 16.1a). Their distance and nature was the subject of the famous Shapley‐ Curtis debate. In the mid‐1920s, Edwin Hubble recognized Cepheid variable stars within a few of these objects, and their observed faintness implied an enormous distance (Section 16.1b). Thus, the true nature of the spiral nebulae was finally unveiled: They are distant galaxies like our own Milky Way Galaxy, and the Universe is far larger than previously thought. Galaxies are the fundamental units of the Universe; each one contains billions of stars spread over a diameter exceeding 10,000 light‐years. » There are various types of galaxies. Spiral galaxies contain a thin, rotating disk of stars and gas; new stars tend to form in the spiral arms (Section 16.2a). They also have a spherical bulge of old stars. Elliptical galaxies have no disk or arms, and generally very little gas and dust; they consist almost entirely of old stars (Section 16.2b). They are roughly spherical or elongated in shape, and don’t rotate much. Lenticular (S0) galaxies are a cross between spirals and ellipticals (Section 16.2c). The shapes of irregular and peculiar galaxies are strange. Galaxies frequently occur in clusters of galaxies (Section 16.3a), and these in turn congregate in superclusters separated by large voids (Section 16.3b). » We now have strong indications that much of the matter in the Universe emits little if any detectable electromagnetic radiation, but it has a gravitational influence on its surroundings. One of the first clues was provided by the flat (nearly constant) rotation curves of spiral galaxies, including our own Milky Way Galaxy (Section 16.4a). These are in stark contrast with the orbital speeds of planets in the Solar System, which decrease with increasing distance from the Sun. There is also evidence for such dark matter in pairs, groups, and clusters of galaxies (Section 16.4b). » The exact composition of the dark matter is unknown (Section 16.4c); possibilities include white dwarfs, neutron stars, black holes, brown dwarfs, large “planets,” and neutrinos. Most of the dark matter is probably exotic subatomic particles such as WIMPs (weakly interacting massive particles). Much of what we call the “normal” dark matter (that is, consisting of protons, neutrons, and electrons) is gas at a temperature of about a million degrees; it has now barely been detected, and therefore isn’t really “dark.” Studies of gravitational lensing can help reveal the presence of dark matter (Section 16.5). They suggest that a small fraction of the dark matter in our Milky Way Galaxy may consist of MACHOs (massive compact halo objects), though the vast majority of it seems to be WIMPs. » Clues to the formation and evolution of galaxies are found from observations of very distant galaxies, at large lookback times; we are viewing them as they were when the Universe was far younger than it is today (Section 16.6). Their distances are obtained from the redshifts measured in their spectra and interpreted according to the Doppler formula and Hubble’s law (Section 16.7). The © Cambridge University Press, 2013 Page 1 The Cosmos, 4th ed. by Pasachoff & Filippenko Universe is expanding, with the speeds of different galaxies being proportional to their current separations from us, and the constant of proportionality is called Hubble’s constant. (We will see in Chapter 18 that the expansion is currently accelerating.) » Galaxies at a variety of lookback times are seen in the Hubble Deep Fields and other long‐
exposure photographs of the Universe (Section 16.8). Recently, galaxies with redshifts exceeding 5, 6, and even 7 have been found; we see them as they were only about one billion years after the birth of the Universe. The Hubble Space Telescope’s Advanced Camera for Surveys and Wide Field Camera 3 are more efficient and cover a wider range of wavelengths than the earlier cameras. » Though most elliptical galaxies formed a very long ago, some were produced relatively recently by the merging of two spiral galaxies (Section 16.9). Early in the history of the Universe, there were many small, irregular galaxies that may have coalesced to form large spirals. Massive galaxies can retain the heavy elements synthesized and blown out by supernovae, while small galaxies tend not to become as chemically enriched. » Computer models of the large‐scale structure of the Universe suggest that clusters and superclusters of galaxies formed at the peaks of a somewhat more uniform distribution of dark matter (Section 16.10). Cold dark matter, which moves slowly compared with light, provides a better match to the data than does rapidly moving hot dark matter. Cold dark matter constitutes most of the matter in the Universe, but it cannot be normal matter consisting of protons, neutrons, and electrons. Though cold dark matter may be mostly WIMPS, its exact nature is one of the outstanding mysteries of modern astrophysics. All matter (of which about 84 per cent is cold dark matter and 16 per cent is normal matter) constitutes only 27 per cent of the Universe; the rest (73 per cent) is gravitationally repulsive “dark energy,” to be discussed in Chapter 18. © Cambridge University Press, 2013 Page 2