Chapter 28 Stars and the Universe

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730 CHAPTER 28: STARS AND THE UNIVERSE the main sequence for billions of years. Collisions with large objects from space, such as those thought to mark the ends of past geologic eras, are possible. Fortunately, these events are becoming less likely as the age of the solar system increases. However, the very-long-term future of the universe is not clear. Three Possible Futures The universe seems to have three possible futures. Some scientists propose that the expansion of the universe may be slowing due to gravity. However, it is possible that there is not enough gravity to stop the expansion. In this case, the universe will continue to expand without limit. Other scientists propose that there may be enough gravity to just stop the expansion, leading to a steady state. If the universe has enough gravity to reverse its expanding phase, it could fall back together in a very distant event that some astronomers call the “big crunch.” A good way to understand this is to consider a baseball thrown straight up. Gravity brings the ball back to the ground. However, if you could propel the ball fast enough, it would continue upward into space and never return to Earth. In recent years, astronomers have found evidence that the universe is not only expanding, but that it is expanding at an increasing rate. What force could work against the force of gravity to cause this? It is as if you threw a ball up into the air and it did not fall back to Earth. In fact, it is as if the ball flew upward faster and faster with time. This would be surprising, indeed. Astronomers find these observations just as surprising. Astronomers have named the mysterious cause of this accelerating expansion “dark energy.” However, they cannot explain it. Nor can they explain the source of gravitational force that holds the rapidly spinning galaxies from breaking apart. This force is attributed to the gravitational attraction of “dark matter,” which astronomers think makes up about 90 percent of the matter in the universe. Dark matter and dark energy, the mysteries of science just keep coming.
TERMS TO KNOW Therefore, the ultimate future of the universe depends upon the balance between the expansion of the big bang, gravity, and dark energy. To date, astronomers have not been able to determine which process will dominate. This remains one of many questions that guide scientific investigation. big bang luminosity cosmic background radiation Milky Way Galaxy Doppler effect nuclear fusion galaxy redshift light-year star CHAPTER REVIEW QUESTIONS Base your answers to questions 1–4 on the Earth Science Reference Tables or Figure 28-3. 1. Which star has about the same surface temperature as the sun? (1) Betelgeuse (3) Sirius (2) Polaris (4) Procyon B 2. Which star is cooler, yet many times brighter than Earth’s sun? (1) Barnard’s Star (3) Rigel (2) Betelgeuse (4) Sirius 3. According to the “Luminosity and Temperature of Stars” graph in the Earth Science Reference Tables, the sun is classified as (1) a main sequence star. (3) a blue supergiant. (2) a white dwarf. (4) a red giant. 4. What is the color of a main sequence star that gives off about 100 times as much light as the sun? (1) blue (3) yellow (2) white (4) red CHAPTER REVIEW QUESTIONS 731
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Page 7 and 8: Figure 28-3 When stars are plotted
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Chapter 28 Stars and the Universe

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