Chapter 28 Stars and the Universe

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716 CHAPTER 28: STARS AND THE UNIVERSE Main Sequence Stars When plotted on this graph, most stars fall into distinct groups. The greatest number of stars fall into an elongated group that runs across the luminosity and temperature diagram from the upper left to the lower right. This region of the graph is known as the main sequence. The position of a star along the main sequence is primarily a function of the mass of the star. RED DWARF STARS The smallest stars, such as Barnard’s Star, are red dwarf stars, which are barely large enough to support nuclear fusion. They are red in color because they are relatively cool. These stars are so dim that even the relatively close red dwarfs are difficult to see without a telescope. In fact, about 80 percent of the night stars closest to Earth are too dim to be visible to the unaided eye. This leads astronomers to infer that red dwarf stars are more numerous than all other groups of stars. However, we do not see them because they are so dim. Small stars last longer than larger stars. The lower temperature and pressure in these stars allow them to conserve hydrogen fuel and continue nuclear fusion much longer than larger stars. The combination of small size and slow production of energy makes them very dim. BLUE SUPERGIANT STARS At the other end of the end of the main sequence are the blue supergiants. These massive stars do not last as long as the smaller stars. The extreme conditions of temperature and pressure at the center of these stars cause rapid depletion of their large quantities of hydrogen. Some of them are a million times brighter than the sun. They are also much hotter than the sun, giving them a blue color. These largest stars are not nearly as common as the smaller stars, in part because they burn out quickly. The most massive stars last less than one-thousandth of the life of the sun. Rigel, a bright star in the winter constellation Orion, is 10,000 times as luminous as the sun. The blue color of Rigel
is apparent if you compare it with Betelgeuse, another bright star in Orion. Betelgeuse is a red giant star on the opposite side of the same constellation. Most other stars fall into one of the three groups on the temperature-luminosity chart. White dwarfs, red giants, and the supergiants are the most common star groups outside the main sequence. HOW DO STARS EVOLVE? Different sizes of stars have different life cycles. However, the evolution of stars can be illustrated by considering a star about the size of the sun. Birth of a Star HOW DO STARS EVOLVE? 717 Star formation begins when a cloud of gas and dust (mostly hydrogen) begins to draw together under the influence of gravity. There are two sources of this material. Some of it is hydrogen and helium left over from the formation of the universe about 14 billion years ago. The rest is the debris from the explosions of massive stars that formed earlier in the history of the universe. This initial phase takes place over a period on the order of 50 million years. (The process is faster for larger stars and slower for smaller stars.) As the material draws together, heat from the collapse of the matter and from friction causes the temperature to increase until there is enough heat and pressure to support nuclear fusion. At this time, the star becomes easily visible since it produces and radiates great quantities of energy. The condensation process can be observed with binoculars or a small telescope in the constellation Orion. Several young stars below the belt of Orion can be seen shining through a giant cloud of gas that surrounds them.
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Page 3 and 4: What Would We Answer? If scientists
Page 5 and 6: of energy. However, nuclear fusion
Page 7: Figure 28-3 When stars are plotted
Page 11 and 12: HOW DO ASTRONOMERS STUDY STARS? Sta
Page 13 and 14: Figure 28-5 Objects that do not giv
Page 15 and 16: colors. You may need to rotate the
Page 17 and 18: Sun 100,000 light-years is an addit
Page 19 and 20: Figure 28-9 The sun and nearby gala
Page 21 and 22: theory proposes that at the time of
Page 23 and 24: TERMS TO KNOW Therefore, the ultima
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Page 27: Open-Ended Questions The graph belo

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