1. Astronomy Ch 20 The Universe What is the universe? What is the universe? Sum of all space, matter, and energy past, present, future http://hubblesite.org/explore_astrono my/hubbles_universe_unfiltered/ http://hubblesite.org/explore_astrono my/hubbles_universe_unfiltered/ http://hubblesite.org/explore_astrono my/hubbles_universe_unfiltered/ http://hubblesite.org/explore_astrono my/hubbles_universe_unfiltered/ Where do we live in the universe: Our Solar System, Milky Way Galaxy, Local Group Cluster, Virgo Super Cluster

2. Astronomy Ch 20 The Universe Section 1: The Life and Death of Stars Section 1: The Life and Death of Stars What Are Stars? Stars are huge spheres of very hot gas that emit light and other radiation. Stars are formed from clouds of dust and gas, or nebulas, and go through different stages as they age. (p. 693) What Are Stars? Stars are huge spheres of very hot gas that emit light and other radiation. Stars are formed from clouds of dust and gas, or nebulas, and go through different stages as they age. (p. 693)

3. Stars have influenced cultures for thousands of years by helping people mark the passage of time and by providing markers for navigation in the night sky. Stars have influenced cultures for thousands of years by helping people mark the passage of time and by providing markers for navigation in the night sky.

4. Stars are born in a nebula (left). After billions of years, most stars become old and lose their outer layers of gas. As a star begins to die, it may become a planetary nebula (right). Stars are born in a nebula (left). After billions of years, most stars become old and lose their outer layers of gas. As a star begins to die, it may become a planetary nebula (right).

5. light-year Stars are located at various distances from Earth. We use the light-year (ly) to describe the distance from Earth to far-away objects such as stars and galaxies. A light-year is the distance that light travels in one year, or about 9.5 × 1012 km. Stars are located at various distances from Earth. We use the light-year (ly) to describe the distance from Earth to far-away objects such as stars and galaxies. A light-year is the distance that light travels in one year, or about 9.5 × 1012 km.

6. Stars are powered by nuclear fusion reactions. A star is a huge sphere of very hot hydrogen and helium gas that emits light. A star is held together by the enormous gravitational forces that result from its own mass. Inside the core of a star, the pressure is more than a billion times the atmospheric pressure on Earth. The temperature in this incredibly dense core is hotter than 15 million kelvins. A star is a huge sphere of very hot hydrogen and helium gas that emits light. A star is held together by the enormous gravitational forces that result from its own mass. Inside the core of a star, the pressure is more than a billion times the atmospheric pressure on Earth. The temperature in this incredibly dense core is hotter than 15 million kelvins.

7. Energy from a nuclear fusion reaction in the core may take tens of thousands of years to reach a star’s surface. When the energy reaches the surface, the energy is released into space as electromagnetic radiation

8. Studying Stars The telescope allowed astronomers to study stars in more detail for the first time. Greeks noticed that stars had color and divided stars by their apparent brightness apparent Astronomers did not learn about the nature of stars until the optical telescope was invented. Astronomers did not learn about the nature of stars until the optical telescope was invented. As science and technology have improved, telescopes have become more powerful and have thus allowed us to see more. As science and technology have improved, telescopes have become more powerful and have thus allowed us to see more.

9. Some stars appear brighter than others. Some stars appear brighter than others. The brightness of a star depends on the star’s temperature, size, and distance from Earth. The brightness of a star depends on the star’s temperature, size, and distance from Earth. The brightest star in the night sky is Sirius in the constellation Canis Major. Sirius appears so bright because it is close to Earth, only about 8.7 light-years away. The surface temperature of Sirius is about 10,000 K. The brightest star in the night sky is Sirius in the constellation Canis Major. Sirius appears so bright because it is close to Earth, only about 8.7 light-years away. The surface temperature of Sirius is about 10,000 K. The sun’s surface is only 6,000 K, but the sun is so close to Earth that it prevents us from seeing other stars during the day. The sun’s surface is only 6,000 K, but the sun is so close to Earth that it prevents us from seeing other stars during the day.

10. We learn about stars by studying energy. Stars produce various types of electromagnetic radiation: from visible light to X rays to radio waves. Scientists use optical telescopes to study visible light and radio telescopes to study radio waves emitted from astronomical objects. Earth’s atmosphere blocks other wavelengths, so telescopes in space are used to study a wider range of the electromagnetic spectrum than telescopes on the ground can detect.

11. Section 1: The Life and Death of Stars The Life Cycle of Stars In a way that is similar to other natural cycles, stars are born, go through stages of development, and eventually die. (p. 698) The Life Cycle of Stars In a way that is similar to other natural cycles, stars are born, go through stages of development, and eventually die. (p. 698)

12. This graph shows the intensity of light at different wavelengths for the sun and for two other stars.

13. The color spectra of most stars contain dark lines. Dark lines are caused by gases in the outer layers of the stars that absorb the light at these wavelengths. Astronomers match the dark lines in starlight to the known lines of elements found on Earth to determine what elements make up a star..

14. Stars change in many ways over their life cycle. Small and medium stars are born in giant nebulas of dust and gas and often end up as white dwarfs billions of years later. The images in this life cycle are not shown to scale.

15. Massive stars supernova -If the core that remains after such a supernova has occurred has enough mass, the remnant can become a supernova -If the core that remains after such a supernova has occurred has enough mass, the remnant can become a neutron star. Neutron stars are small in diameter, but they are very massive. Just a teaspoon of matter from a neutron star would weigh more than 100 million tons on Earth. Neutron stars can be detected as neutron star. Neutron stars are small in diameter, but they are very massive. Just a teaspoon of matter from a neutron star would weigh more than 100 million tons on Earth. Neutron stars can be detected as pulsars, or spinning neutron stars that pulsate radio waves.If the leftover core is great enough, it will collapse to form something else—a pulsars, or spinning neutron stars that pulsate radio waves.If the leftover core is great enough, it will collapse to form something else—a black hole, which consists of matter so massive and compressed that nothing, not even light, can escape its gravitational pull. Because no light can escape a black hole, black holes cannot be seen directly. Black holes can, however, be detected indirectly by observing the radiation of light and X rays from objects that revolve rapidly around them. black hole, which consists of matter so massive and compressed that nothing, not even light, can escape its gravitational pull. Because no light can escape a black hole, black holes cannot be seen directly. Black holes can, however, be detected indirectly by observing the radiation of light and X rays from objects that revolve rapidly around them.

16. Section 2: The Milky Way and Other Galaxies Galaxies While the nearest stars are a few light-years away from Earth, the nearest galaxy to our own is millions of light-years from Earth. A galaxy is a collection of millions to billions of stars. The deeper scientists look into space, the more galaxies they find. There may be more than 100 billion galaxies. If you counted 1,000 galaxies per night, it would take 275,000 years to count all of them.

17. Gravity holds galaxies together. Without gravity, everything in space might be a veil of gas spread out through space. But because of gravity, clouds of gas come together and collapse to form stars. As the first stars in a galaxy age, they throw off gas and dust or become supernovas. New stars then form. The gas, dust, and stars form galaxies because of gravity. Just as Earth revolves around the sun because of gravity, the solar system revolves around the center of the galaxy because of gravity. It takes our solar system about 226 million years to complete one orbit of our galaxy.

18. Galaxies are often found together in clusters. Galaxies are not spread out evenly through space. Galaxies are grouped together and bound by gravity in clusters like the cluster shown in Figure 3. The Milky Way galaxy and the Andromeda galaxy are two of the largest members of the Local Group, a cluster of more than 30 galaxies. New members of the Local Group, are being discovered as more telescopes, such as the Hubble Space Telescope that is shown in Figure 2, become available. Clusters of galaxies can form even larger groups called superclusters. A typical supercluster contains thousands of galaxies that contain trillions of stars in individual clusters. Superclusters can be as large as 100 million light-years across. They are the largest known structures in the universe.

19. Section 2: The Milky Way and Other Galaxies Types of Galaxies Galaxies can be divided into three major types: spiral, elliptical, and irregular. The three types of galaxies have many stars, but differ in structure. We live in the Milky Way galaxy.

20. The Milky Way is a spiral galaxy. Our galaxy is a huge spiraling disk of stars, gas, and dust. This gas and dust is called interstellar matter The Milky Way has a huge bulge in the center that contains primarily old red stars The spiral arms contain young blue stars

21. Section 2: The Milky Way and Other Galaxies How Galaxies Change Over Time By studying closer galaxies that might be similar to ancient ones, scientists can slowly piece together the puzzle of how galaxies evolve. (p. 706)

22. Section 3: Origin of the Universe What Is the Universe? The universe consists of all space, matter, and energy that exists—now, in the past, or in the future. (p. 708) What Is the Universe? The universe consists of all space, matter, and energy that exists—now, in the past, or in the future. (p. 708)

23. We see the universe now as it was in the past. We see the universe now as it was in the past. Astronomers need large units of measure to express distances. A light-year, which is approximately 9.5 × 10 12 km, Remember that while a year is a unit of time, a light-year is a unit of distance. Astronomers need large units of measure to express distances. A light-year, which is approximately 9.5 × 10 12 km, Remember that while a year is a unit of time, a light-year is a unit of distance. It takes time for light to travel in space. When we say the sun is 8 light-minutes away, we are expressing not only its distance from Earth, but also that we see the sun as it was 8 min ago. We never see it as it is in the present. It takes time for light to travel in space. When we say the sun is 8 light-minutes away, we are expressing not only its distance from Earth, but also that we see the sun as it was 8 min ago. We never see it as it is in the present. When we see distant objects, we see them as they were when they were younger. When we see distant objects, we see them as they were when they were younger. Astronomers can compare how galaxies age by looking at many galaxies at various distances and thus at different ages. Astronomers can compare how galaxies age by looking at many galaxies at various distances and thus at different ages.

24. Section 3: Origin of the Universe What Happened at the Beginning? Scientists theorize that the universe formed during a cataclysmic event known as the big bang. (p. 710) What Happened at the Beginning? Scientists theorize that the universe formed during a cataclysmic event known as the big bang. (p. 710) scientists estimate that the universe is about 13.7 billion years old.

25. In 1965, scientists Arno Penzias and Robert Wilson were making adjustments to a new radio antenna that they had built. In 1965, scientists Arno Penzias and Robert Wilson were making adjustments to a new radio antenna that they had built. They could not explain a steady but very dim signal from all over the sky in the form of radiation at microwave wavelengths. They finally realized that the signal was the cosmic background radiation predicted by the big bang theory. They could not explain a steady but very dim signal from all over the sky in the form of radiation at microwave wavelengths. They finally realized that the signal was the cosmic background radiation predicted by the big bang theory.

26. Radiation dominated the early universe. Radiation dominated the early universe. According to the big bang theory, immediately after the big bang, the universe was extremely hot and made up of pure energy. There was a period of rapid expansion that caused the energy to cool and allowed sub-atomic particles, such as protons, electrons, and neutrons, to form. The first stars were born about 400 million years after the big bang. According to the big bang theory, immediately after the big bang, the universe was extremely hot and made up of pure energy. There was a period of rapid expansion that caused the energy to cool and allowed sub-atomic particles, such as protons, electrons, and neutrons, to form. The first stars were born about 400 million years after the big bang.

27. Expansion implies that the universe was once smaller. Expansion implies that the universe was once smaller. Most galaxies are moving away from each other. Most galaxies are moving away from each other. Long ago, the entire universe might have been contained in an extremely small space. Long ago, the entire universe might have been contained in an extremely small space.

28. During the past 100 years, other theories for the origin of the universe have been proposed. A few are still being studied, but the big bang theory is the one that is best supported by the current evidence including the cosmic background radiation and observations of the movement of distant galaxies During the past 100 years, other theories for the origin of the universe have been proposed. A few are still being studied, but the big bang theory is the one that is best supported by the current evidence including the cosmic background radiation and observations of the movement of distant galaxies

29. Section 3: Origin of the Universe Predicting the Future of the Universe Scientists use their increasing knowledge of the universe to hypothesize what might happen to the universe in the future. (p. 715) Predicting the Future of the Universe Scientists use their increasing knowledge of the universe to hypothesize what might happen to the universe in the future. (p. 715)

30. They depend on a mixture of theories and precise observations of very faint and distant objects. They depend on a mixture of theories and precise observations of very faint and distant objects. These observations depend on technology, such as telescopes These observations depend on technology, such as telescopes New space telescopes that collect infrared radiation and X rays are being built and launched. New space telescopes that collect infrared radiation and X rays are being built and launched. Data in these regions of the electromagnetic spectrum may provide important clues about the beginning and future of the universe. Data in these regions of the electromagnetic spectrum may provide important clues about the beginning and future of the universe.

31. One example of new, more sensitive technology is the Chandra X-Ray Observatory, which was launched into orbit around Earth in 1999. One example of new, more sensitive technology is the Chandra X-Ray Observatory, which was launched into orbit around Earth in 1999. This observatory can take photographs in the X-ray part of the electromagnetic spectrum. The presence of X rays indicates matter at temperatures of more than one million degrees. This observatory can take photographs in the X-ray part of the electromagnetic spectrum. The presence of X rays indicates matter at temperatures of more than one million degrees.

32. According to Einstein’s theory of relativity, mass curves space, much in the same way that your body curves a mattress when you sit on it. According to Einstein’s theory of relativity, mass curves space, much in the same way that your body curves a mattress when you sit on it. In 1919, observations of a total solar eclipse showed that Einstein was correct. In the direction of the sun, stars that could be seen only during the eclipse were in slightly different positions than expected. In 1919, observations of a total solar eclipse showed that Einstein was correct. In the direction of the sun, stars that could be seen only during the eclipse were in slightly different positions than expected.

33. The future of the universe is uncertain. The future of the universe is uncertain. The universe is still expanding, but it may not do so forever. The combined gravity of all of the mass in the universe is also pulling the universe inward, in the direction opposite to the expansion. The competition between these two forces leaves three possible outcomes for the universe: The universe is still expanding, but it may not do so forever. The combined gravity of all of the mass in the universe is also pulling the universe inward, in the direction opposite to the expansion. The competition between these two forces leaves three possible outcomes for the universe: 1.The universe will keep expanding forever. 2.The expansion of the universe will gradually slow down, and the universe will approach a limit in size. 3.The universe will stop expanding and start to fall back on itself.