2. UNIT NINE: Matter and Motion in the Universe  Chapter 26 The Solar System  Chapter 27 Stars  Chapter 28 Exploring the Universe

4. Chapter Twenty-Eight: Exploring the Universe  28.1 Tools of Astronomers  28.2 Galaxies  28.3 Theories about the Universe

5. Section 28.3 Learning Goals  Explore theories about the origin of the universe.  Discuss the evidence in support of the Big Bang theory.  Describe planetary systems.

6. 28.3 Doppler Shift  In the 1800s, Christian Doppler discovered that when the source of a sound wave is moving, its frequency changes.  The change in sound you hear is caused by a Doppler shift.  Doppler shift is related to the net change between the source of the waves and the observer.

7. 28.3 Doppler Shift  Doppler shift also occurs with electromagnetic waves, such as visible light, X-rays, and microwaves.  This phenomenon is an important tool used by astronomers to study the motion of objects in space.

8. 28.3 The expanding universe  The faster the source of light is moving away from the observer, the greater the redshift.  The opposite (blueshift) happens when an object is moving toward the observer.

9. 28.3 The expanding universe  Edwin Hubble discovered that the farther away a galaxy was, the faster it was moving away from Earth.  This concept came to be known as the expanding universe.

10. 28.3 The Big Bang theory  The theory that the universe was expanding implies the universe must have been smaller in the past than it is today.  It implies that the universe must have had a beginning.  Astronomers today believe the universe exploded outward from a single point.  This idea is known as the Big Bang theory.

12. 28.3 The Big Bang theory  The Big Bang theory says the universe began as a huge explosion between 10 billion and 20 billion years ago.  According to this theory, all matter and energy started in a space smaller than the nucleus of an atom.

13. 28.3 Evidence for the Big Bang theory  In the 1960s, Arno Penzias and Robert Wilson were trying to measure electromagnetic waves given off by the Milky Way.  The “noise” these scientists found was the cosmic microwave background radiation predicted by the Big Bang theory.

14. 28.3 Evidence for the Big Bang theory  The proportion of hydrogen to helium is consistent with the physics of the Big Bang.  If the universe were significantly older, there would be more heavy elements present compared with hydrogen and helium.

15. 28.3 Planetary systems  A star with orbiting planets is called a planetary system.  Scientists now believe that planets are a natural by-product of the formation of stars.

16. 28.3 How the solar system formed  Scientists think that the solar system was formed out of the same nebula that created the Sun.

17. 28.3 Binary stars  A binary star is a system with two stars that are gravitationally tied and orbit each other.  About half of the 60 nearest stars are in binary (or multiple) star systems.

18. Optional* Investigation 28C  Key Question: How long does it take for starlight to reach Earth? The Time Machine

19. Searching the Cosmos  Astrophysicist Dr. Hakeem Oluseyi (Oh-lu-SHAY-ee) is fascinated by stars. A physics and space science professor at the Florida Institute of Technology, he has invented several new instruments to give astronomers a closer look at the cosmos.