The Sun: Our Very Own Star

1. Chapter 10 Bellringer Henry David Thoreau once said, “The sun is but a morning star.” What do you think this quotation means?

2. Chapter 20 Section 2 The Sun: Our Very Own Star Objectives • Describe the basic structure and composition of the sun. • Explainhow the sun generates energy. • Describe the surface activity of the sun, and identify how this activity affects Earth.

3. Chapter 20 Section 2 The Sun: Our Very Own Star The Structure of the Sun • The sun is basically a large ball of gas made mostly of hydrogen and helium held together by gravity. • Although the sun may appear to have a solid surface, it does not. The visible surface of the sun starts at the point where the gas becomes so thick that you cannot see through it. • The sun is made of several layers, as shown on the next slide.

4. Chapter 20 Section 2 The Sun: Our Very Own Star

5. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun • The sun has been shining on the Earth for about 4.6 billion years. Many scientists once thought that the sun burned fuel to generate its energy. • The amount of energy that is released by burning would not be enough to power the sun. If the sun were simply burning, it would last for only 10,000 years.

6. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • Burning of Shrinking? Scientists later began thinking that gravity was causing the sun to slowly shrink and that gravity would release enough energy to heat the sun. • While the release of gravitational energy is more powerful than burning, it is not enough to power the sun. If all of the sun’s gravitational energy were released, the sun would last only 45 million years.

7. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • Nuclear Fusion Albert Einstein showed that matter and energy are interchangeable. Matter can change into energy according to his famous formula: • E  mc2 • (E is energy, m is mass, and c is the speed of light.) • Because c is such a large number, tiny amounts of matter can produce a huge amount of energy.

8. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • The process by which two or more low-mass nuclei join together, or fuse, to form another nucleus is called nuclear fusion. • In this way, four hydrogen nuclei can fuse to form a single nucleus of helium. During the process, energy is produced. • Scientists now know that the sun gets its energy from nuclear fusion.

9. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • Fusion in the Sun During fusion, under normal conditions, the nuclei of hydrogen atoms never get close enough to combine. • The reason is that the nuclei are positively charged, and like charges repel each other, just as similar poles on a pair of magnets do.

10. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • In the center of the sun, however, temperature and pressure are very high. • As a result, hydrogen nuclei have enough energy to overcome the repulsive force, and hydrogen fuses into helium, as shown on the next slide.

11. Chapter 20 Section 2 The Sun: Our Very Own Star

12. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • Energy produced in the center, or core, of the sun takes millions of years to reach the sun’s surface. • Energy passes from the core through a very dense region called the radiative zone. The matter in the radiative zone is so crowded that light and energy are blocked and sent in different directions.

13. Chapter 20 Section 2 The Sun: Our Very Own Star Energy Production in the Sun, continued • Eventually, energy reaches the convective zone. Gases circulate in the convective zone, which is about 200,000 km thick. • Hot gases in the convective zone carry the energy up to the photosphere, the visible surface of the sun. • From the photosphere, energy leaves the sun as light, which takes only 8.3 minutes to reach Earth.

14. Chapter 20 Section 2 The Sun: Our Very Own Star Solar Activity • The churning of hot gases in the sun, combined with the sun’s rotation, creates magnetic fields that reach far out into space. • The constant flow of magnetic fields from the sun is called the solar wind. • Sometimes, solar wind interferes with the Earth’s magnetic field. This type of solar storm can disrupt TV signals and damage satellites.

15. Chapter 20 Section 2 The Sun: Our Very Own Star Solar Activity, continued • Sunspots The sun’s magnetic fields tend to slow down activity in the convective zone. When activity slows down, areas of the photosphere become cooler than the surrounding area. • These cooler areas show up as sunspots. Sunspots are cooler, dark spots of the photosphere of the sun. Some sunspots can be as large as 50,000 miles in diameter.

18. Chapter 20 Section 2 The Sun: Our Very Own Star Solar Activity, continued • Climate Confusion Sunspot activity can affect the Earth. Some scientists have linked the period of low sunspot activity, 1645-1715, with a period of very low temperatures that Europe experienced during that time, known as he “Little Ice Age.”

19. Chapter 20 Section 2 The Sun: Our Very Own Star Solar Activity, continued • Solar Flares The magnetic fields responsible for sunspots also cause solar flares. Solar flares are regions of extremely high temperatures and bright-ness that develop on the sun’s surface. • When a solar flare erupts, it sends huge streams of electrically charged particles into the solar system. Solar flares can interrupt radio communications on the Earth and in orbit.

20. Chapter 20 Section 4 Planetary Motion Bellringer A mnemonic device is a phrase, rhyme, or anything that helps you remember a fact. Create a mnemonic device that will help you differentiate between planetary rotation and revolution. Record your mnemonic device in your science journal.

21. Chapter 20 Section 4 Planetary Motion Objectives • Explainthe difference between rotation and revolution. • Describe three laws of planetary motion. • Describehow distance and mass affect gravitational attraction.

22. Chapter 20 Section 4 Planetary Motion A Revolution in Astronomy • Each planet spins on its axis. The spinning of a body, such a planet, on its axis is called rotation. • The orbit is the path that a body follows as it travels around another body in space. • A revolution is one complete trip along an orbit.

23. Chapter 20 Section 4 Planetary Motion Earth’s Rotation and Revolution

24. End of Chapter 20