1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Sun: Our Very Own Star Bellringer Henry David Thoreau once said, “The sun is but a morning star.” What do you think this quotation means? Chapter J3

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Describe the basic structure and composition of the sun. Explain how the sun generates energy. Describe the surface activity of the sun, and identify how this activity affects Earth. Objectives Section 2 The Sun: Our Very Own Star Chapter J3

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Sun: Our Very Own Star Chapter J3

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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  mc 2 (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. Section 2 The Sun: Our Very Own Star Chapter J3

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 The Sun: Our Very Own Star Chapter J3

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Sunspots Section 2 The Sun: Our Very Own Star Click below to watch the Visual Concept. You may stop the video at any time by pressing the Esc key. Visual Concept Chapter J3

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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.” Section 2 The Sun: Our Very Own Star Chapter J3

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu 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. Section 2 The Sun: Our Very Own Star Chapter J3