1. Structure of the Sun 24.3 The Sun  Because the sun is made of gas, no sharp boundaries exist between its various layers. Keeping this in mind, we can divide the sun into four parts: the solar interior; the visible surface, or photosphere; and two atmospheric layers, the chromosphere and corona.

2. Structure of the Sun 24.3 The Sun  Photosphere The photosphere is the region of the sun that radiates energy to space, or the visible surface of the sun. It exhibits a grainy texture made up of many small, bright markings, called granules, produced by convection. Most of the elements found on Earth also occur on the sun. Its temperature averages approximately 6000 K (10,000ºF).

3. Structure of the Sun

4. 24.3 The Sun  Chromosphere The chromosphere is the first layer of the solar atmosphere found directly above the photosphere. It is a relatively thin, hot layer of incandescent gases a few thousand kilometers thick. Its top contains numerous spicules, which are narrow jets of rising material.

5. Chromosphere

6. Structure of the Sun 24.3 The Sun  Corona The corona is the outer, weak layer of the solar atmosphere. The temperature at the top of the corona exceeds 1 million K. Solar wind is a stream of protons and electrons ejected at high speeds from the solar corona.

7. 24.3 The Sun  Sunspots Sunspots appear dark because of their temperature, which is about 1500 K less than that of the surrounding solar surface. A sunspot is a dark spot on the sun that is cool in contrast to the surrounding photosphere.

8. Sunspots

9. 24.3 The Sun  Solar Flares During their existence, solar flares release enormous amounts of energy, much of it in the form of ultraviolet, radio, and X-ray radiation. Solar flares are brief outbursts that normally last about an hour and appear as a sudden brightening of the region above a sunspot cluster. Auroras, the result of solar flares, are bright displays of ever-changing light caused by solar radiation interacting with the upper atmosphere in the region of the poles.

10. Aurora Borealis

11. The Solar Interior 24.3 The Sun  Nuclear Fusion During nuclear fusion, energy is released because some matter is actually converted to energy. Nuclear fusion is the way that the sun produces energy. This reaction converts four hydrogen nuclei into the nucleus of a helium atom, releasing a tremendous amount of energy. It is thought that a star the size of the sun can exist in its present stable state for 10 billion years.

12. Energy Transfer as Heat 17.2 Heating the Atmosphere  Heat is the energy transferred from one object to another because of a difference in the objects’ temperature.  Temperature is a measure of the average kinetic energy of the individual atoms or molecules in a substance.

13. Energy Transfer as Heat 17.2 Heating the Atmosphere  Three mechanisms of energy transfer as heat are conduction, convection, and radiation. Conduction is the transfer of heat through matter by molecular activity.  Conduction Convection is the transfer of heat by mass movement or circulation within a substance.  Convection

14. Energy Transfer as Heat

15. 17.2 Heating the Atmosphere  Electromagnetic Waves The sun emits light and heat as well as the ultraviolet rays that cause a suntan. These forms of energy are only part of a large array of energy emitted by the sun, called the electromagnetic spectrum.

16. Electromagnetic Radiation 24.1 The Study of Light  Electromagnetic radiation includes gamma rays, X-rays, ultraviolet light, visible light, infrared radiation, microwaves, and radio waves.  The electromagnetic spectrum is the arrangement of electromagnetic radiation according to wavelength.

17. Electromagnetic Spectrum

19. Visible Light Consists of an Array of Colors

20. Energy Transfer as Heat 17.2 Heating the Atmosphere  Radiation Radiation is the transfer of energy (heat) through space by electromagnetic waves that travel out in all directions. Unlike conduction and convection, which need material to travel through, radiant energy can travel through the vacuum of space.

21. Energy Transfer as Heat 17.2 Heating the Atmosphere  Radiation All objects, at any temperature, emit radiant energy. Hotter objects radiate more total energy per unit area than colder objects do. Objects that are good absorbers of radiation are good emitters as well.

22. What Happens to Solar Radiation? 17.2 Heating the Atmosphere  When radiation strikes an object, there usually are three different results. 1. Some energy is absorbed by the object. 2. Substances, such as water and air, are transparent to certain wavelengths of radiation. 3. Some radiation may bounce off the object without being absorbed or transmitted.

23. Solar Radiation

24. What Happens to Solar Radiation? 17.2 Heating the Atmosphere  Reflection and Scattering Reflection occurs when light bounces off an object. Reflection radiation has the same intensity as incident radiation. Scattering produces a larger number of weaker rays that travel in different directions.

25. What Happens to Solar Radiation? 17.2 Heating the Atmosphere  Absorption About 50 percent of the solar energy that strikes the top of the atmosphere reaches Earth’s surface and is absorbed. The greenhouse effect is the heating of Earth’s surface and atmosphere from solar radiation being absorbed and emitted by the atmosphere, mainly by water vapor and carbon dioxide.

26. What Happens to Solar Radiation?

27. Why Temperatures Vary 17.3 Temperature Controls  Land and Water Land heats more rapidly and to higher temperatures than water. Land also cools more rapidly and to lower temperatures than water.