1. CHAPTER 22 THE SUN and ITS SOLAR SYSTEM
I. The Sun

3. Topic 1: Studying the Sun
It is dangerous to look mat the sun because it can cause blindness.
The telescope was the instrument (by Galileo) used to make the first scientific observations of the sun.
A solar telescope is a special telescope that projects a large image of the sun into a dark underground room. With special glasses similar to those used by welders, solar physicists can safely watch the sun’s image and observe changes that occur on the sun’s surface.

4. Topic 1: Studying the Sun
(d) Satellites have made it possible to study the sun and its radiation without the interference of Earth’s atmosphere. Orbiting Solar Observatories (OSO-1 through OSO-8) returned data from the sun from Helios A and Helios B were placed in long, oval orbits that carried them inside the orbit of Mercury. The Solar Maximum Satellite (or Solar Max) launched in 1980 functioned until 1989 and Ulysses launched in 1990 will fly over the sun’s pole.

5. Topic 2: Properties of the Sun
Our sun is an average-sized star. Its diameter of km is about 110 times Earth’s diameter. The sun’s volume could hold more than 1 million Earths. Its mass is 745 times greater than all the planets together.
BACKGROUND: For a particle travelling at the speed of light, it would take 8 minutes and 20 seconds (on our clock) to travel between the sun and Earth.

6. Topic 2: Properties of the Sun
(b) Temperatures on the sun are extreme. Its surface temperature is about 5500oC and its interior temperature may be as high as oC.

7. Topic 3: The Sun’s Atmosphere
(a) The sun’s atmosphere is divided into three regions. The photosphere is the apparent yellow surface of the sun. It is about 400 km thick and appears to be made of millions of individual cells, called granules about 1500 km across with a bright center and dark edges. Granules are the tops of the columns of gases that form in the region below the photosphere. The gases are rising at the centre of the granule and sinking back down at the edges. Individual granules last about 8 minutes before sinking.

8. Topic 3: The Sun’s Atmosphere
(a) The photosphere is the lower, denser (2.8 x 10-8 g/cm3) part of the sun’s atmosphere Above the photosphere is the sun’s outer, less dense atmosphere. At the lower part of the outer atmosphere us the chromosphere, colored red by glowing hydrogen. It extends thousands of km above the photosphere.

9. Topic 3: The Sun’s Atmosphere
(a) Above the chromosphere is the corona. It has so little gas that it would be considered a vacuum on Earth. It surrounds the sun to a height of more than 1 million km. It is seen during a total eclipse as a faint, pearly light.

10. Topic 3: The Sun’s Atmosphere
(b) Solar prominences are huge, red, flamelike arches of material that occur in the corona. Their light is the result of changes that occur in cooler, denser parts of the corona. They may last for many hours and some extend millions of km above the photosphere.

11. Topic 4: Sunspots
(a) Sunspots are dark spots on the photosphere. Some are barely visible and others are larger than Earth’s diameter. Some last a few hours while others remain visible for a few months. They typically have a dark centre (umbra) and a lighter rim (penumbra).

12. Topic 3: The Sun’s Atmosphere
(a) Sunspots occur in pairs. One is the a north magnetic pole and the other a south magnetic pole. The concentration of magnetic forces at these locations slows down solar activity and causes the photosphere to cool by as much as 1500oC.

13. Topic 4: Sunspots
(b) Sunspots appear to move left to right across the sun’s surface. This motion is caused by the sun’s rotation. Since the sun is made of gases, its rotation rate is not uniform. At the equator the sun takes about 25 Earth days for one rotation and 27 days at the poles.

14. Topic 4: Sunspots
(c) The number of sunspots visible on the photosphere changes daily. During peak sunspot activity, over 100 may be counted. During periods of low sunspot activity, several days may pass when no spots are visible. These changes occur in a sunspot cycle that averages about 11 years from one period of peak activity to the next.

15. Topic 5: The Solar Wind and Magnetic Storms
(a) Solar wind is the constant stream of electrically charged particles given off by the corona. These particles fly into space in all directions, some at an average speed of 400 km/s by the time they reach Earth. Solar events produce huge gusts of solar wind.

16. Topic 5: The Solar Wind and Magnetic Storms
(b) Great tears, called coronal holes, sometimes appear in the corona. Some extend halfway around the sun, and many do not close for many months. Solar wind pours from coronal holes in a great torent of particles.

17. Topic 5: The Solar Wind and Magnetic Storms
(b) Solar flares are another source of solar wind bursts. They are outbursts of light that arise suddenly in areas of sunspot activity. Most flare up in a few minutes then fade rapidly. The number of solar flares increases as the number of sunspots increase.

18. Topic 5: The Solar Wind and Magnetic Storms
(c) As the solar wind blows past Earth, some particles interact with Earth’s magnetic field and upper atmosphere, causing auroras, or northern and southern lights. Magnetic storms occur on Earth when the particles thrown out by coronal holes and solar flares are added to the constant solar wind produced by the corona.

19. Topic 5: The Solar Wind and Magnetic Storms
(c) At such times auroras may be seen in middle latitudes as well as in polar areas, and compass needles may give inaccurate readings. Electrical surges may disrupt telephone reception and damage unprotected electrical appliances. Radio reception in AM, citizen’s band (CB), and shortwave frequencies may be affected.

20. Topic 6: Source of the Sun’s Energy
(a) The sun is mostly hydrogen. Four hydrogen nuclei have a mass of amu. In fusion 4 hydrogen nuclei join to form a helium nucleus that has a mass of only about amu. This mass difference is converted into electromagnetic radiation which is transmitted into space.

21. Topic 6: Source of the Sun’s Energy
(a) Astronomers calculate that about 4 million metric tons of matter are being changed to energy every second in the sun million metric tons of hydrogen becomes 560 million tons helium.

22. Topic 6: Source of the Sun’s Energy
(b) The mass of the sun is so great that this process can continue for another 5 billion years.