History of Sun

THE SUN

Less than 5 billion years ago, in a distant spiral arm of our galaxy, called the Milky Way, a small cloud of gas and dust began to compress under its own weight. Particles within the cloud's center (core) became so densely packed that they often collided and stuck (fused) together. The fusion process released tremendous amounts of heat and light which could then combat the compressing for ce of gravity; eventually, the two forces reached equillibrium. The balance of fusion reactions versus gravitational collapse which occurred in this little cloud is fondly refe rred to as a star, and this story is about the birth and life of the closest star to Earth, the Sun.Our Sun is one of at least four hundred billion stars in the Milky Way galaxy, and it lives 8 kiloparsecs (2.5 billion billion billion miles) from the center of the galaxy. All stars in our galaxy and other galaxies come in different sizes and colors, and our sun is a medium sized star known as a yellow dwarf. The cloud from which it formed, fortunately for us, did not use all of its gas and dust to make the Sun; that which was left over, less than one percent of the original material, formed the 9 planets.
The Sun has been fusing hydrogen into helium and hence providing us with its rad iant energy for 4.5 billion years, and it is expected to continue to do so for another 3 to 4 billion years more. And then what? As the sun gets older, it will fuse more and more hydrogen in its core. Once all of the hydrogen is turned into helium, the star stops fusing hydrogen and loses its abi lity to combat gravity. Then gravity begins to compress the Sun under its own weight again. The introduction of more compression causes the new helium particles inside of the core to collide hard enough so that they can stick together and fuse. The core thus begi ns to fuse helium into carbon to make enough energy to maintain its balance with the crushing force of gravity. The making of carbon, however, gives off more energy than did the making of helium. The energy being pumped out of the core radiates through the outer layers of the sun called the envelope. The introduction of too much energy into the envelope heats up the envelope particles so much that the envelope expands (for the same reasons that steam rises). At this point in its life, the Sun's envelope will expand to engulf all of the inner solar system out to Mars. The temperature will drop in the envelope as well, as the particles become so spread out that they no longer are colliding enough to create tremendous heat. A drop in temperature in a star can b e seen in the change in the color of a star; cooler stars are redder than hotter, bluer stars. Thus, at this stage of its life, the Sun will be called a red giant.
When the envelope expands too far away from the Sun's core, the envelope will begin to float off of the core and into space. This floated-off envelope material is known as a planetary nebula. Since the bulk of the Sun is envelope material, when this material floats off, gravity does not work as hard to crush the remaining core, and the core stops fusing. The particles of carbon in the core are still very densely packed, however, and so the core is very hot, but tiny -- about the size of the Earth. This leftover hot and tiny core will be called a white dwarf.
But for now, the Sun maintains itself as a yellow dwarf star, giving off radiation in all wavelengths of light including light we can and cannot see. It is the largest object in the solar system, yet is one of hundreds of billions of stars in our enormous galaxy.

Reference Table of Stats
	Mass	Diameter	Distance from Earth	Temperature(Surface) (Core)
	2x10^30kg	1,390,000km	149,600,000km	5770K 15,000,000K

brief history of the Sun which we included in these pages gives you a comprehensive and perhaps complicated introduction to the Sun as a star in our galaxy, the Milky Way. Stars are very different objects than are planets. They are very large and are giving off immense amounts of radiation. As we pointed out on the Sun history page, most of the stuff in the Solar System is in the Sun, leaving very little for the planets, moons, and us! It is difficult to imagine how large a star is without seeing it up close; however, we can do a quick experiment to calculate the size of the closest star to us, our Sun.


ACTIVITY: How big is the Sun?
This activity requires a sunny day and a cheerful disposition towards math!
Be sure to remind students that looking at the Sun can cause permanent eye damage -- never look at the Sun directly!
Materials: 8 1/2 x 11 inch sheets of paper, yard(meter)sticks, tape, pencils, index cards, straight pin

1. On a sheet of paper, draw two parallel lines very very very close together, about a pencil lead width. Measure the distance between them and record it.

2. Prick a tiny hole in the center of an index card, fold over one end, and tape that to the top (at 0 mark) of a yard or meter stick.

3. Hold the paper out underneath the card with one edge resting up against the yard (meter) stick., and the shadow of the card falling squarely on the paper. Slide the paper up or down along the edge of the stick so that the sunlight coming through the hole in the index card just fills the space between the little parallel lines drawn on the paper.

4. Record where on the yard (meter) stick the edge on which the paper was aligned when the pin-prick of light fit inside the parallel lines (should be around 8.5 inches or 218 mm).Now some quick relations involving space and math! Ask students if they have ever made scale models of planes, boats, people...How does scale modeling work? Students should understand the idea of ratios, factors of difference between two sizes. In the case of the Sun, we know that the the distance between the two lines, or the size of the little light spot, was recorded by the students in the first step and should be around 2mm. We also know that the students recorded how far away from the source of the light (the prick hole) the spot is by the number they recorded for the distance they needed to place the paper from the prick hole to make the spot fill the lines. So, they should then have a 2mm spot which is around 218mm away from the source hole. If we wanted to move the paper further away, could we guess by how much the spot would grow? What if the paper were over 93 million miles away?!
The ratio between the distance to the spot and the width of the spot is the value of the recorded distance divided by the recorded spot size. Students should use their own recorded values, but for this explanation, we will use the ones we guessed above.
Distance to the spot is 218mm. Size of the spot is 2mm. So, 218 divided by 2mm is 109. The number 109 is the ratio of the two measurements and will be our guide in determining the size of the Sun. Since we are using the sunlight as a guide, and we have found a direct relationship (ratio) between the distance of the light and the size of the light spot, we can make direct conclusions about the Sun itself. (Similarly, if we were using a flashlight, we could make determinations about it). If the distance of the Sun is around 93 million miles, then the size of the Sun will be found like so: Distance to Sun is 93 million miles (93,750,000 to be exact). Size of the Sun is unknown, but the ratio from the observations is 109. So, 93,750,000 miles divided by size of the Sun is 109. Another way to say that is 93,750,000 miles divided by 109 is the size of the Sun. Ha ha! So...

5. Students find their ratio from dividing the distance they moved the paper from the index card by the size of the little sunlight spot. They then can use the ratio relation described above to find out the size of the Sun!



DISCUSSION:Is the size of the Sun surprising when students consider the size of the Earth is 8,000 miles across? Is the Sun really anything similar to the Earth? Was the ratio relation easy to understand and use? Did students find the math difficult or powerful? How could students use the ratio relation to measure other things? What about the size of the Moon? Try holding a pencil out at arm's length towards the full moon. The pencil width should just wink out all of the moon's face. If you know the width of the pencil and the distance between it and your eyes, now you can find the ratio! Divide the distance to the moon (250,000 miles) by this ratio, and you have found the size of the moon!

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The Sun and the SeasonsIf you ask students what a particular season means to them, they'll probably mention the weather usually associated with it. Summer is hot. Winter is cold. Spring and Fall are in between. (For those of us living close enough to the poles, winter means snow, too!) A common misconception (even among a disturbingly large number of college graduates) is that the Earth is closer to the Sun during the summer, causing summer's warmer temperatures. This model, of course, is inconsistent with the fact that while the Northern Hemisphere has summer, the Southern Hemisphere has winter. Furthermore, while it is summer for the Northern Hemisphere, the Earth is actually slightly farther away from the Sun than during the Northern Hemisphere winter.
During the Northern Hemisphere's summer, the North Pole is tilted towards the Sun. During the winter, it is tilted away. This tilt causes the Sun to appear higher in the sky during the summer than during the winter. The higher Sun causes more hours of daylight and more intense, direct sunlight, or hotter conditions on the surface of the Earth. Questions to ask the class include: How is summer different from winter? What changes as winter gives way to spring? What changes are there as summer becomes fall? What about when winter approaches?
It is important to note that even without the tilt of the Earth, there would still be variations in temperature from one location to another, caused mainly by the curvature of the earth. Locations closer to the equator would still, on the average, be warmer than locations closer to the poles. Light and heat (radiation) from the Sun would still strike polar regions at more of an angle than nearer the Equator. This angle tends to "spread out" the same amount of energy over a larger area, thereby decreasing its intensity and the amount of heat it brings to the Earth. The activities addressing this topic demonstrate and test this assertion.

ACTIVITY: Energy from the SunBe sure to remind students that looking at the Sun can cause permanent eye damage -- never look at the Sun directly!
Solar radiation is emitted in various forms which travel at the speed of light. Light travels through space as waves of different lengths. Our eyes can only see radiation as visible light, but radiation also occurs as radio waves, infrared rays, ultraviolet rays, X- rays and gamma rays. Together these waves make up the electromagnetic spectrum. The sunlight reaches the Earth mostly as visible, infrared, and radio light; the upper atmosphere blocks most of the other wavelengths. When the light reaches the ground it is absorbed into the grass, rocks, people, soil, etc. and becomes heat energy. How else could we "feel" sunlight if it did not have the ability to change into heat?
Questions to ask: How much energy is the sunlight creating on the Earth? How can we measure the warmth from the Sun? What might influence fluctuations in warmth? What experiences have students had which help in these predications? Will soil be warmer or colder than air temperature? Does this change during the day? During the year?

Materials: thermometers; soil; sunlight; index cards; stick; pencils

1. Students (in small groups) choose an appropriate spot to insert a thermometer(s) in the earth. With a stick (not the thermometer!) make a narrow hole in the soil and carefully insert the bulbous end of the thermometer. With an index card, make and place a sign signifying experiment areas. Students should predict a temperature for the soil.

2. Return at various intervals to read and record the time and temperature. Read and record the temperature of the air at the same time as the recordings of the earth temperature and compare these.

DISCUSSIONStudents discuss results of these experiments. What caused the highest or hottest temperature? Was this related to the time of day? What were students able to discover from their measurements? How accurate were their predictions? What factors helped them to predict well? What conclusions can they make about the effect of the Sun's rays on the Earth? If done at intervals over a period of time, did earlier experiments help their predictions? What would happen if the experiment were done in a couple of months?


ACTIVITY: How Angle Spreads SunlightBe sure to remind students that looking at the Sun can cause permanent eye damage -- never look at the Sun directly!
The previous activity allowed students to observe the energy of the sunlight. However, since the angle of the sunlight influences the amount of energy it creates, it is important for students to observe this effect to understand why the winter on the Earth is colder than the summer is. This activity shows how light falling upon a tilted surface is less intense than if it were falling directly, demonstrating this "spreading out" of light by measuring how quickly and by how much the sunlight can warm two sheets of paper: one tilted, one not.

Materials: Two sheets of black construction paper; two pieces of cardboard or plywood; bricks or blocks to prop up board; masking tape; two thermometers.

1. Cut an inch-wide slit in the middle of each piece of black construction paper. Tape one of these sheets of black construction paper to each of the cardboard or plywood boards. Place a thermometer into each slit such that the bulb is between the board and the paper, and the scale can be read without removing the thermometer. Tape the thermometers in place. Leave the assembled thermometers (we'll call them "contraptions"!) in the shade long enough so that they read the same outside temperature.

2. Bring the contraptions out into the Sun. Tilt one up so that it faces the Sun and the Sun's rays fall nearly perpendicular to the board. The other one should lie flat on the ground, or even tilted slightly backwards from the Sun if the Sun is especially high in the sky.

3. Periodically (every minute or so) record the temperature on each thermometer until the temperatures level off and stop climbing. Beware the effects of clouds and wind, as well as the shadows of over-anxious students! Let the thermometers sit for a few minutes. Record their final temperatures.

DISCUSSIONWhich paper's temperature rose more quickly? What was the angle of the Sun when the temperature was the highest for each thermometer? Can the students make a relation between the angle of the sunlight and the affect on the Earth from seeing the affect on the tilted paper? How high is the Sun at noon in the summer versus how high the Sun is at noon in the winter? Try to make a connection between the tilt of the Earth in the summertime and the summer temperatures as well as the tilt of the Earth in the wintertime and the winter temperatures. What if the Earth was not tilted? Would there be any seasons?