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Overview
You step outside on a crisp, clear night and gaze up at a sky full of sparkling stars. You wonder which one to wish upon--they're all beautiful, but so far away. Why not try wishing upon the star that's closest to our own planet Earth, the one we see almost every day, the one that provides the light and heat we need to survive? Our sun is just one of the 100 billion or so stars in our galaxy, and there are billions of other galaxies in the universe. It may seem like the sun is close to us, but it's about 150 million kilometers (93 million miles) away. It's bigger than anything we can imagine--about 1.4 million kilometers (870,000 miles) in diameter. A million Earths could fit inside it! The sun may be only one among billions of other stars in the universe, but it's the one that makes our life on Earth possible. How? By providing energy in many forms--solar power, fossil fuels, wave power, wind power. Without heat and light from the sun, Earth would be just another dark, cold place in space where life as we know it couldn't exist. Where does the sun get all this energy? The sun's mass is approximately 300,000 times more than Earth's, and the greater an object's mass, the greater the pressure at its center. Charles's law tells us that when you squeeze--or compress--a gas, it gets hot. Most of the sun's mass is composed of hydrogen gas atoms, and about 100 years ago, physicists came up with the hypothesis that the sun's tremendous mass squeezed the hydrogen atoms until they ignited, releasing heat and light energy that eventually made it through space to us. Based on calculations of the mass of the sun, they figured that the sun would burn itself out in 6,000 years. Evolutionary biologists and geologists knew from their own studies that life on Earth had been around much longer than 6,000 years, so the research continued. Decades later, a new hypothesis arose. Think about the hottest oven you can imagine, then turn up the temperature to about 25,000,000*F. That's how hot it gets in the center of the sun. At that temperature the hydrogen nuclei are moving so fast that when they crash into each other they stick together to form helium nuclei. The "fallout" from this crash is a tremendous amount of energy, released mainly in the form of heat and light. This reaction at the nuclear level is called nuclear fusion. Scientists calculate that there is enough hydrogen in the sun to continue the fusion reaction and provide heat and energy for at least another five or six billion years. What is the range of temperatures on Earth? Compare this with temperatures on other planets in our solar system. Do any other planets have a similar range that would allow for human survival? What kinds of energy resources do we have on Earth? How are they related to the sun? Are any of our energy resources in danger of being used up? How can we conserve our energy resources?

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