The Sun

What is 25 million degrees Fahrenheit at its center (14 million degrees Celsius) and made of gas? You guessed it, the Sun! The Sun is a huge, hot ball of gas that is the center of our solar system. The Sun is so large that a million Earths could fit inside of it. While the sun has a relatively low average density, about 1.4 times the density of water, its large size causes it to have an incredible amount of mass. The large mass of the Sun means that it has a very powerful gravitational pull. (Note: The average density of the Sun is 1.4 times the density of water; the average density of the Earth is 5.5 times the density of water.)

The Sun is 93 million miles (150 million km) from our planet, but it still supplies us with almost all of the energy that we consume. The Sun's energy reaches the Earth in the form of heat, light and radiation. Without this energy, life could not exist on or planet. For example, all the fruits, vegetables and grains we eat use photosynthesis to turn energy from the Sun into energy for us. When we eat these plants we get the energy we need to walk, run, talk, throw, etc. We also eat animals that eat plants. The energy gets transferred from the plants to the animals and then to humans. Take a hamburger, for example. The lettuce, tomato, onion, bun, ketchup and mustard all come from plants. The egg in the mayonnaise comes from a chicken that gets its energy from eating plants. The hamburger comes from a cow that gets its energy from eating plants. As we can see, everything we eat gets its energy from the Sun. Without the Sun we would not be able to get the food we need to live.

So, what about other sources of energy? From where does the electricity we use in our houses and schools come? What about the energy to power our computers, lights, cars, trains, boats and planes? Most of the energy we use in our homes comes from burning fossil fuels such as coal and oil. Our transportation devices, such as automobiles, trains, boats and planes, almost all use fossil fuels (usually oil). Fossil fuels are made from decayed plants and animals from millions of years ago. From where do you think all those plants and animals got their energy? That's right — their energy came from the Sun. Other types of energy, such as solar, wind and hydroelectric, all require the Sun to provide power. Without the Sun, the Earth would be a frozen and dark ball with no life.

Figure 2. The layers of the Sun, from the center out: core, radiative zone, convection zone, photosphere, chromosphere and corona.

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The Sun is not on fire like we think of fires on Earth. Oxygen is not required; the Sun is a big nuclear fusion reaction. If the Sun did behave like a fire on Earth, it would have burnt out long ago. So what keeps the Sun alive? The Sun is mostly made of hydrogen gas. This hydrogen is fused with other hydrogen to form helium. This process creates an immense amount of energy that we see as light and feel as heat. How long do you think it takes energy from the Sun to reach our planet? Because the Sun is so far away, it takes the energy from the Sun eight minutes to reach us on Earth.

Figure 3. Engineers designed the Arecibo radio telescope in Puerto Rico - the largest single-dish telescope on the planet.

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The Sun does not have a solid surface where we could stand, like on our planet. It has many different layers (see Figure 2). At the center is the core where the fusion of hydrogen takes place. Then, the energy passes through the radiative (or radiation) zone and the convection zone. The outermost layers of the Sun include the photosphere, chromosphere, and the corona. The visible surface of the Sun is the photosphere; it is from here that the light from the Sun comes. The next layer is the chromosphere, which is a thin layer of transparent hot gas around the photosphere. The outermost layer of the Sun is known as the corona. Humans can only view the corona with the naked eye during a solar eclipse (even then, one should never look directly at the Sun, as it can cause permanent eye damage).

So, how have we learned so much about the Sun? Since people should not look directly at the Sun, we must create special devices that can look at the Sun for us and gather information for us. These devices include telescopes, cameras, sensors and satellites. These scientific instruments are designed and built by engineers. Huge ground-based telescopes are built by civil, mechanical and electrical engineers (see Figure 3). Aerospace, mechanical and electrical engineers design the satellites that look at the Sun and gather data from nearer the Sun. Without this scientific equipment we would not know everything we know about the Sun.

From: https://www.teachengineering.org/view_lesson.php?url=collection/cub_/lessons/cub_solar/cub_solar_lesson02.xml