The Sun: Structure & Life Cycle

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  • 0:03 Our Star, the Sun
  • 1:01 Structure of the Sun
  • 3:12 Life Cycle of the Sun
  • 5:04 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Learn some secrets of our star, the sun, and discover what's inside of it. Learn about the history and future of the sun, then test your new knowledge with a quiz!

Our Star, the Sun

From any spot on Earth, the sun looks relatively small--so small you can easily block out its brilliance with the palm of your hand. But in spite of its diminutive size as perceived from Earth, the sun cranks out an unbelievable amount of energy. If you're standing outside, the area immediately around you is receiving the energy equivalent of ten 100-watt bulbs from the sun, every second!

The apparent small size of the sun is a result of it being 93 million miles away from us. If Earth was transported to the sun, it could fit inside over a million times. However, Earth would fare poorly at such a close distance, as surface temperatures there are over 10,000 degrees Fahrenheit. If Earth had such poor luck to be transported inside the sun, the scalding temperature of over 27 million degrees Fahrenheit would destroy our beloved planet immediately.

Structure of the Sun

Our sun is actually relatively small compared to some of the other stars in the universe. Despite its small size, the sun is bursting with activity - sometimes quite literally.

The sun is made up of a plasma that is about 75% hydrogen and 25% helium. There are three layers that make up the inside of the sun. The innermost layer, known as the core, extends to about one-quarter of the sun's radius. In here, pressures and temperatures are so high that hydrogen is fused into helium through a process known as fusion. Hydrogen fusion produces the thermonuclear energy that keeps the sun burning and sends radiation out into the universe. Because fusion happens at the core of the sun, we have light on Earth.

The next layer is called the radiative zone. This layer is more than twice as thick as the core, extending to about 70% of the sun's radius. In the appropriately named radiative zone, energy created by fusion radiates out towards the surface of the sun.

Energy leaving the radiative zone next enters the convective zone. In here, the plasma interior of the sun begins to travel in circular motions as the material experiences cycles of heating and cooling. Extremely hot plasma rises to the surface of the sun, where it cools before sinking again. As the plasma heads back towards the radiative zone, it begins to heat up and the cycle repeats.

The sun's atmosphere is also made of three layers. The bottommost layer is the photosphere. The photosphere emits the sun's visible light. The splotchy coloration of this layer is due to temperature differences resulting from convective cells called granules. The dark areas, known as sunspots, are large relatively cool regions. Above the photosphere is the chromosphere, a less dense region of atmosphere that contains jets of rising gas. The top layer, known as the corona, extends several million miles out into space. This layer contains rapidly moving particles called prominences, the flame-like structures that originate from the sun's photosphere.

Life Cycle of the Sun

More than 4.6 billion years ago, the sun was merely a collection of hydrogen and helium gases in a dark, cold area known as a nebula. Gravitational attraction between dense pockets of gas led to the formation of a protostar, a dense ball of gases.

As a protostar, the sun radiated thermal energy as the gas ball continued to condense under the force of gravity. Eventually the temperature of the gas ball reached over 1.8 million degrees Fahrenheit. At this point, temperatures and pressures were high enough to fuse hydrogen nuclei together to form helium, and the sun began its life as a star.

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