Discover a main sequence star, how it

Discover our sun, the provider of energy for life on Earth. Learn how the sun was born, how it lives as a main sequence star, how it will age into a red giant and how it will die as a white dwarf.

Birth of the Sun

The sun was born as a star in a nebula 4.

6 billion years ago.

Nebula Birth Place of Sun

Look in the sky at night. It is filled with over 100 billion stars. But only one of those stars is precious to us, and we don’t see it at night. That star is our sun – the source of energy for life on Earth. The sun is so important to life on Earth that it has been worshipped and studied for centuries.

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In ancient times, people believed the sun was a burning ball of fire created by the gods. Later, people thought it was a solid object or a liquid ball. Now we believe it is a huge ball of hydrogen undergoing nuclear fusion.Look at our universe. The sun is huge! Over one million Earths could fit inside the sun. Have you ever worried that our sun will die? Did you ever wonder how it was born?Our sun is the closest star to Earth. To understand how the sun was born, you need to know how a star is born.

A star’s life cycle is much like our human life cycle. A star is born, lives for a while, and dies. Our sun’s life began about 4.

6 billion years ago like any other star in the universe, in a nebula, a giant interstellar cloud of dust and gas so far away we would need 100,000 years to reach it. This cloud was perfectly content and quiet. Then something happened that caused the cloud to collapse and start spinning until it formed a flat disc. Most of the material that was originally in the cloud ended up in one ball in the center of the disc.

For 100,000 years the ball was squeezed tighter, and the temperature increased until it was hot enough to perform nuclear fusion in its core. Voila, a star is born. The rest of the material that didn’t settle in to the center of the disc formed the planets, moons, asteroids and other parts of the solar system.

Main-Sequence Star

Once nuclear fusion started, our sun was big enough and bright enough to be a main-sequence star. This stage of the sun’s life is a very stable time for the sun. The pressure of gravity forcing everything in toward the center of the sun is equal to the force of the energy from fusion that the sun is doing in its core pushing outward.

Gravity pressure in a main-sequence star is equal to the force of fusion energy in the core.
Sun Gravity Equal to Fusion Force

So the sun carries on its life as a main-sequence star, burning up the hydrogen fuel in its core.

As it does so, the sun’s energy increases. This increase in energy isn’t too good for Earth. In a billion years or so, the sun’s energy will be 10% more than it is today. This will cause an extreme greenhouse effect on Earth and all the water in our atmosphere will dry up.In 3.5 billion years, the sun will be 40% brighter than it is today and all oceans and glaciers and snow on Earth will evaporate. Nothing will be able to survive.

In six billion years, the sun will run out of fuel. When this happens, there will be no energy pushing the core out, and the pressure of all the layers of the sun will push inward. This causes the core to become unstable and collapse under its own weight, which causes the core to get hotter. As the core gets hotter and denser, the other layers of the sun grow. The sun will expand so much that it will engulf the two planets closest to it, Venus and Mercury. It may possibly take over Earth as well. The sun is no longer in the main sequence stage of life but is now in its red giant phase.

During the red giant phase, increased pressure in the core allows the sun to fuse even heavier nuclei. In the case of the sun, the helium nuclei are fused into carbon.

Death of the Sun

Near the end of life, the sun is reduced to a sphere of carbon called a white dwarf.
Sun Red Giant Stage

When all the helium of the sun has fused into carbon, the sun’s core stabilizes but the other layers continue to grow.

Eventually the other layers get blasted off and all that will remain of the mighty sun is a ball of carbon. The sun is now a white dwarf, and the outer regions of it are a planetary nebula. Since it is no longer doing any nuclear fusion, the white dwarf will cool down slowly until it becomes the same temperature as the rest of the universe, just a few degrees above absolute zero.

Lesson Summary

There are billions of stars in our universe, but one dominates our galaxy: the sun. The sun begins its life like any other star, from the dust and gas in a swirling cloud. The particles condense together until there is enough pressure to begin nuclear fusion. At this point in the sun’s life-cycle, it is a star in the main sequence phase of life. After millions of years the sun will run out of fuel and enter a red giant phase, where it will grow until it engulfs Venus and Mercury and possibly Earth.

Eventually it will pass out of the red giant phase and become a white dwarf before cooling down to the temperature of the rest of space.

Learning Outcomes

After watching this lesson, you should be able to:

  • Summarize the life cycle of the sun, including birth and death
  • Describe the main sequence and red giant phase of the sun
  • Define nebula and nuclear fusion

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