Astronomy in Motion: History of 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.





Distance from Earth




Fun in the Sun: Sun Related Activities

Everything under the Sun: Sun Related Resources
WWW encyclopedias Books Clubs and Events

Introduction to Astronomy in Motion Sun pages

About Astronomy in Motion

HEAD Science Education Page

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