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Star Cycle
by Daniel

Space keeps expanding and needs stars to fill in those empty gaps. That is where the star cycle comes in.

Star cycles take a very long time. These cycles have four steps involved.

The first stage is the stellar nursery. This is where they are born. We can see that in our own galaxy and in others close by that nurseries sit in the heart of giant molecular clouds. These clouds start out very lumpy. The lumps collapse much quicker than the thinner parts of the cloud. The lumps are regions of greater density, and have more mass than other parts. Gravity pulls these lumps together quicker. Each fragment that is almost a complete star is a protostar. At that time, collapsing causes the protostar to get extremely hot and slow down the collapse. This is where the begin to glow.

Planetary Nebula
What happens in the second, third, and fourth stages depend on what happens in the nursery. Stars form when hydrogen atoms ar pulled together into huge spheres. Clusters of interstellar particles pull in many other kinds of particles. This increases the size of a star. The brightest stars have masses 100 times more than the sun. High mass stars have 5 gasses inside them. Hydrogen, helium, carbon, neon, and oxygen. Intermediate stars have 3, hydrogen, helium, and carbon. Low mass stars have only hydrogen and helium. Low mass stars also exhaust the supply of hydrogen in it's core. The sun is a low mass star. These stars die before or during the hydrogen burning stage, mainly because central tempetures never get hot enough to fuse helium into carbon.

In the second stage, stars can turn into sun like stars, huge stars, or giant stars. Many stars go no farther than this stage, but some do.

In the third stage, those that were huge or sun like, most likely turn into giant. If they were giant in the second stage, they will turn into red super giants. The sun would take about 5 billion years to become a red giant. Bigger stars, such as red super giants, can produce elements heavier than iron.

Someday, all stars get to the fourth and final stage. Smaller stars will turn into white dwarfs, but larger ones will explode into black holes. Usually, the stars that do collapse into a black hole, are over 40 times larger than the sun. Black Holes are still very mysterious. There gravity pulls objects in, but no one really knows what happens to everything it pulls in. When stars explode, they can be seen for miles. Throughout the milky way, people see exploding stars everywhere. Massive stars burn much brighter, causing them to use more energy. Because of this, the bigger the star, the shorter it's life is. Scientists have discovered stars involved or nearing icstinction all throughout our galaxy, and many other galaxies. They have also seen stars that are forming or just very young. We don't know why white dwarfs don't collapse.

Though we know a lot about these cycles, they are still very mysterious. But as time goes on, we will continue to learn more about star cycles.

Credits:

http://www.airynothing.com/high_energy_tutorial/basic_astro/life_cycle01.html

http://www.angelfire.com/moon/astronomy/stars.html

http://www.windows.ucar.edu/tour/link=/sun/Solar_interior/Nuclear_Reactions/Fusion/Fusion_in_stars/star_life.html

http://map.gsfc.nasa.gov/m_uni/uni_101stars.html

http://spaceinfo.jaxa.jp/note/hoshi/e/hos01_e.html

http://www.airynothing.com/high_energy_tutorial/basic_astro/life_cycle02.html

http://aspire.cosmic-ray.org/labs/star_life/starlife_main.html

http://library.thinkquest.org/C0110277/stars/life_cycle.htm

http://www.telescope.org/pparc/res8.html

http://chandra.harvard.edu/xray_sources/white_dwarfs4.html

http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/starbirth.shtml

http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/

http://www.kidscosmos.org/kid-stuff/stars-facts.html

Starbirth1 Picture: http://www.enchantedlearning.com/subjects/astronomy/stars/lifecycle/starbirth.shtml

Nebula Picture: http://aspire.cosmic-ray.org/labs/star_life/starlife_main.html

Planetary nebula picture: http://map.gsfc.nasa.gov/m_uni/uni_101stars.html