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Download The Life Cycle of Stars Introduction Stars are huge spheres of very
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The Life Cycle of Stars Introduction Stars are huge spheres of very hot gas that emit light and other radiation. They are formed from clouds of dust and gas, or nebulas, and go through different stages as they age. They are located various distances from Earth. We use a light-year to describe the distance from Earth to far away objects such as stars and galaxies. A light-year is the distance that light travels in one year, or about 9.5 x 1012 km. Stars are powered by nuclear fusion reactions. A star is held together by the enormous gravitational forces that result from its own mass. Inside the pressure is more than a billion times the atmospheric pressure on Earth. The temperature is hotter than 15 million kelvins. Fusion takes place in the core. Fusion combines the nuclei of hydrogen atoms into helium nuclei. When hydrogen nuclei collide, they fuse to form deuterons, which have one proton and one neutron. Another proton collides with a deuteron to form a helium isotope. Each time that two particles fuse, energy is released. The core of the star is the hottest which can be around 10,000,000 K. We can tell how hot a star is based on its color. For example, the sun appears yellow because the peak wavelength of the sun’s energy is near yellow on the spectrum. Yellow also corresponds to a temperature near 6,000 K. Hot stars emit more energy at every wavelength than cool stars do. Hot stars emit the most energy at blue wavelengths. A red star will have the lowest temperature. The Life Cycle of Stars Stars are born, go through stages of development, and eventually die. About 90% of all stars in our galaxy, including the sun, are in midlife, or converting hydrogen to helium in their interiors. About 5 billion years ago, a thin cloud of dust and gas, or nebula, collapsed inward as it was pulled by the force of its own gravity. As the nebula collapsed, it began to spin. This small, spinning cloud formed a protostar. About 30 million years after the nebula started to collapse, the center of the nebula reached a temperature of 15 million kelvins. Electrons were stripped from hydrogen atoms to leave hydrogen nuclei, which are protons. At very high pressures, protons may get as close to each other as 10-15 m. At such a small distance, the strong nuclear force overpowers electrical repulsion and the protons fuse. The protons combine to form helium through this process of nuclear fusion. The onset of fusion marks the birth of a star such as our sun. Fusion reactions in the sun’s core generate energy that produces an outward pressure that balances the inward force due to gravity. Because of these balanced forces, the sun has maintained a stable size for 5 billion years. The sun is currently converting hydrogen into helium. Over time, the percentage of the core that is helium becomes larger. Eventually, the core will exhaust about 25% of its hydrogen, and the number of fusion reactions will decrease. When this happens, the sun will begin to die. Scientists estimate, however, that the sun will exist for another 5 billion years. 1 The sun will become a red giant before it dies. A red giant is a large, reddish star late in its life cycle. As the number of fusion reactions decreases, the pressure from the release of energy in the core of the sun drops, and the core will contract causing its temperature to rise. The outer layers will expand, and the sun will become a red giant. The star is red because its surface is relatively cool. But the core is hot enough to convert helium into carbon and oxygen. When the core of a red giant sun depletes most of its helium, it will contract further, which will cause the outer layers to expand again. At this point, the temperature at the core is not high enough to fuse heavier elements. The outer layers will expand out from the core and will eventually leave the star. The remnants will become a white dwarf, a small, dim, and very dense star about the size of Earth. White dwarfs no longer fuse elements, so they slowly cool. All stars that have a mass of 1.4 solar masses or smaller, or a mass of less than 1.4 times the mass of our sun, will have a similar life cycle. In fact, most stars in our galaxy will end as white dwarfs. Massive stars evolve faster than smaller stars do. They also develop hotter cores that create heavier elements through fusion. The formation of an iron core signals the beginning of a supergiant star’s death because, unlike fusing lighter elements, fusing iron atoms to make heavier elements requires adding energy rather than releasing it. When fusion requires more energy than it can produce, there is no longer any outward pressure to balance the gravitational force. The core collapses because of its own gravity and then rebounds with a shock wave that violently blows the star’s outer layers away from the core. The resulting huge, bright explosion is called a supernova. A supernova is defined as a gigantic explosion in which a massive star collapses and throws its outer layers into space. The remnant of a supernova can become a neutron star. Neutron stars are only a few dozen kilometers in diameter, but they are very massive. A neutron star is as dense as matter in the nucleus of an atom, about 1017 kg/m3. Just a teaspoon of matter from a neutron star would weigh more than 100 million tons on Earth. Neutron stars can be detected as pulsars, or spinning neutron stars that are sources of pulsating radio waves. If the leftover core of a supernova has a mass that is greater than 3 solar masses, it will collapse to form something else—a black hole, which consists of matter so massive and compressed that nothing, not even light, can escape its gravitational pull. Because no light can escape a black hole, black holes cannot be seen directly. Black holes can, however; be detected indirectly by observing the radiation of light and X rays from objects that revolve rapidly around them. The Hertzsprung - Russell diagram (H-R diagram) is a tool that astronomers use to help them understand how stars change over time. The vertical axis shows the luminosity (the total energy output at every wavelength) or the brightness of the stars. The horizontal axis shows the surface temperature of the stars, with hotter temperatures on the left side of the diagram. Once a star is stable—fusing hydrogen into helium in its core—it appears on a diagonal line in the H-R diagram called the main sequence. Most stars are main-sequence stars. The position of the star on the main sequence depends on the initial mass of the star. Red giant stars are both cool and bright, so they appear in the upper right. White dwarf stars are both faint and hot, so they appear in the lower left. Our sun which is in the middle of the H-R diagram will eventually become a white dwarf and will be located with the other white dwarfs on the diagram. Taken from: Physical Science with Earth and Space Science, Holt, Rinehart and Winston, NewYork, 2008. 2 Life Cycle of Stars Questionnaire 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. Name _____________________________ Date _____________________ Period ____ List the phases of the birth of a star (like our sun) in order: _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ Our sun has a _____________________ color spectrum which corresponds to what temperature? __________________________ Around 90% of the stars in our galaxy mainly undergo what fusion reactions? ___________________________________________________________ The temperature required for fusion to occur in a star is? ______________________________ What forces etc. cause a nebula to ultimately become a star? _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ What is special about 10 million kelvin with respect to the strong force? _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ When will our sun start dying? _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ Our sun will evolve or change to become what after this process occurs in #7? Describe it. _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ After this, our sun will undergo what changes and become what at the end of its life? _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ If our sun only makes helium, where is the higher numbered elements made? (Explain) _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ What is a neutron star? _________________________________________________________________________________ _________________________________________________________________________________ 3 12. What is a pulsar? _________________________________________________________________________________ _________________________________________________________________________________ 13. On the graph called the HR diagram astronomers use show how stars change over time? Explain the relationship the graph is making. _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 14. Define black hole _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 15. ON the HR diagram, our sun is listed as what type of star? ________________________________________________ 16. The coolest stars appear where on the H-R diagram? _______________________________________________________ 17. These cool stars include what type of stars? _______________________________ 18. Where do white dwarfs appear on the H-R diagram? ___________________________________________________ 19. A red star will have what type of temperature according to the article? _______________________________________ 20. Define light year _________________________________________________________________ 21. What is a nebula (plural nebulae) ? _________________________________________________________________________________ _________________________________________________________________________________ 22. Why is it correct to say we are made of star stuff? _________________________________________________________________________________ _________________________________________________________________________________ 4