Ch 9 (The Formation & Structure of Stars)

Ch 9 (The Formation & Structure of Stars) - Chapter...

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The Formation and Structure of Stars Chapter 9
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Stars exist because of gravity. They form because gravity makes clouds of gas contract. Once they form, stars spend long, stable lives generating nuclear energy at their centers and balancing their own gravity with the pressure of the hot gas in their interiors. In the end, stars die because they exhaust their fuel supply and can no longer withstand the force of their own gravity.
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The goal of this chapter is to answer these five essential questions about stars. 1. How are stars born? 2. How do stars make energy? 3. How do stars maintain their stability? 4. How long can a star survive? 5. What evidence do astronomers have that theories of star formation are correct?
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9-1 The Birth of Stars 9-1 The Birth of Stars There is a correlation between young stars and clouds of gas and dust. Where you find the youngest groups of stars, you also find large clouds of gas illuminated by the hottest and brightest of the new stars. How do these cold clouds of gas that float between the stars contract, heat up, and become stars?
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The Interstellar Medium The space between the stars is not completely empty, but filled with very dilute gas and dust. About 75% of the gas is hydrogen, and 25% helium, but there are traces of carbon, nitrogen, oxygen, calcium, sodium and heavier atoms. Roughly 1% of the mass is made up of microscopic dust about the size of the particles in cigarette smoke. The dust is made of carbon and silicates (rocklike minerals) mixed with or coated by frozen water. This dust is called interstellar dust.
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Sometimes the interstellar medium is easily visible as a cloud of gas and dust. Such a cloud is called a nebula (plural: nebulae). There are three kinds of nebula. Emission nebula, reflection nebula, and dark nebula. The interstellar medium consists of two components: cool, dense clouds and hot, low-density gas.
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Emission Nebulae The Fox Fur Nebula NGC 2246 The Trifid Nebula The star must be hotter than about B1 (25,000 K). They have a distinctive pink color produced by the blending of the red, blue and violet Balmer lines. Emission nebulae are produced when a hot star excites the gas near it to produce an emission spectrum.
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Reflection Nebulae A reflection nebula is produced when starlight scatters from a dusty nebula. (The light we see is starlight that got reflected by the nebula.) Therefore its spectrum is just the absorption spectrum of starlight. A reflection nebula is blue for the same reason the sky is blue. More blue light is scattered than red light. (This shows that the dust particles must be very small.)
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the view of more distant stars or nebulae. Dark Nebulae
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Ch 9 (The Formation & Structure of Stars) - Chapter...

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