chapter16BB.pt

chapter16BB.pt - Star Birth Where do stars form...

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Star Birth
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Where do stars form?
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Star-Forming Clouds Stars form in dark clouds of dusty gas in interstellar space The gas between the stars is called the interstellar medium
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Composition of Clouds We can determine the composition of interstellar gas from its absorption lines in the spectra of stars 70% H, 28% He, 2% heavier elements in our region of Milky Way
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Molecular Clouds Most of the matter in star-forming clouds is in the form of molecules (H 2 , CO,…) These molecular clouds have a temperature of 10-30 K and a density of about 300 molecules per cubic cm
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Molecular Clouds Most of what we know about molecular clouds comes from observing the emission lines of carbon monoxide (CO)
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Interstellar Dust Tiny solid particles of interstellar dust block our view of stars on the other side of a cloud Particles are < 1 micrometer in size and made of elements like C, O, Si, and Fe
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Interstellar Reddening Stars viewed through the edges of the cloud look redder because dust blocks (shorter- wavelength) blue light more effectively than (longer-wavelength) red light
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Interstellar Reddening Long-wavelength infrared light passes through a cloud more easily than visible light Observations of infrared light reveal stars on the other side of the cloud
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Observing Newborn Stars Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed
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Observing Newborn Stars Observing the infrared light from a cloud can reveal the newborn star embedded inside it
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Glowing Dust Grains Dust grains that absorb visible light heat up and emit infrared light of even longer wavelength
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Glowing Dust Grains Long-wavelength infrared light is brightest from regions where many stars are currently forming
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Why do stars form?
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Gravity versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons
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Mass of a Star-Forming Cloud A typical molecular cloud (T~ 30 K, n ~ 300
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chapter16BB.pt - Star Birth Where do stars form...

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