Feb_28 - Stars Thursday, February 28 Thu, Feb 28: Stars...

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Stars Stars Thursday, February 28 Thursday, February 28
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Thu, Feb 28: Stars Tue, Mar 4: Planets Problem Set #7 due Problem Set #7 returned Tue, Mar 11, 1:30 pm 1:30 pm Final Exam Final Exam
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What is a star? Examples of stars: Sun Betelgeuse
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Pleiades: known traditionally as the “seven sisters” (6 bright stars, 1 fainter) With his telescope, Galileo saw many more stars.
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Average density of ordinary matter in the universe = 0.04 ρ crit = 3.6 × 10 -28 kg/m 3 Density of Sun = 1400 kg/m 3 = 4 quadrillion quadrillion × average Stars are lumps of ordinary matter compressed to small size and high density.
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What is a star? A luminous ball of gas powered by nuclear fusion nuclear fusion in its interior.
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“Why do stars shine?” Stars are dense (Sun is 40% denser than liquid water). Stars are opaque (because they’re made of ionized gas). Stars are hot (surface temperatures > 2000 Kelvin).
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Hot, dense, opaque objects emit light! Today, we call hot, dense, opaque objects that emit light “ stars stars ”. Flashback slide!
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Question: Why do stars shine? Short answer: Stars shine because they are hot. Follow-up question: Why don’t stars cool down?
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There’s a continuous fossil record of life on Earth for over 3 billion years. Sun’s luminosity can’t have been wildly variable – if it had, life would have scorched or frozen.
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source to replace the energy carried away by photons. What’s the power source?
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Feb_28 - Stars Thursday, February 28 Thu, Feb 28: Stars...

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