20 - Today Announcements: HW#8 due Friday 4/9 at 8:00 am....

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ISP209s10 Lecture 20 -1- Today • Announcements: – HW#8 due Friday 4/9 at 8:00 am. • The size of the Universe (It’s expanding!) • The Big Bang • Video on the Big Bang • NOTE: I will take several questions on exam 3 and the final from the videos we watch. Sleep at your own risk.
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ISP209s10 Lecture 20 -2- How do we measure properties of a star? How do we know what our sun (and other stars) are made of ? How do we know the temperature? How do we know the size? From the spectrum of the EM radiation from the star (“Blackbody radiation”)
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ISP209s10 Lecture 20 -3- Some Clicker Questions - #1 What happens to a star if its surface temperature is increased and its size remains the same? A) It only gets brighter B) It only gets more red C) It gets brighter and more blue D) It only gets dimmer E) It gets dimmer and more red Hint: recall how color correlates with temperature
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ISP209s10 Lecture 20 -4- Some Clicker Questions - #1 What happens to a star if its surface temperature is increased and its size remains the same? A) It only gets brighter B) It only gets more red C) It gets brighter and more blue D) It only gets dimmer E) It gets dimmer and more red
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ISP209s10 Lecture 20 -5- A Stars Energy Source – nuclear fusion The sun generates its energy by a set of fusion reactions where 2 nuclei are “stuck” together and release energy . (the strong force in action) Conditions required for nuclear fusion (two things): High temperature : the central temperature of the sun is 15 million Kelvin. This is necessary to overcome the repulsion between the positively charged protons. High density : the probability of collisions must be high. Note: the Sun is balanced just right. It does not burn too fast or two slowly for us to have a potentially comfortable existence.
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ISP209s10 Lecture 20 -6- The pp-chain in the Sun time
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ISP209s10 Lecture 20 -7- A star is born 1) Big clouds of gas (nebula) accumulate by chance thanks to gravity 2) Atoms heat up as more and more material falls towards the center under gravity as the gas cloud collapses to a smaller size 3) Eventually, the gas is hot enough that nuclear fusion reactions occur 4) Heat generated by fusion “balances” any further collapse (thermal pressure)
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ISP209s10 Lecture 20 -8- Stars • The mass of a star determines most properties of a star: lifetime, color, size, luminosity • Massive stars are very bright and hot, but they don’t last very long. • Stars are a balance between
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20 - Today Announcements: HW#8 due Friday 4/9 at 8:00 am....

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