Lec19 - Chapter 15 Surveying the Stars How do we measure...

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Chapter 15 Surveying the Stars
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How do we measure stellar luminosities?
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The brightness of a star depends on both distance and luminosity.
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Luminosity: Amount of power a star radiates (energy per second = watts) Apparent brightness: Amount of starlight that reaches Earth (energy per second per square meter)
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Thought Question Alpha Centauri and the Sun have about the same luminosity. Which one appears brighter? A. Alpha Centauri B. The Sun
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Thought Question Alpha Centauri and the Sun have about the same luminosity. Which one appears brighter? A. Alpha Centauri B. The Sun
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The amount of luminosity passing through each sphere is the same. Area of sphere: 4 π (radius) 2 Divide luminosity by area to get brightness.
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The relationship between apparent brightness and luminosity depends on distance: Luminosity Brightness = 4 π (distance) 2 We can determine a star’s luminosity if we can measure its distance and apparent brightness: Luminosity = 4 π (distance) 2 × (brightness) B = L 4 π D 2 L = 4 D 2 B
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Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright. B. It would be only 1/6 as bright. C. It would be only 1/9 as bright. D. It would be three times brighter.
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Thought Question How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/3 as bright. B. It would be only 1/6 as bright. C. It would be only 1/9 as bright. D. It would be three times brighter.
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So how far away are these stars?
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Parallax is the apparent shift in position of a nearby object against a background of more distant objects.
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Apparent positions of nearest stars shift by about an arcsecond as Earth orbits Sun.
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Parallax and Distance p = parallax angle d (in parsecs) = 1 p (in arcseconds) d (in light-years) = 3.26 × 1 p (in arcseconds)
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Most luminous stars: 10 6 L Sun Least luminous stars: 10 –4 L Sun ( L Sun is luminosity of Sun)
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How do we measure stellar temperatures?
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