# 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?
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)
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
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
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.
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.
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)
How do we measure stellar temperatures?

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