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chapter15

# chapter15 - Surveying the Stars How do we measure stellar...

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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)
These two stars have about the same luminosity -- which one appears brighter? A l p h a C e n t u r i T S 0% 0% 1. Alpha Centauri 2. The Sun 0 of 5 10

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Luminosity passing  through each sphere is  the same Area of sphere:          4  (radius) π 2 Divide luminosity by  area to get brightness
The relationship between apparent brightness and  luminosity depends on distance:                                   Luminosity      Brightness =                                  4  (distance) π 2     We can determine a star’s luminosity if we can measure  its distance and apparent brightness:       Luminosity  =   4  (distance) π 2  x (Brightness)

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How would the apparent brightness of Alpha Centauri change if it were three times farther away? I t w o u l d b e n y 1 / 3 a . 6 9 h r i m 0% 0% 0% 0% 1. It would be only 1/3 as bright 2. It would be only 1/6 as bright 3. It would be only 1/9 as bright 4. It would be three times brighter 10 0 of 5
So how far 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  angle  depends on  distance
Parallax is  measured by  comparing  snapshots  taken at  different times  and measuring  the shift in  angle to star

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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)
The Magnitude Scale m = apparent magnitude , M = absolute magnitude apparent brightness of Star 1 apparent brightness of Star 2 = (100 1/5 ) m 1 - m 2 luminosity of Star 1 luminosity of Star 2 = (100 1/5 ) M 1 - M 2

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How do we measure stellar temperatures?
Every object emits  thermal radiation  with a  spectrum that depends on its temperature

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An object of  fixed size grows  more luminous as  its temperature  rises
1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average

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chapter15 - Surveying the Stars How do we measure stellar...

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