Chapter 11 WEB - Characterizing Stars How bright are stars...

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Characterizing Stars How bright are stars? How distant are stars? How hot are stars? How massive are stars? What is an HR diagram?
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What can we measure about a star? Brightness Color Position Image: Spectrum: Spectral lines Overall shape
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Which star looks like it is giving off more light? Which star is actually giving off more light at its surface?
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The brightness of a star in the sky depends on both distance and luminosity
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Luminosity: Total amount of power a star radiates: in all directions (energy/second) Apparent brightness: Amount of starlight that reaches Earth (energy/second/area). Measures how bright star appears to us.
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The Sun and Alpha Centauri have about the same luminosity. Which one appears brighter? A. The Sun B. Alpha Centauri Survey Question
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The Sun and Alpha Centauri have about the same luminosity. Which one appears brighter? A. The Sun B. Alpha Centauri Survey Question Closer star appears brighter!
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Rank the amount of Sun’s energy reaching each equal-size square (1-3) from greatest to least: A. 1>2>3 B. 3>2>1 C. 3>1>2 D. 2>3>1 E. all the same (i.e., 1=2=3) Survey Question 1 2 3
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Rank the amount of Sun’s energy reaching each equal-size square (1-3) from greatest to least: A. 1>2>3 B. 3>2>1 C. 3>1>2 D. 2>3>1 E. all the same (i.e., 1=2=3) Survey Question 1 2 3
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Inverse Square Law
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A 100-watt light bulb has a luminosity of 100 watts. Compared to an up-close view from one foot away, if you move 10 feet away the bulb appears: A. 9 times fainter. B. 10 times fainter. C. 11 times fainter. D. 100 times fainter. E. 1000 times fainter. Survey Question
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A 100-watt light bulb has a luminosity of 100 watts. Compared to an up-close view from one foot away, if you move 10 feet away the bulb appears: A. 9 times fainter. B. 10 times fainter. C. 11 times fainter. D. 100 times fainter. E. 1000 times fainter. Survey Question
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How do we measure a star’s luminosity? luminosity 4 ! (distance) 2 app. brightness = We can determine a star’s luminosity if we can measure its brightness and distance. Luminosity is a great parameter to describe stellar evolution (changes stars undergo throughout their lifetime).
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How do we measure a star’s distance?
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How do we measure a star’s distance? Stellar parallax
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Parallax is given in arcseconds (1/3600 degree) or smaller Pluto has angular diameter of about 0.1 arcsecond This is roughly equivalent to a (40 mm) ping-pong ball viewed at a distance of 50 miles (80 km).
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How do we measure a star’s distance? If a star has a parallax angle of 1 arcsec we define it to be at the distance of 1 parsec = 206 265 AU
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Stellar parallax & Parsec Lecture Tutorial Parsec p. 37 through 39
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You are an astronomer working on Mauna Kea. You observe the same star field every month for 6 months and you notice that two stars have huge parallaxes, with star !
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