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Astronomy 1 – Winter 2011
Lecture 20; February 25 2011
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Previously on Astro-1• The Sun
– Internal structure– Energy source– Neutrinos and the solar neutrino problem– Sunspots and the sun cycle
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Today on Astro-1• Introduction to stars• Measuring Distances• Inverse square law: luminosity vs brightness• Colors and spectral types• Masses of stars
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The closest stars to the sun.
But how do we know the distance to them?
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Parallax
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Parallaxmeasuring the distance to a stard = 1/pp = parallax in arcsecd = distance in parsecs1 pc = 3.26 ly
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Parallaxmeasuring the distance to a stard = 1/pp = parallax in arcsecd = distance in parsecs1 pc = 3.26 ly
Remember 1pc is the distance at which 1AU subtends 1 arcsec
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ExampleA star has a parallax of 0.1”. What is its distance?d = 1/pp = parallax in arcsecd = distance in parsecs
d = 1/0.1=10 pc
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Question 20.1 (iclickers!)
•Consider two stars, star 1 and star 2. Star 1 has a parallax of 0.05 arcsec. Star 2 has a parallax of 0.40 arcsec. How far away are the two stars?•A) Star 1: 5 pc, Star 2: 40 pc•B) Star 1: 1/5 pc, Star 2: 1/40 pc •C) Star 1: 10 pc, Star 2: 25 pc •D) Star 1: 20 pc, Star 2: 2.5 pc
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To put in perspective:If 1pc = distance from SB to NYC, distance from Sun to Earth ~ 10m! Less than this room!
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Inverse square lawdetermining the luminosity of a star
b = L/4πd2 b = brightness of star as we see itL = luminosity of star (wattage)d = distance to star
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The Inverse-Square Law Radiation from a light source illuminates an area that increases as the square of the distance from the source. The apparent brightness decreases as the square of the distance. The brightness at d = 2 is 1/(22) = 1/4 of the brightness at d = 1, and the brightness at d = 3 is 1/(32) = 1/9 of that at d = 1.
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Question 20.2 (iclickers!)
•At the distance of the Earth from the Sun (1 AU) the intensity of sunlight is 1370 watts/m2. What is the intensity at the distance of Saturn from the Sun (10 AU)?A.13,700 watts/m2 B.1370 watts/m2 C.137 watts/m2 D.13.7 watts/m2
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The Starsdistances – from parallaxluminosites – from b = L/4πd2
temperatures — from color and spectrumHot ColdO B A F G K M L T
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Spectra of stars with different surface temps
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Hydrogen absorption line:electron jumps from n = 2to n = 4 orbit
Spectra of stars with different surface temps
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If surface temperature is low,few hydrogen atoms havetheir electron in the n = 2 orbit:hence no absorption
Spectra of stars with different surface temps
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If surface temperature is high,most hydrogen atoms havelost their electron completely:hence no absorption
Spectra of stars with different surface temps
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If surface temperature isnot too hot and not too cold,many hydrogen atoms havetheir electron in the n = 2 orbit:hence strong absorption
Spectra of stars with different surface temps
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Question 20.3 (iclickers!)•A star has a radius half of that of the Sun and a luminosity equal to 60% of that of the Sun. What’s the star’s surface temperature? The surface temperature of the Sun is 5800K.A.7220 KB.6650 KC.4660 KD.3610 K
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The mass of stars
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To determine stellar mases we rely on binary star systems. As seen from Earth, the two stars that make up this binary system are separated by less than 1/3 arcsecond. For simplicity, the diagram shows one star as remaining stationary; in reality, both stars move around their common center of mass
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http://astro.ph.unimelb.edu.au/software/binary/binary.htm
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H-R diagramwith masses
The main sequence is a mass sequence!
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• Parallax is a tool to measure distances
• The Inverse-Square Law relates luminosity and brightness
• Low luminosity stars are more common than more luminous ones
• Colors and spectral types measure a star’s temperature
• The Hertzsprung-Russell (H-R) diagram is a graph plotting luminosity vs temperature
• Most stars belong to the main sequence. Other important classes are giants, supergiants and white dwarfs.
• Spectral typing can be used to determine distances
• Masses can be determined for binaries. The main sequence is a mass sequence!!
Summary
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The End
See you on monday!