Chapter 17 - Physics1902 Stars:Distances,...

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Physics 1902 Stars: Distances,  Magnitudes and Types Lecture W4.b Chapter 17
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If the stars should appear but one night every thousand years how  man would marvel and stare. Ralph Waldo Emerson
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Distance to stars is measured by parallax As earth orbits, changes angle to a star
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Measuring Stellar Positions Early measurements were made with a Transit Circle Most measurements for the past 100 years have been made with photographic plates
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Transit Circle of Groombridge (~1817) Mount is precisely aligned east-west RA measured by timing passage of star across centerline Declination read from 4 foot circles
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Modern Measurements of Parallax The Hipparchos satellite has measured positions of 118,000 stars with a precision of 0.001 arc seconds. For information see http://wwwhip.obspm.fr/hipparcos/SandT/hip-S
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so star appears to move in sky
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Why don’t we see this: Aristotle: Geocentric Model: Why the earth must be stationary: Suppose that the earth revolved around the sun. In observing two distant stars, α > β: Since the separation between the stars doesn't change, the earth must be stationary . What is wrong with this argument?
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What will be the shape of the apparent motion for a star at the pole? on the ecliptic? How big is motion? 1 parsec = distance of a star whose apparent change in position would subtend an angle of 1" 3 × 10 16 m. 3 light-years D = 1/angle
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Parallax   Distance What's 1”? Roughly a dime at 10 km. First star distance was 61-Cygni (measured by Bessel). Closest star is Proxima Centauri, which is a binary with Centauri, with parallax of .76” What is distance?
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D = 1/.76 = 1.32 pc or D=2*150000000/(.76/3600/360*2*π) ~4x10 16 m~4 light years Parallaxes can be determined to ± .004” Hence distances can be measured out to 100pc: very small compared to galaxy size ( 20 kpc)
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The solar neighbourhood: 
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Distance: Closest star (α Centauri) is at a distance of ~1.3 pc. Sirius is at about 5 pc. Time: One million years (1 Myr) is fairly short Speed: Speed of light is standard (c=300000 km/s). Low speeds are 100 km/s = c/3000
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This note was uploaded on 10/05/2010 for the course PHYS 1902 taught by Professor Staff during the Winter '10 term at Carleton CA.

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Chapter 17 - Physics1902 Stars:Distances,...

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