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Chapter 17

# Chapter 17 - Physics1902 Stars:Distances MagnitudesandTypes...

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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
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
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
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?
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?
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:
Some Numbers: 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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Some Numbers: Mass: Sun has mass of 2*10 30 kg: will refer to this as M o (so Earth has a mass of one millionth M o Brightness/Luminosity: Sun has (absolute) magnitude 5 Brightest normal stars are about M=-8 dimmest things we can see are about m=28
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Chapter 17 - Physics1902 Stars:Distances MagnitudesandTypes...

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