ay45c1-page8 - Huygens also thought that Sirius(modern mV =...

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Unformatted text preview: Huygens also thought that Sirius (modern mV = 1:45) was as bright as a candle at 80 m. So, if it was same intrinsic brightness as Sun: s DSirius = 127000  29000 AU DSun (1=80)2  1 light year. 2 We now know that the actual distance to Sirius is  550000 AU (8.8 l.y.). (It is much brighter intrinsically than the sun, and Huygen's calibration of it in candles was also not accurate.) 1.2.5 Stellar distances from Parallax across Earth's orbit The rst accurate stellar distances came from parallax across the Earth's orbit: Earth Distant stars 1 AU close star Sun Earth 6 months later Since If this angle is _ 1 arc sec the star is at a distance of 1 _ [parallax second] parsec  1  rad 1 arcsec = 1 arcsec  3600 arcsec 180 1 = 206265 rad.  So 1 pc  206265 AU (= 3:26 ly). With modest astrometric telescopes (and lots of e ort) direct parallaxes are possible out to 10{50 pc (0.1 to 0.02 arcsec). Beyond that, indirect methods must be used. 8 ...
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