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Lecture 3

ASTA02H3 Lecture Notes - Lecture 3: Scientific Notation, Planck Constant, Parsec

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Astronomy Lecture 3
Finding How Far the Stars Are
D (parsec) = 1/p (arcseconds)
A Parsec is equal to 206, 265 AU or 3.26 ly
By definition 1 parsec (pc) is the distance of a star subtends an angle equal to
one arc second
Note the dependence of parsec to the size of the orbit of the Earth
First Sucessful Measurement of Parallax
Fredrich Bessel was the first person to measure the parallax of the star
In 1838, Bessel measured the parallax of Cygni-61 to be 0.313 arc-second
The actual value is closer to 0.286 arc-second
Distances between the stars
Parsec is a natural unit as it turns out that the average distance between two individual
stars is about a parsec
Measuring Parallax
Measuring parallax is a difficult task as the subtended angle is always very small
The angle substended by Alpha Centauri is only 0.76 arc-second
The Blurring of the light of the stars by the atmosphere of the Earth makes the parallax
measurement very difficult and uncertain
Hipparcos Satellite was launched in 1989 to measure stellar parallaxes
Hipparcos (High Precision Parallax Collecting Satellite)
Reminds us of Hipparchus
The goal was to measure the parallax of 120, 000 stars with a precision of 2-4 millarcsec
Hipparcos Facts
Observed for 3.5 years
Three catalogues: one with 118, 218 stars with 20 times the accuracy of ground-based
observations (Hipparcos Catalogue)
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