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Lecture 6

ASTA02H3 Lecture Notes - Lecture 6: Proper Motion, Hipparcos, Minute And Second Of Arc


Department
Astronomy
Course Code
ASTA02H3
Professor
Diana Valencia
Lecture
6

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ASTA02- Lecture 6 (Jan. 24th)
- Distances to Stars
Distance is the most important and most difficult quantity to measure in astronomy
Distances are necessary for estimating:
Total energy emitted by an object (Luminosity)
Masses of objects from their orbital motions
True motions of stars
- Method of Trigonometric Parallaxes
Stellar parallax: nearby stars appear to move with respect to more distant background
stars due to the motion of the Earth around the Sun
The line of sight to the star in December is different than in June
Formula:
tanƟ=x/d
Distance from Earth to Sun is 1 Astronomical Unit (AU)
Ɵ is in arcseconds
Distance is in parsecs
Because even the nearest stars are very far away (0.7 parsecs away)
The greater the distance, the smaller the parallax, thus the distance measurement is less
precise
- Gaia Mission
The follow-up to Hipparcos
Measure positions and motions for about 1 billion stars (with accuracy of 20 micro
arcsec)
About 20 million stars are being measure with a distance precision of 1%
Study exoplanets
- Star motion
To the naked eyes, the stars appear fixed to the sky
In reality, stars are in constant motion
Proper motion: the apparent angular motion of a star across the sky with respect to
more distant stars
Typical proper motion is ~0.1 arcsec/year
Largest: 10.25 arcsec/yr (Barnard Star)
Proper motions are cumulative (they build up over times)
The longer you wait, the larger the apparent angular motion
Modern measurement of proper motions:
Compare images of the sky taken 20, 50 years apart and measure how much the
starts have moved relative to distant background stars
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