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205
Surveying the Stars
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Parallax
The apparent change in the
direction of the remote object due
to a change in the vantage point of
the observer is called parallax.
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Stellar Parallax
Astronomical objects are far away  we need a large baseline
The largest baseline we have is the orbit of the Earth around the Sun.
So, the first thing we need to know is the distance between the Sun and
the Earth.
This can be measured from the distance of the other planets measured
by radar observations  RUNG 1 of the Cosmic Distance Ladder.
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Parallax Measurements
Distance d = 206265 D/p D: baseline (AU) p: angle in arcseconds
Degree  Minute  Second
1 arcsec  d = 206265 AU = 1 parsec = 3.26 lightyears
1/10 arcsec  d = 10 parsecs = 32.6 lightyears
1 arcsec  50,000 TL coin from 5 kms.
So, stellar parallaxes are not visible to the naked eye  RUNG 2
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Nearest Stars:
Alpha Centauri  triple star system
a double and a faint star  Proxima Centauri  1.3 pc = 4.2 LY
Next: Sirius : 2.6 pc = 8 LY
With the ground based observations we can see about 1000 stars
within a 20pc (70LY) radius.
Why Hubble Space Telescope??
20pc  60pc,
1,000 stars  20,000 stars
New: Hipparcos mission  distances to 118,000 stars are measured.
PHYS
205
Proper Motion of Stars
Our Sun goes around the Milky Way galaxy every 200,000,000 years.
Similarly all other stars are also moving.
Proper motion: The change in the position of stars in the sky.
One year is too short to measure the proper motion.
Intervals of 20-50 years are taken (with photographs).
Smaller proper motion, farther away the star  RUNG 3.
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205
Barnard’s Star
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205
Space Velocity
Vs: True velocity of a star
Vr: Radial velocity of a star, measured by Doppler shift.
Vt: Tangential velocity of a star measured by the proper motion.
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205
Doppler Shift