Surveying the Stars

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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.
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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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Barnards Star
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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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Doppler Shift