# PHYS 205 Surveying the Stars PHYS 205 Parallax

• Slides: 9

PHYS 205 Surveying the Stars

PHYS 205 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.

PHYS 205 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.

PHYS 205 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

PHYS 205 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 20 pc (70 LY) radius. Why Hubble Space Telescope? ? 20 pc 60 pc, 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 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.

PHYS 205 Barnard’s Star

PHYS 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.

PHYS 205 Doppler Shift