Why can't astronomers measure the parallax of a star that is a million light-years away?

1 Answer
Feb 21, 2016

Because they are too far.


A star at 3.26 light years (1 parsec) has a parallax of 1 second of arc. So a star at one million of light-years away, would have a parallax of

#3.26xx10^-6# seconds of arc. The Hipparcos satellite (the best we have) only can determine parallaxes of #10^-3# seconds of arc.