How is a parallax used to measure the distances to stars?
As Earth orbits the Sun individual stars seem to move their position against the celestial background. The nearer a star is to is, the greatest that apparent move is. That apparent change in the stars position is known as its parallax. A star close enough to show a change of 1 second of an arc is said to be at a distance of one parsec. No star is actually that close. Proxima Centauri, the nearest start to us after the Sun, is 0.75 of a second of an arc. One parsec is equivalent to 3.76 light years. The farther away a star is, the smaller its parallax. Stars over 50 light years away have a parallax that is too small to measure, even with the most powerful of telescopes. Only about 1000 stars have an accurately measured parallax. Beyond that, the absolute magnitude of a star is used to estimate its distance, which relates to its brightness.
