Proper Motion

2013-10-08T21:49:47Z (GMT) by Adric Riedel
<p>Proper Motion (in the sense of "intrinsic to the object") is the apparent angular motion of stars across the sky, due to their orbits around the center of the Milky Way Galaxy. Lacking any depth perception, astronomers have to use different techniques to measure a star’s radial (directly toward or away from us) and transverse (across the sky) motions. Proper motion is that transverse motion.</p> <p>Historically, proper motions (the transverse motion) have been used to find potential nearby stars. The idea is that the stars that appear to be moving the fastest should be the closest. Consider the powerpoint animation: your brain wants to put the slow-moving stars in the background, and the fast moving ones closer. It’s not perfect, of course; every star is actually on its own unique orbit around the center of the Milky Way Galaxy. Proxima Centauri, the closest star, has only the 17th highest proper motion in the NLTT catalog; Barnard’s Star, the fastest-moving star, is the second-closest star system. Still, the 3.8″/yr motion of alpha Centauri AB (Proxima was unknown until 1915) combined with its brightness- combined, they are the fourth-brightest point of light in the sky- was enough to single it out to 17th century astronomers as a potential nearby star worthy of measuring a parallax to.</p> <p>For reference: The background is a blurred, rotated crop of an HST image of NGC 5866, the Spindle Galaxy http://hubblesite.org/newscenter/archive/releases/2006/24/image/a/<br>Technically speaking, the powerpoint shows off a "parallax scroller", where several layers of stars moving at different speeds produce the illusion of depth.</p>