![]() It is related to angular spacing across for seemingly single but really globular groups of stars. A much larger ly is used for stellar distances, Parsec is a little larger. So the distance to the sun is by definition one AU. 1 parsec 2.0626 (105) AU The unit parsec is much bigger than AU(astronomical unit). The smallest AU used for in-between distances of orbiters in a star system, like solar system. Or, in other words, an arcseconds spans a larger and larger physical size (as described by James K) out to ~15 Gly (where it spans roughly 28,000 lightyears), after which it spans a smaller and smaller size. A parsec is bigger than an astronomical unit, so it can sometimes be helpful for nearby stars. The result - which is obtained by integrating the Friedmann equation - is that galaxies become smaller and smaller out to a certain distance (roughly 15 billion lightyears), after which the second effect starts to dominate and they start to grow in size. On the other hand, we look further and further into the past, and hence see galaxies at a time when they looked larger and larger. Thus, we have two competing effects: On the one hand, galaxies become smaller and smaller with distance, as expected and as decribed in James K's answer. And because the Universe expands, everything was closer together in the past, so we see distant galaxies as they were when they were closer and hence spanned a larger angle on the sky. ![]() I just wanted to mention an effect which comes into play if you look really, really far away:īecause light moves at a finite speed, we see galaxies (and other things) as they were in the past. James K's answer is right on, and probably what you're after. ![]()
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