Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/3/84 (WLS Mods); site astrovax.UUCP Path: utzoo!watmath!clyde!burl!ulysses!princeton!astrovax!wls From: wls@astrovax.UUCP (William L. Sebok) Newsgroups: net.astro Subject: Re: Measurement of Light Years and Distance Message-ID: <476@astrovax.UUCP> Date: Sun, 21-Oct-84 14:15:46 EDT Article-I.D.: astrovax.476 Posted: Sun Oct 21 14:15:46 1984 Date-Received: Mon, 22-Oct-84 01:58:26 EDT References: <872@ihuxp.UUCP> Organization: Princeton Univ. Astrophysics Lines: 76 > Can anyone tell me how the distance in light years is measured? > How do we know how far something is? Does the spectrum shift > of light have something to do with it? This is a typical astronomy graduate student generals question. The distance to galaxies is measured by a rather shaky ladder of measurements. It goes something like this: a) distance to the nearest stars is measured with parallaxes. This is the apparent yearly back and forth of nearby stars relative to more distant stars caused by the earths revolution around the sun. b) distances to somewhat more distant clusters of stars is measured by a method called "statistical parallaxes". When the proper motions (apparent angular motions) and redshifts (i.e. velocities away from the observer) of a number of the stars in the cluster are combined with the assumption that these stars are moving parallel to each other in space, a distance can be derived. The statistical parallax method is used to measure the distance to the Hyades star cluster. c) Relative distances between different star clusters is computed by a method call "spectoscopic parallax". This really isn't a parallax at all, of course. This method assumes that the relationship between the real brightness (luminosity) and color of stars on the main sequence (the state in which stars spend most of their lives) is the same in both clusters. If the luminosity is the same then the difference in apparent brightness yields the ratio of distances. Using spectoscopic parallaxes other star clusters can be compared to the Hyades and their distances derived. d) Some of these clusters will have Cepheids. Cepheid stars have the property that the period in which they vary is related to their luminosity Again if a star's luminosity is known its apparent brightness yields its distance. Using star clusters with distances determined by the spectroscopic parallax method that contain cepheids, the relationship between Cepheid period and luminosity can be determined. Cepheids can then be used to determine the distance to the closest galaxies (like Andromeda, for example). e) Unfortunately the Cepheid method does not let one measure galaxies out to very far. Some other methods are applied to get further. For example the brightest HII region in the arms of a galaxy is assumed to be some constant luminosity. Another method is to assume that the brightest stars in a galaxy have constant luminosity. Evidence backing this up is all ad hoc. That is, for galaxies of known distance, the luminosities of these properties do not vary too much. f) due to the expansion of the universe, distant galaxies recede from us at a rate proportional to their distance. Thus their redshift can be used as a distance indicator. However there is also a random velocity component to galaxy motion which for nearby galaxies swamps the effect of distance on redshift. Thus redshifts can only be used as a distance indicator for distant galaxies. Using a combination of the methods of part e) one can get out to galaxies far enough away to calibrate the relationship between redshift and distance. All of this really should occupy the chapter of a book. I have grossly oversimplified and have completely ignored the corrections one applies to use these methods properly. I have also ignored other many methods which supply crosschecks (and controversy) to the "conventional" plan I have laid out (this article is already getting long enough). Some other astronomers on the net could describe some of these other distance measuring methods. It is the hope that the Space Telescope will tighten the distance scale by allowing each of these links in the chain to stretch across a greater range in distances, allowing, for instance, the measurement of real parallexes of star clusters containing Cepheids and the measurement of Cepheids in more distant galaxies. -- Bill Sebok Princeton University, Astrophysics {allegra,akgua,burl,cbosgd,decvax,ihnp4,noao,princeton,vax135}!astrovax!wls