Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!swrinde!emory!athena.cs.uga.edu!mcovingt From: mcovingt@athena.cs.uga.edu (Michael A. Covington) Newsgroups: sci.electronics Subject: Re: How to tell distance of Stars Message-ID: <1991May19.045844.8211@athena.cs.uga.edu> Date: 19 May 91 04:58:44 GMT References: <8937@crash.cts.com> <17660159@hpfcdj.HP.COM> <1991May15.114054.9013@nmrdc1.nmrdc.nnmc.navy.mil> Organization: University of Georgia, Athens Lines: 38 There's some confusion here. The main ways to measure the distance of stars are: (1) For nearby stars, parallax. View the star with the earth at one side of its orbit, and again with the earth at the other side of its orbit 6 months later. The star shifts against the background. This is sufficient to give us the distances of a few hundred or maybe thousand of the nearest stars. (2) By studying data obtained from (1) it was possible to determine the true light output of the stars (computed from apparent brightness and distance). It was further determined that for a special class of stars called Cepheid variables, the period of variation is related to the true light output. (3) Some Cepheids in other nearby galaxies can be seen and their apparent brightness and period measured. From this the distance of the galaxy can be computed. (4) For virtually all the galaxies whose distance can be measured or estimated by (3) and related methods, it turns out that recessional velocity (as indicated by redshift) is proportional to distance (Hubble's Law or hypothesis). So from the redshift of a distant galaxy we can ESTIMATE its distance. Notice that (4) is much less certain than the first 3. For some objects, such as the quasars, we know *only* that they have a huge redshift, and we *assume* that they are therefore a great distance away. Some astronomers, notably Halton Arp, dispute this; they say that the redshift is simply unexplained, or possibly due to idiosyncratic motion not related to the overall expansion of the universe. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------