fcm@ihuxp.UUCP (F.C. McAtee) (10/18/84)
>>Newsgroups: net.astro >>Subject: StarDate: October 16: A Supernova in Andromeda >> >>Scientists at the time believed that S Andromedae lay only a few >>THOUSAND light-years from Earth...... >> >>Since Andromeda was now known to be MILLIONS of >>light-years away..... >> 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? As you can see, I am not very knowledgeable on the subject. -- F. C. McAtee AT&T Consumer Products, Inc. ...ihnp4!ihuxp!fcm
td@alice.UUCP (Tom Duff) (10/19/84)
The problem of measuring astronomical distance always puzzled me as well. It turns out that it's all VERY indirect. The distance to nearby (less than a few parsecs) objects can be measured by parallax. Because there is a slight perspective shift when observing nearby stars from opposite ends of the earths orbit, we can effectively triangulate their positions by measuring their angular shift relative to the more distant (and therefore relatively stationary) stars. The use of parsecs as a unit of distance is an artifact of this means of measurement. One parsec is the altitude of an isosceles triangle whose base is the diameter of the earth's orbit and whose opposite-the-base angle is one second of arc. This is the distance to a star whose stellar parallax is one second. Now that I think of it, I'm not sure how the diameter of the earth's orbit would be measured. I guess you could use the parallax from simultaneous distant observations of planets to fix their orbits and then use Kepler's laws to compute the size of earth's orbit. (But, how did Kepler do the measurements to establish his laws in the first place?) Anyway, the next step is to notice that the period of certain variable stars (called Cepheid variables after the constellation in which the first one was noticed) whose distance we can measure by parallax is a function of their absolute brightness (i.e. the luminous flux at their surface.) Since we can measure the period and apparent brightness of Cepheid variables, we can use the fact that apparent brightness decreases by an inverse square law to compute the distance to many stars whose parallax is too small to measure. In fact, distances to nearby galaxies can be measured by observation of particularly bright Cepheids within them. This is how it was first established that the spiral nebulae were much farther away than any star in the galaxy, and that there were in fact other galaxies. (Imagine being the first person to recognize that the universe isn't just thousands of light-years wide, but millions!) The next step (due to Edwin Hubble?) is to notice that all the galaxies whose distance we can measure seem to be moving away from us with a velocity that varies directly with their distance (i.e. the universe is expanding.) We can measure their velocities by looking at the amount of Doppler shift of certain characteristic lines of their spectra. This is how the distances of the most distant objects in the universe are measured. As I said, this is an extremely indirect way to measure distance. Each step depends on lines of reasoning that may have flaws. For example, the diminution of stellar brightness might not be governed by an inverse square law. If space is filled, however tenuously, with small dust particles that absorb light, the attenuation will have an exponential term that will certainly dominate at stellar distances. Furthermore, those Cepheids that seem to be in other galaxies may not be related to them at all and may just but superimposed from our point of view (but see below.) Also, it was not known for many years that there are two different kinds (or `populations') of Cepheid variables, which obey different period/luminance laws. The best `plausible reasoning' argument I know for all of this is as follows: Stars and galaxies are not distributed uniformly across the night sky. The seem to be organized into recursive clusters (i.e. clusters of clusters of clusters of ...) Unless this clustering is some sort of cosmic practical joke on the human race, the measured distances to objects within these clusters should also cluster. And they do, so well that when astronomers notice an object within a cluster has a measured distance very different from its neighbours, it is taken as a sign that something very strange and worthy of detailed examination is afoot.
wls@astrovax.UUCP (William L. Sebok) (10/21/84)
> 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
gino@voder.UUCP (Gino Bloch) (10/27/84)
> 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? > F. C. McAtee Not a trivial question. In short: distances are measured by a long chain of difficult measurements, deductions, and extrapolations, where the term `noise-to-signal ratio' would be more appropriate than the usual term. At greater length: the first step is parallax, the apparent shift in position of objects viewed from opposite sides of the earth's ORBIT. This is good to a few hundred light years with decreasing accuracy. After that, knowledge of such things as brightness as a function of spectral class and other characteristics is used. The latter leads to distance to nearer galaxies, such as Andromeda (M31). Properties of galaxies as a whole lead to estimates for farther galaxies and clusters. Finally, the red-shift observed in this regime leads to extrapolation of distances to the farthest galaxies. The above just scratches the surface to give a flavor of what goes on. Read some good books on astronomy (I have no recommendations at my finger- tips, sorry). That will be fun, I think (but then I love astronomy). -- Gene E. Bloch (...!nsc!voder!gino)