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The Question

(Submitted November 02, 1998)

I am a student and very interested in astronomy. I would like to ask about the different ways in astronomy that we can measure the distances? I know that we can measure some distances by using the parallax method. Are there other ways in astronomy which we can determine the distance? And what are they?

The Answer

You have asked a very good question, since finding distances to far away objects is a very important, and often very difficult, part of astronomy. There are only a few stars whose distances can be measured using parallax. Other methods used to find distances to objects too far away to use a direct method use some estimation of the real brightness of the object. If you know how bright something is and compare to how bright it appears to be you can get a distance (since objects further away are fainter in a calculatable way).

Within our solar system, the most accurate way is to bounce radar off the nearby planets. In this way we can get very precise distances for the Moon, Venus, and Mars. Also, since we know the relative distances of the planets from each other, we can find the absolute distances of the ones further out by determining the absolute distances of the nearest planets..

Progressing to larger distances, for stars that are too far away to use the parallax method, it is common to use pulsating stars called Cepheid variables. The good thing about these stars is that they have a well-defined relation between their intrinsic brightness and the period with which they vary. (Bright ones go through one cycle of period light variations slower than do faint ones.) This relation has been well calibrated for nearby Cepheids, therefore if we observe Cepheids in distant galaxies we can get their distances by timing their periods. You may have read in the newspapers a few years back about how Wendy Freedman at Mount Wilson used Hubble telescope observations of Cepheids in the Virgo cluster of galaxies to revise the extra-galactic distance scale.

In really far away galaxies where you cannot even distinguish individual stars, one has to rely on more indirect methods. For instance, based on the shape and color of a given galaxy, you can guess-timate its intrinsic brightness, and then from its observed brightness get the distance.

There is more discussion of distance estimates on our web site. This is our local search engine, within which you can type something like: "distance determination".

http://imagine.gsfc.nasa.gov/cgi-bin/search/search.pl?swindex=ask_astro

John Cannizzo
J. Allie Hajian
("Ask an Astrophysicist")

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