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