The Cosmic Distance Scale
The following series of web-based exercises takes students on an interactive
tour measuring cosmic distances. The exercises measure a succession of
distance ratios, systematically stepping out in scale from the familiar
world of everyday objects to larger and larger things, ultimately exploring
the expansion rate of the universe as a whole.
- The first lab begins in the appropriate
historical context,
retracing the steps used by Eratosthenes and Aristarchus for
finding the size of the Earth, the size of and distance to the Moon,
and the size of and distance to the Sun, all without telescope
technology. The measurement scripts no longer work for this version of
this lab. Please check our
Clearinghouse of labs for similar exercises.
- The second lab studies astronomical parallax, simulating
the apparent motion of stars as the Earth goes around the sun
in its orbit, a technique which first gave the distances to stars (in terms
of the Earth-Sun distance) in the nineteenth century. Because
the angular motion is very tiny this method can
access only nearby stars but with satellite data (or using other
intermediate steps) it can reach far enough
to calibrate the brightnesses of
a particular class of stars, Cepheid variables, which are in
turn bright enough to measure the distances to entire galaxies
outside the Milky Way and thereby calibrate the extragalactic
distance scale. Not tested for a long time.
- The third lab constructs a Hubble diagram of
galaxy distances and redshifts; coupled with the Cepheid
distance calibration, this yields a measurement of Hubble's constant
and an estimate of the expansion rate and age of the universe.
A similar Hubble diagram extended to much higher redshift with
Type Ia supernovae hints at departures from a linear Hubble law,
a consequence of the evolution of the expansion over cosmic time.
This page is maintained by Dr. Ana Larson .
Suggestions, comments, and questions should be sent to:
larson@astro.washington.edu
Last update: 02/16/09
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