Cepheid variable stars as distance indicators.

Certain stars that have used up their main supply of hydrogen fuel are unstable and pulsate.

RR Lyrae variables have periods of about a day. Their brightness doubles from dimest to brightest.

Typical light curve for a Cepheid variable star.

Cepheid variables have longer periods, from one day up to about 50 days. Their brightness also doubles from dimest to brightest.

From the shape of the ``light curve'' of a Cepheid variable star, one can tell that it is a Cepheid variable. The period is simple to measure, as is the apparent brightness at maximum brightness.

Cepheids as distance indicators

Cepheids are important beyond their intrinsic interest as pulsating stars. Astronomers have found that there is a relation between the period of a Cepheid and its luminosity.

This enables astronomers to determine distances:

The same applies to RR Lyrae variable stars. Once you know that a star is an RR Lyrae variable (eg. from the shape of its light curve), then you know its luminosity.

Where did this period-luminosity relation come from?

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Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@uoregon.edu