Variable stars

Stars in certain regions of the HR diagram 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.

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. Astronomomers have found that their is a relation between the period of a Cepheid and its luminosity.

This enables astronomers to determine distances:

Find the period.
This gives the luminosity.
Measure the apparent brightness.
Determine the distance from the luminosity and brightness.
- We know
- So

This is useful because Cepheid variables are very bright, so we can see them a long way away.

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