Distances to Galaxies | Cepheid Variables | How to Look for Cepheid Variables
 | The Period-Luminosity Relation | Find the Cepheids in M100
2. Cepheid Variables
Astronomers first determine the distance to nearby stars as accurately as possible, then use bootstrapping techniques to estimate distances to successively more distant celestial entities. This bootstrapping technique is often called the ``distance ladder.'' For a very nice (and concise) summary of the numerous methods used, see Ned Wright's ``The ABC's of Distances'' page. We will focus on only one method, Cepheid variable stars.

Variable stars are a type of star that we have not discussed thus far. As the name implies, these are stars that vary in brightness. In the case of Cepheid stars these changes in brightness are brought about by the star's contraction and expansion, or pulsations.
The pulsations of a Cepheid variable are periodic, meaning they repeat on a regular basis. The pulsation periods range from about a day to about two months.

Why are Cepheids so valuable as distance indicators? It is well known that the apparent brightness of an object decreases as the square of its distance. As you have learned, if you know both the apparent and intrinsic brightness (or luminosity) of a star, you can calculate how far away it must be. While it is easy to measure the apparent brightness of a star, the luminosity must be determined indirectly. Cepheids are special in that the period at which Cepheids pulse is related to their average luminosity. This is known from Cepheids in nearby clusters whose distances we saw how to find in the last lab.  More luminous Cepheids pulse more slowly. By measuring the period of a Cepheid we can calculate its luminosity. Therefore, by comparing the luminosity with the apparent brightness, we can determine its distance. Cepheids are relatively bright stars, which also makes them desirable distance indicators--brighter stars can be seen to greater distances.

The Period-Luminosity Relation was discovered by a deaf woman named Henrietta Leavitt who was working as a research assistant at Harvard.  Leavitt was part of a team of women known as "computers" because all they did was make computations. In the days before modern computers, all the scientific calculations had to be done by hand. Professor Edward Pickering of Harvard insisted on hiring women to do this work, saying that women were more efficient than men. These women became known as "Pickering's Harem." Leavitt used her access to all this data to gain insights into variable stars. In 1912, she made the connection between a Cepheid variable's pulsation period and its average luminosity, allowing astronomers everywhere to measure distances that were unheard of before.

For more information on Henrietta Leavitt and other women in astronomy, click on the following links: Leavitt Bio, "Miss Universe" article.
                             

         Henrietta Leavitt                                            The Harvard Women, with Frank Hinkely and Edward King (Leavitt is sixth from the left)