Astronomers made the first direct, conclusive measurement of a pulsating Cepheid stars size as it swelled and shrank a method that promises better estimates of the universes age, size and rate of expansion.

Using Variable Stars to Measure Distance

Three videos from NASA illustrate how astronomers use cepheid variables to judge vast distances. Cosmic Distances : Demonstration of how objects at different distances appear to shrink. Scientists use this knowledge, and the size of cepheid variable stars, to judge distances.

Cepheid Pulses : The regular pulsing of cepheid variable stars lets scientists pick them out from other stars.

Cepheid Variable in M100 : A regularly-pulsing cepheid variable star in the outskirts of galaxy M100 lets astronomers determine how far away the galaxy is.

The rate at which Cepheids pulsate is closely related to their true brightness or luminosity. By comparing the supergiant stars apparent brightness (as seen from Earth) with their true brightness, astronomers can calculate their distance. That means Cepheids serve as "cosmic yardsticks" to measure distances in the universe.

Galaxy M100 (below) is the home of a pulsating Cepheid variable star, located in the center of the three top images.

By making the first direct measurement of a Cepheid stars changing diameter, and thus its distance, "we believe can get this yardstick properly measured" by calibrating the relationship between a Cepheids pulsation rate and brightness, Kulkarni said. "This is a first step in nailing down the Hubble Constant."

"Over the next few years, we will begin having a much better idea of the expansion rate and age of the universe," said Ben Lane, a Caltech planetary sciences graduate student.

The findings were published September 28 in the journal Nature by Kulkarni, Lane, former Caltech graduate student Marc Kuchner, scientist Andrew Boden and Michelle Creech-Eakman, a postdoctoral scholar at NASAs Jet Propulsion Laboratory (JPL) in Pasadena, California.