New Method Measures Astronomical Distances

A new method for measuring cosmic distances could pinpoint objects in the universe up to 300 million light-years away and beyond.

The just-realized technique was announced today at a meeting of the American Astronomical Union in Pasadena, Calif., and could help astronomers in their quest to track the universe's expansion rate.

Until now, astronomers have partly relied on giant stars called short-period cepheids that brighten and dim every few days to calculate distances to objects in the universe. This light blinking directly relates to the star's true brightness, and astronomers can compare the true and apparent luminosity (as seen from Earth) to determine the distance to that object. But beyond 100 million light-years from Earth, the stars' signals get lost among other bright stars.

(A light-year is the distance light will travel in a year, which is about 6 trillion miles, or 10 trillion km.)

The new method involves so-called ultra long period cepheids (ULP cepheids), which are much brighter and so stand out at even farther distances.

Krzysztof Stanek of Ohio State University and his colleagues searched the literature, finding records for 18 such ULP cepheids, ranging from 12 to 20 times the mass of the sun and located in nearby galaxies. The distances to these nearby galaxies are well known, so the astronomers used that knowledge to calibrate the distance to the ULP cepheids.

They found that they could use ULP cepheids to determine distance with a 10 to 20 percent error, a rate typical of other methods used for measuring cosmic distances. The researchers hope to reduce that error as more ULP cepheids are recorded.

Now, the researchers are using the Large Binocular Telescope in Tucson, Ariz., to look for more ULP cepheids. Stanek said they've found a few good candidates in the galaxy M81, but those results have yet to be confirmed.

The research was funded by the National Science Foundation.