A new way to measure cosmic distances

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 9 June, 2009

A rare and often overlooked type of star – cepheid variables – could allow astronomers to measure cosmic distances three times further than previously possible, to 300 million light years and beyond.

Cepheid variables – giant stars that pulse in brightness – have long been used by astronomers as reference points for measuring distances in the nearby Universe, but beyond 100 million light years from Earth, their signal gets lost among other bright stars. Now, a new study of a special type of cepheid – one that pulsates very slowly with a period of a month or longer – could allow astronomers to measure distances up to three times farther.

The ULP cepheids were located in nearby galaxies such as the Small Magellanic Cloud. Image: Credit: NASA, ESA and A. Nota (STScI/ESA).

There are already several methods for calculating the distances to stars, but astronomers often have to combine methods to indirectly determine a distance. The method of determining distances is often referred to as the cosmic distance ladder, with each new method a higher rung above another, but with errors adding up at each rung. Therefore, any single method that can skip rungs on the ladder is a prized tool for probing the Universe.

In a press briefing at the American Astronomical Society meeting in Pasadena on Monday, astronomers announced that a slowly pulsing bright class of cepheid might aid the effort to calculate the size and age of the Universe with greater precision than ever before achieved. Principal investigator Krzysztof Stanek’s new technique uses so-called “ultra long period cepheids” (ULP cepheids). “We believe they could provide the first direct stellar distance measurements to galaxies in the range of 50-100 megaparsecs (150 million - 326 million light-years) and well beyond that,” he says.

Until now, researchers have paid little attention to this type of cepheid, because it wasn’t certain how well they conformed to the period-luminosity relationship. But, says Stanek, the fact that they are larger and brighter than most stars means they stand out from the crowd, making them good distance markers after all.

The Ohio State group have started using the Large Binocular Telescope to search for more ULP cepheids in order to help measure the Universe and also learn more about how very massive stars evolve. Image: Aaron ceranski.

Stanek, with Ohio State doctoral students Jonathan Bird and Jose Prieto, uncovered 18 ULP cepheids from the literature, each located in a nearby galaxy, such as the Small Magellanic Cloud. Since the distances to these nearby galaxies are well known, they were used to calibrate the distances to the ULP cepheids with a 10-20 percent error – a rate typical of other methods that make up the cosmic distance ladder.

“We hope to reduce that error as more people take note of ULP cepheids in their stellar surveys,” Bird said. “What we’ve shown so far is that the method works in principle, and the results are encouraging.”

Astronomers have long suspected that ULP cepheids don’t evolve the same way as other cepheids, but in this study the Ohio State team found the first evidence of a ULP cepheid evolving as a more classical cepheid does, growing hotter and cooler many times over its lifetime. In between these cycles the outer layers of the star become unstable, which causes the changes in brightness. ULP cepheids are thought to go through this period of instability only once, and going in only one direction from hotter to cooler. But in the new study it turns out that one of the ULP cepheids – a star in the Small Magellanic Cloud dubbed HV829 – is clearly moving in the opposite direction, pulsing every 84.4 days compared with 87.6 days forty years ago. If the period is shrinking, the star itself is also shrinking and getting hotter.

The Ohio State group have started using the Large Binocular Telescope to search for more ULP cepheids in order to help measure the Universe and also learn more about how very massive stars evolve.