Cepheid variables are a class of stars with a luminosity that changes as a function of time. Cepheids are well-studied astronomical bodies—they were first discovered in 1784—and subsequent work has been able to identify a precise relationship linking their luminosity and pulsation period. Because of this well-defined relationship, they have been employed as standard candles and used to accurately measure intergalactic distances. In fact, it was calculations based on Cepheids that led Hubble and Humason to formulate Hubble's law and conclude that the Universe was expanding.

Despite all that we know, however, there is a problem when it comes to Cepheids. Calculations of their mass based on two different stellar theories leads to two very different numbers. In fact, using the theory of pulsating stars will result in a mass prediction 20 percent less than that arrived at using the theory of stellar evolution. This problem has troubled astrophysicists since the 1960s.

To resolve this discrepancy, astronomers need an independent measurement of the mass of a Cepheid to see which theory—if either—is correct. Prior attempts at mass measurements have given estimates to within only 15 to 30 percent accuracy, not enough to resolve the discrepancy between the two theories. An independent measurement, to get the right precision, requires a Cepheid to be one half of a pair of binary eclipsing stars that orbit each other in a plane that would be seen edge-on from Earth. Since neither Cepheids or binary eclipsing stars in such orbital arrangements are common, a combination of both is exceedingly rare.

This week's edition of Nature contains a letter reporting on just such a rare find. OGLE-LMC-CEP0227 is a stellar pair in the Large Magellanic Cloud that orbits us in the right orientation and contains a Cepheid star. According to the paper, the Cepheid variable star pulsated every 3.8 days and the two orbited each other every 310 days.

By measuring the orbits of both stars, as well as the contraction-and-expansion of the Cepheid over the entire orbit, astronomers were able to determine the Cepheid's mass to within 1 percent error. The observed mass matched exactly with that predicted by the theory of stellar pulsation; the larger mass predicted by the theory of stellar evolution was shown to be largely in error.

Future work by the group will involve looking for more such binary systems. With more data on hand, they believe they can much more accurately pin down the distance between Earth and the Large Magellanic Cloud, a result that will greatly increase the accuracy of our cosmic-scale distance estimates.

Nature, 2010. DOI: 10.1038/nature09598 (About DOIs).