A team of astronomers from Chile and Poland have made the most accurate measurement yet of a Cepheid, which is a member of a class of pulsating, variable stars that are larger and brighter than the Sun but that have remained a mystery since they were first discovered in the 18th century. The researchers say the findings will help determine the mass of such stars. The study was funded in part by the OGLE ('The optical gravitational lensing experiment') project, which was supported by the European Research Council.
Classical Cepheid Variables or simply Cepheids expand and contract like other stars, taking anything from a few days to months to complete their cycle. The time taken to brighten and grow fainter again is longer for stars that are more luminous yet shorter for the dimmer ones. This precise relationship makes the study of Cepheids one of the most effective ways to measure the distances to nearby galaxies and from there to map out the scale of the whole Universe. Despite their importance, however, Cepheids are still not fully understood, and predictions of their masses derived from the theory of pulsating stars have, until now, remained highly inaccurate.
A team of astronomers, led by Grzegorz Pietrzynski from the Universidad de Concepción in Chile and the Obserwatorium Astronomiczne Uniwersytetu Warszawskiego in Poland, measured the mass of a Cepheid with an accuracy far greater than any earlier estimates using the HARPS spectrometer — which uses precise radial velocity to make measurements — on the 3.6-metre telescope at the European Southern Observatory's (ESO) La Silla Observatory in Chile. 'This new result allows us to immediately see which of the two competing theories predicting the masses of Cepheids is correct,' enthused Dr Pietrzynski.
To resolve this mystery concerning the star's mass, astronomers needed to find a double star containing a Cepheid where the orbit could be seen edge-on from Earth. In these cases, known as eclipsing binaries, the brightness of the two stars dims as one component passes in front of the other, and again when it passes behind the other star. In such pairs astronomers can determine the masses of the stars to a high degree of accuracy. Unfortunately neither Cepheids nor eclipsing binaries are common, so the chance of finding such an unusual pair seemed very low.
However, the researchers found themselves with a lucky break, as Wolfgang Gieren from Universidad de Concepción explained: 'Very recently we found the double star system we had hoped for among the stars of the Large Magellanic Cloud. It contains a Cepheid variable star pulsating every 3.8 days. The other star is slightly bigger and cooler, and the two stars orbit each other in 310 days. The true binary nature of the object was immediately confirmed when we observed it with the HARPS spectrograph on La Silla'.
The observers carefully measured the brightness variations of this rare object as the two stars orbited and passed in front of one another. They also used HARPS and other spectrographs to measure the motions of the stars towards and away from the Earth — both the orbital motion of both stars and the in-and-out motion of the surface of the Cepheid as it swelled and contracted.
This very complete and detailed data allowed the observers to determine the orbital motion, sizes and masses of the two stars with very high accuracy — far surpassing what had been done before for a Cepheid. The mass of the Cepheid is now known to about 1% and agrees exactly with predictions from the theory of stellar pulsation. However, the larger mass predicted by stellar evolution theory was shown to be significantly in error.