London, Nov 25 (ANI): Astronomers have measured the mass of a Cepheid with accuracy far greater than any earlier estimates - allowing them to immediately see which of the two competing theories predicting the masses of Cepheids is correct.
Grzegorz Pietrzynski Pietrzynski (Universidad de Concepcion, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland) led the study.
Classical Cepheid Variables, usually called just Cepheids, are unstable stars that are larger and much brighter than the Sun. They expand and contract in a regular way, taking anything from a few days to months to complete the cycle. The time taken to brighten and grow fainter again is longer for stars that are more luminous and shorter for the dimmer ones.
This property makes them 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.
Unfortunately, despite their importance, Cepheids are not fully understood. To resolve this mystery, astronomers needed to find a double star containing a Cepheid where the orbit happened to be seen edge-on from Earth.
In such pairs astronomers can determine the masses of the stars to high accuracy.
"Very recently we actually found the double star system we had hoped for among the stars of the Large Magellanic Cloud. 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," said Wolfgang Gieren.
The mass of the Cepheid is now known to about 1percent 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.
The team hopes to find other examples of these remarkably useful pairs of stars to exploit the method further. They also believe that from such binary systems they will eventually be able to pin down the distance to the Large Magellanic Cloud to 1 percent, which would mean an extremely important improvement of the cosmic distance scale.
The study appears in the 25 November 2010 edition of the journal Nature. (ANI)