Accurately weighing a pulsating giant

Accurately weighing a pulsating giant: "

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).