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50-Year Mystery of Strange Pulsating Stars Solved

Since the 1960s, a class of special pulsating stars has stymied scientists attempting to weigh them. The two main ideas for how to measure these stars' masses have so far produced vastly different results.
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Since the 1960s, a class of special pulsating stars has stymied scientists attempting to weigh them. The two main ideas for how to measure these stars' masses have so far produced vastly different results.

Now astronomers report the discovery of a peculiar star system that may provide a Rosetta stone of sorts, allowing researchers to finally determine which method of weighing the stars is accurate.

These pulsating stars, called Cepheid variables, hold a place of honor in astronomy because they allow scientists to measure intergalactic distances. Cepheids pulsate becoming brighter and dimmer on a steady schedule because their gas heats and cools, and expands and contracts, in a cycle.

The history of Cepheid-weighing

For years, scientists have determined the masses of Cepheid variables using two opposing theories, with wildly different results. One method is based on the theory of how stars evolve over time, while the other is based on physical properties of variable stars.

Calculations from the first method suggest these stars weigh 20 to 30 percent more than calculations from the second process suggest. Until now, astronomers have been unable to reconcile the difference.

"We have measured the mass of a Cepheid with an accuracy far greater than any earlier estimates," said team leader Grzegorz Pietrzyński of the University of Warsaw in Poland. "This new result allows us to immediately see which of the two competing theories predicting the masses of Cepheids is correct.

A unique system

The team found a special Cepheid variable star known as OGLE-LMC-CEP0227 that is rotating around a companion star in such a way that the two take turns passing in front of each other from Earth's point of view.

This system, known as an eclipsing binary, is different from standard binary systems because of its angle toward our planet: The stars' orbits occur in a plane along our line of sight. As one star blocks the other, the background star's light is obscured, and the change in brightness allows astronomers to learn a great deal about both stars.

Such a system is incredibly rare the new pair of stars were discovered outside our Milky Way galaxy in a neighboring galaxy, approximately 160,000 light years away, called the Large Magellanic Cloud.

Using the European Southern Observatory s telescope at La Silla Observatory in Chile, as well as other telescopes, the research team determined a precise mass for the two individual stars, as well as their sizes and orbital motions. The measurements provide a value for the Cepheid star's mass with an accuracy of 1 percent  a huge improvement over previous methods.

The new calculations support the mass measurements produced by the stellar pulsation theory.  Predictions obtained by the theory of stellar evolution, on the other hand, differ significantly from the new findings.

Standard candles

Unlike the sun, Cepheid variables brighten and dim in a predictable cycle ranging from a few days to a few months. The well-defined relationship between their brightness and their pulsation period means that by timing their light pulsations, astronomers can determine how bright the stars are intrinsically.

Scientists then compare the star's intrinsic brightness to its apparent brightness that is, how bright it appears to be from our vantage point here on Earth. Because a star will appear dimmer the farther away we are from it, astronomers can obtain a reliable measurement of a Cepheid star's distance from this brightness measurement. This useful feature of Cepheids has earned them the nickname " standard candles."

This allows astronomers to use them to gauge the distance between galaxies and the rate of expansion of the universe.

The researchers hope the newfound ability to determine these stars' true masses could help make these cosmological measuring sticks even more useful.

The discovery is detailed in the Nov. 25 issue of the journal Nature.