If Delta Cephei were a star of the celebrity rather than the cosmic variety, today's tabloids would be filled with headlines exposing its secret relationship.

Researchers announced today in a report in The Astrophysical Journal that Delta Cephei, a star that astronomers have been studying for 230 years, has a hidden companion. It turns out that Delta Cephei is a binary star, meaning that it is one member of a pair of stars that orbit around the same center.

"I had been trying to quantify the precision at which the instrument I was using could measure Delta Cephei and then after about three years of measurements all of a sudden I realized that there was an additional signal that was completely unexpected," says Richard Anderson of Johns Hopkins University, who discovered the companion star, in an intervew.

Delta Cephei was the first of a group of stars known as Cepheids to be discovered, and one of the most well-studied of all stars. Its partner likely escaped detection over the past two centuries because of its "small mass," according to Anderson. It is only about 10 percent of the mass of Delta Cephei, but that is still several times more massive than our own sun.

Researchers commonly use Cepheids as sort of cosmic yardstick. Cepheids have unique properties that allow researchers to precisely calculate their intrinsic luminosity, or how much light they emit. The intrinsic luminosity of a star is similar to the wattage of a lightbulb in that its value is constant. But of course, if you hold a lightbulb right in front of your face it will burn your eye, yet if it is farther away you see it as much dimmer.

"So if you know how much light that lightbulb emits and you can see how bright it appears, then that tells you something about the distance," says Anderson.

Similarly, researchers use the intrinsic luminosity of Cepheids and their perceived brightness from Earth to calculate their distance. These distances then serve as a standard for measuring distances to other objects in the universe.

Right now, "we need even better distance estimates in the universe in order to better understand cosmology," Anderson says. "By having found this companion, we can help to improve the measurement of this distance and this will help improve the distance scale eventually."

Anderson emphasizes that, although the discovery of Delta Cephei's companion can help researchers measure cosmic distances even more precisely, "it is not going to revolutionize cosmology." Any improvements that it allows will be small, though useful, he says.

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