A star that is visible to the naked eye in the night skies of Southern Arizona has properties nearly identical to the sun's, researchers announced Thursday.

A study done at the Fairborn Observatory in the Patagonia Mountains and at the Lowell Observatory in Flag-staff found that 18 Scorpii, a star that is more than 270 trillion miles away, is "just about as close to a twin of the sun as you can find," said Jeffrey Hall, an astronomer at Lowell who has been observing the star for more than 10 years.

Hall said that solar variations have some effect on climate, and that researchers are trying to understand how much the sun may have varied during the last couple of hundred or thousand years by observing solar twins.

He said solar variations have probably not been a dominant factor in the global warming of recent decades, but may account for some of it and that understanding the long-range variability of the sun may affect future climate models.
"What we're trying to do in this long-term project is ultimately learn more about our own sun and its impact on the planet," said Gregory Henry, a research astronomer at Tennessee State University who analyzes the data from the Southern Arizona observatory.

While this study focuses on the star and its implications within our solar system, Hall said that 18 Scorpii is a good target for planet-search projects and that researchers have their eyes on stars similar to the sun because if they have Earth-like planets orbiting them, the climates on those planets have the potential to be similar to Earth's.

The researchers are evaluating hundred of stars, but Hall said comparing some of them to the sun is like comparing apples to oranges, while comparing 18 Scorpii to the sun is comparing apples to apples.

"What was surprising was that before we looked at its behaviors, its properties looked like a close solar twin," said Henry of the star's age, mass, chemical composition and temperature. "Then when we studied it, we not only found that it was structurally like the sun, but that it behaved very much like the sun."

Hall said it's unusual to study one star for so long, but necessary because the solar cycle - when sun spots and solar flares rise and fall - is about 11 years long. This is one difference between 18 Scorpii and the sun because the twin's activity cycle lasts about seven years.
The most recent findings of this long-term research program are scheduled to appear in the May issue of The Astronomical Journal.