This star's activity cycle (the phenomenon that causes the periodic rise and fall in the number of sunspots on the Sun) is about seven years long, compared to about 10 years for recent solar cycles.

However, 18 Scorpii exhibits a suite of other characteristics that are essentially the same as the Sun's. These include mass, temperature, chemical composition, and luminosity.

"We found that where the Sun's overall brightness varies by typically 0.1 percent over its activity cycle, 18 Sco likewise varies by about 0.09 percent, which is effectively the same," said Jeffrey Hall of Lowell Observatory. "And just like the Sun, 18 Scorpii gets brighter as it gets more active."

Using complementary observations from Lowell Observatory's Solar-Stellar Spectrograph (SSS) and Tennessee State's Automatic Photometric Telescope (APT) program, the astronomers have measured output from one star that seems indistinguishable from the Sun in all respects: 18 Scorpii, or HD 146233. Identified as a close solar twin in 1997, 18 Scorpii exhibits behavior very similar to the Sun's, as well as an activity cycle similar to the Sun's.

"Since about 1970, solar variations probably account for only a small fraction of observed global warming," said Wes Lockwood of Lowell Observatory. "However, earlier in the 20th century and in the pre-industrial era, natural forces, including solar variations, undoubtedly induced changes in terrestrial climate, and will continue to do so in addition to the impact of human activities. What we really want to know is what is the Sun capable of."

Scientists have only been observing solar variations directly from space since 1978. To estimate what the Sun may have been doing earlier in human history, particularly during periods of unusual lulls in its activity such as the Maunder Minimum of 1645-1715, researchers have used "proxy" indicators, indirect records of solar variations preserved in the terrestrial record. (The Maunder Minimum was a time period noted by an absence of sunspots that coincided with the coldest part of the Little Ice Age).

One method by proxy involves measuring concentrations of certain isotopes such as Beryllium 10 or Carbon 14. Changes in the strength of the solar wind modulate the flux of cosmic rays entering Earth's atmosphere, which in turn, affects the production rate of these isotopes. By measuring the changing isotope concentrations in terrestrial core samples, scientists reconstruct estimates of likely solar activity.

Another way to constrain what the Sun might have done during its activity maxima and minima is to observe what the most nearly Sun-like stars do as their activity varies. The current research is the most recent finding as part of this unique long-term program and is in press in The Astronomical Journal: "The Sun-Like Activity of the Solar Twin 18 Scorpii."

The spectroscopic observations for this study were obtained with the Solar-Stellar Spectrograph (SSS), operated at the 1.1-meter John Hall Telescope at Lowell Observatory's dark sky site near Flagstaff, Arizona. In regular operation for 12 years, the spectrograph allows observations of Sun-like stars as well as the Sun itself.

Meanwhile, co-investigator Gregory Henry of Tennessee State University acquired brightness measurements of 18 Scorpii using an 0.8-meter automatic photometric telescope (APT) at Fairborn Observatory in southern Arizona's Patagonia Mountains.

Observations from this study serve as a useful guide in reconstructing the evolution of solar luminosity, particularly with regard to the Sun's behavior during its occasional quiescent periods. "We know that the solar twin 18 Scorpii has luminosity variations remarkably similar to those of the Sun," said Hall.

"We'll now continue to study additional stars, including some that appear to be in Maunder minimum-like periods, and study their brightness changes. If we can reach a point where we are reasonably confident that the brightness variations exhibited by Sun-like stars really are representative of what the Sun does or doesn't do, we can place some useful limits on how much the Sun did or didn't vary in recent centuries or millennia. In turn, that helps constrain how much solar forcing we can justifiably put into climate models."

The result of this study demonstrates that 18 Scorpii, the one star that seems to best meet all the criteria of a solar twin, is practically a solar clone in its brightness variations over the past 10 years. Though over 270 trillion miles away, 18 Scorpii is providing key insights into the long-term variability of our own star - and the magnitude of its influence on terrestrial climate change.

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