spaceSpace and Physics

This Star Could Be The Key To Understanding The Sun's Behavior


Stephen Luntz

Freelance Writer

clockJan 8 2018, 15:33 UTC

The sunspots in this image are obvious, but over on the right are particularly bright spots. More metals in a star increase the brightness difference between these areas and the ordinary star, in turn increasing the variability in output. NASA/SDO

Due to the Sun's importance to life on Earth, it is hardly surprising astronomers are desperate to understand its behavior. However, there is only so much you can learn from a single example, particularly one so large and powerful it can't be changed by any puny experiment we might run. Fortunately, astronomers have found one star that can serve as a comparison, being almost exactly like the Sun in all ways but one.

Like other middle-aged stars, the Sun is fairly stable. Nevertheless, it has an 11-year cycle, during which sunspot and solar flare activity rises and falls almost to nothing, accompanied by a 0.1 percent variation in solar output. Nor is each cycle the same. During the 17th century, Maunder Minimum sunspots largely disappeared for more than half a century. Our lack of understanding of why this occurred inhibits our capacity to predict the intensity of future cycles.


The ideal experiment changes one feature at a time, leaving everything else the same. The astronomical equivalent would be to find stars that match the Sun on all but one parameter. One suitable model has been identified; HD 173701 is astonishingly similar in mass and age to the Sun, and only a tiny amount smaller and cooler, but has a very different composition. At 120 light-years away, a tiny fraction of the width of the galaxy, it is relatively easy to study.

Stars are mostly hydrogen and helium, at least until the end of their lives when they have fused their raw materials into other elements. However, stars that were not the first to form in their part of the universe incorporate heavier elements from their predecessors, which astronomers call metals, in the process of formation. The proportion of metals in a star's composition is one of the most important things to know about it. HD 173701 has twice the metal content as the Sun.

Published in The Astrophysical Journal, this new study is the most detailed analysis of any Sun-like star over its entire stellar cycle, using observations both from ground-based telescopes and the Kepler Space Craft. The paper reveals HD 173701 takes 7.4 years, rather than 11 to go from peak to peak in activity.

The authors, led by Dr Christoffer Karoff of Aarhus University, report HD 173701's magnetic field varies by nearly twice as much as the Sun's. The authors attribute this to the way additional metals interfere with light radiating from a star's center and increase the difference between brighter and darker parts of the stellar surface. Both of these amplify stellar variability.


The tiny solar variation does not affect Earth's climate anything like as much as fossil-fuel interest want us to believe. Nevertheless, there is a small effect and understanding the solar cycle will help us anticipate this.

spaceSpace and Physics
  • tag
  • sunspots,

  • solar cycle,

  • solar twin,

  • solar variability,

  • metalicity