The M67 'solar laboratory'
The M67 'solar laboratory'

Sustained drops in the energy output of the Sun could be more common than modern experience suggests, according to an international team of astronomers that has studied the activity of a number of Sun-like stars. The results could mean that past changes in global temperatures are more likely to be related to variations in solar activity than previously thought, and could allow us to predict similar changes in future.

Our Sun has a well documented cycle of magnetic activity with a period of about 11 years. This cycle can be observed as a rise and a fall in the number of sunspots and a variation of about 0.15% in the power output of the Sun. Direct observations of sunspot numbers stretch back about 400 years, but the amount of carbon-14 taken up by living things drops during periods of high activity and this can be used to chart solar activity back several thousand years.

Solar laboratory

In 2006 Mark Giampapa of the National Solar Observatory, Arizona, and colleagues used the Very Large Telescope (VLT) in Chile to measure the activity levels of 60 stars in the M67 galactic cluster – nearly 3000 light-years away. "M67 is an ideal solar laboratory in the sense that it has the same age as the Sun and virtually identical chemical abundances," Giampapa told physicsworld.com.

Sunspots cannot be observed directly on distant stars, so the team focused on certain emission lines in the spectra of light emitted by the stars. The width of these lines can be related to the level of magnetic activity in the star, allowing the team to conclude that 7–12% of the stars exhibited activity beyond a typical solar maximum and 17% were below a typical solar maximum.

Now, Giampapa and Ansgar Reiners of Germany's George August University have made further measurements of these high-activity stars and found them to be less like the Sun than first thought. "Those stars that are more active than the Sun at the maximum of its sunspot cycle appear to be more rapidly rotating, which naturally gives rise to enhanced activity," explained Giampapa. In some cases the stars were rotating twice as a fast as the Sun and so are unlikely to be a true representation of our star.

Atypical feature?

The low activity levels cannot be so easily explained and might represent a typical feature in the activity of a Sun-like star. One well documented example of such a period of unusual solar quiescence was the Maunder Minimum of the 17th century, when a significant drop in sunspot numbers coincided with a recorded drop in global temperatures. If Giampapa's research rings true, it could mean that the Sun spends a significant amount of its time in a Maunder Minimum-like state – and it might reveal how likely we are to experience such periods of cooling in the future.

"This interpretation of the Sun is also supported by the terrestrial carbon-14 record showing that the abundance of this isotope is consistent with lower solar activity," Giampapa.

However, Lyndsay Fletcher, a solar physicist at the University of Glasgow not involved in the research, suggests being a little more cautious about how to interpret the findings. She described the result as "genuinely interesting" and that "the evidence is there but it's subtle and it's hard to know how to relate it to the Sun."

Not absolutely identical to the Sun

The problem might lie in the fact that the M67 stars are not absolutely identical to our Sun. "These stars are solar type but they're not exactly the same class. They range from G2 to G6 and the Sun is a G2 star," Fletcher explained.

"It's very challenging research and it looks like they've done an extremely good job but I think there is more work that needs to be done to see whether this interesting suggestion actually bears out," she said.

The research has been accepted for publication in Astrophysical Journal.