Sunspots are one of the ways we can measure the activity level of the Sun. Generally, the more sunspots we observe, the more active the Sun is. We’ve been tracking sunspots since the early 1600s, and we’ve long known that solar activity has an 11-year cycle of high and low activity. It’s an incredibly regular cycle. But from 1645 to 1715 that cycle was broken. During this time the Sun entered an extremely quiet period that has come to be known as the Maunder Minimum. In the deepest period of the minimum, only 50 sunspots were observed, when typically there would be tens of thousands. We’ve never observed such a long period of quiet since, and we have no idea why it occurred.
The problem with trying to understand solar cycles is that you have a sample size of 1. You can track sunspots all you want, but all that tells you is how one particular star behaves. You can never be sure if the Sun’s behavior is typical for a star or unusual. The only way to find out is to look at the cycles of other stars.
Ideally, astronomers would love to count the sunspots, or starspots, of other stars over time, but this isn’t easily done. We can’t observe the surface details of Sun-like stars, and starspots are typically small compared to the size of a star, so they barely decrease the brightness of a star over time. Random fluctuations in solar brightness easily overwhelm any change in brightness due to starspots. But there is a way to measure stellar activity indirectly.
We know that sunspots correlate to the magnetic activity of the Sun. The more magnetically active the Sun is, the more sunspots we typically see. It turns out that this magnetic activity can be measured using spectral lines. In the ultraviolet range, there are two particular lines known as H and K that are affected by magnetic activity. So if you measure the H-K lines of a star over time, you can measure the rise and fall of a star’s activity. It just so happens that Mount Wilson Observatory started measuring the H-K lines of about 400 stars in 1966, and measured them for three decades.
In a recent study, a team of astronomers combined the Mount Wilson observations with more recent data from observatories such as Keck to measure the activity cycle of 59 stars across 50 to 60 years. They were able to confirm the cycles of 29 of them, by observing at least two full cycles. No small feat given that some of the stars have 20-year cycles. They also found that some stars have no cycles at all. But one star was particularly interesting.
Known as HD 166620, the star typically has a 17-year activity cycle. But since 2003 it’s been quiet. This is the first example of another star experiencing a long-term quiet period similar to the Maunder Minimum. HD 166620 is about 80% of the Sun’s mass, and about 6 billion years old. A bit smaller and older, close enough in size and age that the dynamics of the two stars should be similar.
It isn’t clear why HD 166620 has entered a quiet period, but further observations will be able to measure its magnetic activity in greater detail. In time we may also observe the star as it leaves its quiet period and returns to a 17-year cycle. It could provide the data we need to understand why the Sun was once quiet for decades.
Reference: Baum, Anna C., et al. “Five Decades of Chromospheric Activity in 59 Sun-like Stars and New Maunder Minimum Candidate HD 166620.” The Astronomical Journal 163.4 (2022): 183.