Nearby star’s midlife crisis illuminates the future of our own Sun

2022-08-10

Soon after European astronomers developed the first telescopes at the start of the 17th century, they observed dark spots speckling the Sun’s surface. They also handed their modern successors a mystery. From about 1645 to 1715, the spots, now known to be indicators of solar activity, all but disappeared. Gathering sunspot counts and other historical observations, astronomer John Eddy concluded nearly 50 years ago that the Sun had essentially taken a 70-year nap, which he called the Maunder Minimum after an astronomer couple who had previously studied it.

Now, it appears the Sun is not the only star that takes long naps. By building a decades-long record of observations of a few dozen stars at specific wavelengths that trace stellar activity, a team of astronomers has identified another star going through its own Maunder Minimum period. “I am more convinced this is a Maunder Minimum star than anything else I’ve seen,” says Jennifer van Saders, an astronomer at the University of Hawaii, Manoa, who was not involved in the discovery.

The finding, reported in a preprint last month on arXiv, could help explain what triggered the Sun’s strange behavior 400 years ago and suggests more such episodes are likely. “This is the way to study the past and future of the Sun,” van Saders says. She adds the discovery supports a theory she and colleagues have advanced: that such events are an occasional symptom of a critical transition in the magnetic field of Sun-like stars about halfway through their lifetime—a midlife crisis of sorts. Some astronomers speculate that the Sun’s transition helped favor the emergence of life on Earth, and that searching for stars in a similar stage could help identify other solar systems conducive to complex life.

Scientists have known for decades that our Sun’s activity surges and ebbs on a roughly 11-year cycle, which corresponds to how often its magnetic poles flip their orientation. During a solar maximum, sunspots proliferate, marking weak points in the magnetic field, where plasma from the Sun’s atmosphere can lash out in violent loops. Astronomers have spotted young Sun-like stars with similar cycles, and older ones that have totally stable activity. But no one had spotted a cycling star suddenly turning flat.

In 2018, as part of undergraduate research at Pennsylvania State University, University Park, Anna Baum set out to combine observations of the telltale wavelengths from 59 stars taken by the Mount Wilson Observatory and the W. M. Keck Observatory to produce a 50-year chronology of star evolution. During a 7-year gap in data while Keck was upgrading a detector, one star appeared to show a drastic shift. Its activity went from cycling over a 17-year period to being virtually flat, and it’s stayed that way for the past 18 years.

Baum thought at first she’d made an error; perhaps the observatories were even looking at two different stars. But earlier this year, her colleagues came across additional observations that filled in the data gap, capturing the star’s emissions as it switched from active to quiet. The recovered data set “hit the jackpot,” says Jacob Luhn, an astronomer at the University of California, Irvine, and lead author on the preprint.

The discovery reinforces one popular theory about why these extended quiescent periods happen. Stars spin more slowly with age because their solar winds act as “magnetic brakes,” like a child sticking out their arms while revolving in a chair. In 2016, van Saders and her colleague Travis Metcalfe of the White Dwarf Research Corporation noticed that at some point, stars stop hitting the brakes and their velocity stabilizes—a shift, they proposed, that stems from a change in the stars’ magnetic field. Then, last year, Dibyendu Nandi and colleagues at the Center of Excellence in Space Sciences India pinned down the idea with computer simulations that linked the stabilizing of the spin rate to a weakening magnetic field. During this transition, as the star heads toward a “lazy” state in which its activity is flat rather than cycling, random perturbations in its magnetic field can result in temporary cycle shutoffs like the Maunder Minimum, Nandi says.

The theory predicts that this transition state will emerge in middle-aged stars—just like our Sun and this newly identified napping star. “Everything about this discovery has actually corroborated what we’ve been talking about for the last 5 years,” Metcalfe says. “We definitely knew about stars that were not cycling, but we didn’t know how they got there—this is like the missing link in that evolutionary picture.”

Our Sun’s magnetic transition probably began around the same time life on Earth first crawled out of the sea, and that may be no coincidence, Metcalfe suggests. The incoming particles and radiation from active stars damage DNA and promote mutations, speeding evolution. They “may be part of the necessary ingredients to get life started,” he says. But at some point, energetic space weather poses a threat to complex life—“like a giant cosmic reset button that’s always going off,” he adds.

Stars undergoing a transition from cycling to stable could provide the ideal balance of spark and protection to nurture life. “If we’re looking for technological civilizations,” Metcalfe says, “maybe the best place to look is around stars that are in the second half of [their] lifetimes”—in other words, just entering a midlife crisis.

Science, 10 August 2022
; https://science.org