Nearby star also lacks sunspots and might help explain why
From: Penn State University
By Gail McCormick
March 22, 2022 -- The
number of sunspots on our Sun typically ebbs and flows in a predictable 11-year
cycle, but one unusual 70-year period when sunspots were incredibly rare has
mystified scientists for 300 years. Now a nearby Sun-like star seems to have
paused its own cycles and entered a similar period of rare starspots, according
to a team of researchers at Penn State. Continuing to observe this star could help
explain what happened to our own Sun during this “Maunder Minimum” as well as
lend insight into the Sun's stellar magnetic activity, which can interfere with
satellites and global communications and possibly even affect climate on Earth.
The star—and a catalog of 5 decades of starspot activity of 58 other Sun-like
stars—is described in a new paper that appears online in
the Astronomical Journal.
Starspots appear as a
dark spot on a star’s surface due to temporary lower temperatures in the area
resulting from the star’s dynamo—the process that creates its magnetic field.
Astronomers have been documenting changes in starspot frequency on our Sun
since they were first observed by Galileo and other astronomers in the 1600s,
so there is a good record of its 11-year cycle. The exception is the Maunder
Minimum, which lasted from the mid 1600s to early 1700s and has perplexed
astronomers ever since.
“We don’t really know what caused the Maunder Minimum, and we have been looking
to other Sun-like stars to see if they can offer some insight,” said Anna
Baum, an undergraduate at Penn State at the time of the research and first
author of the paper. “We have identified a star that we believe has entered a
state similar to the Maunder Minimum. It will be really exciting to continue to
observe this star during, and hopefully as it comes out of, this minimum, which
could be extremely informative about the Sun’s activity 300 years ago.”
The research team pulled data from multiple sources to stitch together 50 to 60
years of starspot data for 59 stars. This included data from the Mount Wilson
Observatory HK Project—which was designed to study stellar surface activity and
ran from 1966 to 1996—and from planet searches at Keck Observatory which include
this kind of data as part of their ongoing search for exoplanets from 1996 to
2020. The researchers compiled a database of stars that appeared in both
sources and that had other readily available information that might help
explain starspot activity. The team also made considerable efforts to
standardize measurements from the different telescopes to be able to compare
them directly and otherwise clean up the data.
The team identified or confirmed that 29 of these stars have starspot cycles by
observing at least two full periods of cycles, which often last more than a
decade. Some stars did not appear to have cycles at all, which could be because
they are rotating too slowly to have a dynamo and are magnetically ‘dead’ or
because they are near the end of their lives. Several of the stars require
further study to confirm whether they have a cycle.
“This continuous, more than 50-year time series allows us to see things that we
never would have noticed from the 10-year snapshots that we were doing before,”
said Jason Wright, professor of astronomy and astrophysics at Penn
State and an author of the paper. “Excitingly, Anna has found a promising star
that was cycling for decades but appears to have stopped.”
According to the researchers, the star—called HD 166620—was estimated to have a
cycle of about 17 years but has now entered a period of low activity and has
shown no signs of starspots since 2003.
“When we first saw this data, we thought it must have been a mistake, that we
pulled together data from two different stars or there was a typo in the
catalog or the star was misidentified,” said Jacob Luhn, a graduate
student at Penn State when the project began who is now at the University of
California, Irvine. “But we double- and triple-checked everything. The times of
observation were consistent with the coordinates we expected the star to have.
And there aren’t that many bright stars in the sky that Mount Wilson observed.
No matter how many times we checked, we always come to the conclusion that this
star has simply stopped cycling.”
The researchers hope to continue studying this star throughout its minimum
period and potentially as it comes out of its minimum and begins to cycle once
again. This continued observation could provide important information about how
the Sun and stars like it generate their magnetic dynamos.
“There’s a big debate about what the Maunder Minimum was,” said Baum, who is
now a doctoral student at Lehigh University studying stellar astronomy and
asteroseismology . “Did the Sun’s magnetic field basically turn off? Did it
lose its dynamo? Or was it still cycling but at a very low level that didn’t
produce many sunspots? We can’t go back in time to take measurements of what it
was like, but if we can characterize the magnetic structure and magnetic field
strength of this star, we might start to get some answers.”
A better understanding
of the surface activity and magnetic field of the Sun could have several
important implications. For example, strong stellar activity can disable
satellites and global communications, and one particularly strong solar storm
disabled a power grid in Quebec in 1989. It has also been suggested that
sunspot cycles may have a connection to climate on Earth. Additionally, the
researchers said that information from this star could impact our search for
planets beyond our solar system.
“Starspots and other forms of surface magnetic activity of stars interfere with
our ability to detect the planets around them,” said Howard Isaacson, a
research scientist at the University of California, Berkeley, and an author of
the paper. “Improving our understanding of a star’s magnetic activity might
help us improve our detection efforts.”
The curated database of the 59 stars and their starspot activity from this
research has been made available for researchers to further investigate
“This research is a great example of cross-generational astronomy, and how we
continue to improve our understanding of the universe by building upon the many
observations and dedicated research of astronomers that came before us,” said
Wright. “I looked at starspot data from Mount Wilson and Keck Observatory for
my thesis when I was a graduate student, Howard looked at starspot data from
the California Planet Survey for his master’s thesis, and now Anna has stitched
together all the data for a more comprehensive look across the years. We are
all excited to continue studying this and other promising stars.”
https://science.psu.edu/news/Wright3-2022
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