Today in science: Tau Boötis’ pole flip

Giant exoplanet orbiting tau Boötis, through the star's magnetic arcs like solar prominences.
Artist’s concept of the giant exoplanet orbiting the star Tau Boötis, seen through the star’s magnetic arcs. Image via David Aguilar/CfA/

March 13, 2008. On this date, a faint star in the constellation Boötes the Herdsman went down in astronomical history. It’s the publication date of a study of the star Tau Boötis, seen by an international team of astronomers to flip its north and south magnetic poles. These astronomers had been engaged in mapping the magnetic fields of stars. But Tau Boötis was the first star, other than our sun, seen to undergo a magnetic reversal. The study was published in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

Afterwards, astronomers intently watched Tau Boötis for more magnetic turnovers. They discovered that this star undergoes magnetic reversals in periods of about two years. That’s in contrast to our sun, which undergoes a magnetic reversal about every 11 years. Read more about the sun’s magnetic reversals below.

Though it’s not a bright star, you can see Tau Boötis on April evenings. It’s near the blazing yellow-orange star Arcturus, brightest star in your eastern sky in the evening in April, assuming you’re in the Northern Hemisphere. To verify that you’re looking at Arcturus, look for the Big Dipper up high in your northern sky. Follow the arc of the Big Dipper’s handle to Arcturus. Tau Boötis is some 70 times fainter than Arcturus. You’ll see its location on the chart below.

Star chart showing location of Tau Boötis.
The star Tau Boötis can be seen on April evenings.

Now, about our sun’s magnetic reversals. As we said above, the sun’s magnetic polarity flips approximately every 11 years. Magnetic reversals are part of our sun’s normal activity, and – as shown by Tau Boötis in 2008 – it’s likely other stars similar to our sun in the Milky Way galaxy (and other galaxies) also undergo magnetic reversals.

The video below was published on YouTube in December 2013, when the current solar cycle was near its peak. It features solar astrophysicist Alex Young, talking about Solar Cycle 24 and about what a magnetic flip means for Earth.

Now check out another cool video, below. It’s from NASA’s Goddard Space Flight Center and is a visualization, showing the position of the sun’s magnetic fields from January 1997 to December 2013. Magenta lines show where the sun’s overall field is negative and the green lines show where it is positive. A region with more electrons is negative, the region with less is positive. Additional gray lines represent areas of local magnetic variation.

The visualization shows how, in 1997, the sun showed positive polarity on the top and negative polarity on the bottom. Over the next 12 years, each set of lines is seen to creep toward the opposite pole, eventually showing a complete flip.

Fun to think that other stars (most likely) do this, too!

Bottom line: On April 13, 2008, astronomers published the first study showing the pole flip, or magnetic reversal, of a star other than our sun. The star was Tau Boötis, visible on April evenings. This post includes 2 great videos about our sun’s magnetic pole reversals.

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Source: Magnetic cycles of the planet-hosting star Tau Boötis

March 13, 2019

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Deborah Byrd

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