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Faint star in Bootes makes history

Tonight … a word about a faint star in the constellation Bootes the Herdsman that made astronomical history in the year 2007. In that year, an international team of astronomers, led by Jean-Francis Donati and Claire Montau of France, caught the star Tau Boötis flipping its north and south magnetic poles. These astronomers had been mapping the magnetic fields of stars. It was the first time a magnetic reversal had been observed on any star other than our sun.

Astronomers intently watched Tau Boötis for more magnetic turnovers, and it appears this star undergoes magnetic reversals in periods of about two years. They’re hoping Tau Boötis will enable them to understand how magnetic engines drive stars, including our sun.

Tau Boötis is faintly visible in a dark country sky. Look eastward on these April evenings for the blazing yellow-orange star Arcturus, the brightest in your eastern sky. 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.

You’ll need a dark country sky and a moon-free night to see Tau Boötis with the unaided eye. This star is nearly 70 times fainter than Arcturus. On these northern spring evenings, the star Muphrid shines to the upper right of Arcturus, and Tau Boötis lodges to the upper right of Muphrid.

Artist view of the giant exoplanet orbitng tau Bootis, through the star's magnetic arcs. Image via David Aguilar, CfA, via cfht.hawaii.edu.

Artist view of the giant exoplanet orbitng tau Bootis, through the star’s magnetic arcs. Image via David Aguilar, CfA, via cfht.hawaii.edu.

The Tau Boötis is interesting for another reason. It harbors a planet that’s several times the mass of Jupiter and has an orbital period of only three and one-third days.

In 2014, astronomers found water vapor in this planet’s atmosphere. They’re hoping this star and its planet can shed some light on the relationship between stellar magnetic cycles and planetary climate.

More recently, the International Astronomical Union selected the planet Tau Boötis b and its host star as part of a process for giving proper names – for the first time selected by the public – to exoplanets and some host stars. Astronomy clubs around the world participated in this process, and the IAU announced the new names in mid-December, 2015. More than half a million votes from 182 countries and territories contributed to the IAU’s new official designations of 14 stars and 31 exoplanets orbiting around them.

Tau Boötis b was supposed to be on the list, but – late in the game – the IAU annulled its new name, judging it didn’t conform with the IAU standards for naming exoplanets.

The IAU says Tau Boötis b will get a new proper name as part of a public process in the near future.

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Bottom line: Tau Boötis is a faint star seen to flip its north and south magnetic poles. The star has a planet, Tau Boötis b, that was part of a public naming process by the International Astronomical Union … almost.

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Bruce McClure