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Surprise on Barnard’s Star

M-class red dwarf stars are of increasing interest in terms of astrobiology. If we can devise weather models that allow for regions of relative stability, a planet locked tidally to its star at a fraction of the distance from Mercury to the Sun could produce conditions suitable for life. The National Geographic show ‘Extraterrestrial,’ shown again the other night, projects just such an environment, and imagines life forms that might evolve there.

But red dwarfs are tricky because they’re flare stars. In their early lives, they spin more quickly than they will when they enter their dotage; the rapid spin can produce magnetic fields that, in turn, create flares. Life on a planet circling a younger red dwarf would have to adapt to flares that can double the star’s brightness within a matter of seconds. Some believe this makes Proxima Centauri an unlikely candidate for life-bearing planets.

And what about Barnard’s Star, so tantalizingly close (5.9 light years) to our own Sun? Study a nearby red dwarf and you can infer its age from its rate of spin. As Ken Croswell points out in A Flare for Barnard’s Star (originally published on Astronomy.com on November 11), variations in that star’s light suggest a rotation of once every 130 days. The rotation is slow, pegging the star as old enough — possibly 11 or 12 billion years — that it should no longer be emitting flares.

Fortunately, surprises are what make astrophysics such a joy. Thus the news that Barnard’s Star has indeed released a major flare, observed in July of 1998 but only analyzed four years later. Diane Paulson (Goddard Space Flight Center) and colleagues will publish this work in a future issue of Publications of the Astronomical Society of the Pacific. The team’s work indicates the flare’s temperature reached 8000 K, more than double Barnard’s usual temperature.

So are flares in older M-class stars a rarity, or do we need to revise our assumptions? Ponder this if you have an interest in amateur astronomy, for this is yet another case where regular observation over long periods of time may re-write the book on accepted theory. The Paulson paper “Optical Spectroscopy of a Flare on Barnard’s Star” is available here in preprint form. Also, be aware that the original paper written by the American astronomer Edward Emerson Barnard identifying this star as nearby is also available online, and is a key document for those of us who collect classic papers, loosely defined as those that enrich and sometimes overthrow accepted thinking.

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