November 21, 2019

Astro-Challenge: Monitoring Luyten’s Flare Star.

Artist’s conception of a flare star in action.

(Credit: NASA).

It’s ironic that the most common type of star also lies hidden from view in the night sky.  Our Sun and others like it make up a paltry ~20% of the fusion-burning stellar engines in the Milky Way; the vast majority of stars are red dwarfs with less than %50 the mass of our Sun. And although Alpha Centauri’s C companion Proxima lies just over 4 light years distant, not a single red dwarf is visible to the naked eye. We’ve written about other red dwarfs in the range of a backyard telescope, such as Groombridge 34 & Omicron Eridani; this week, we’d like to turn your attention to a curious specimen in the constellation Cetus.

UV Ceti, also known as Luyten’s Flare Star, is one of the most abrupt variables known.  First discovered during a proper motion survey conducted by astronomer Williem Jacob Luyten in 1948, Luyten’s Flare Star also goes by the designation of Luyten 726-8 or Gliese 65. The system is a pair of red dwarf stars with an average apparent magnitude of about +12 locked in a 26.5 year orbit varying from 2 to 8.8 astronomical units in separation. (UV Ceti is in fact Luyten 726-8B).

Though this system generally sits below +11th magnitude, it has been the site of some of the most intense flares observed. On September 24th, 1952, observers in Europe noted an increase in brightness from this star from magnitude 12.3 to 6.8 in only 20 seconds time! Anyone who visually monitors variable stars knows just how amazing that is; this 5.5-fold increase in magnitude was tremendous (over 100 times in brightness) and according to Burnham’s Celestial Handbook, lasted only 3 minutes. UV Ceti has undergone similar outbursts greater than +9th magnitude in more recent decades, but the 1952 flare shows just what its capable of.

So, just what’s going on here? M- Class dwarfs are tempestuous stars, and experience magnetic re-connection events, much like are seen on our own Sun. Both stars in the Gliese 65 system are at the lower end of the red dwarf mass scale at about 10% the mass of our own Sun and very active; a large flare from a red dwarf star may unleash up to 10,000 times the equivalent energy as seen from its counterpart dispatched from our own Sun. Needless to say, that doesn’t bode well for any possibilities for life around such an energetic star; any would-be planet in the habitable zone of such a system would be periodically bathed in surface-sterilizing radiation. As we study such systems, it’s become apparent that the relatively sedate behavior of our own Sun may be why we’ve evolved here to contemplate the universe in the first place.

A wide field view of the location of UV Ceti. (Simulations created by the author in Starry Night).

You can monitor UV Ceti for yourself; another outburst similar to the 1952 one would be an unforgettable sight, similar to the outburst of EV Lacertae in 2008. The star is well placed high to the south during the evening early in the year and its position is;

Right Ascension: 01 hours 39’ 02”

Declination: -17° 57’ 02”

The system varies from a 1.2”-9” arc second separation which is currently between minimums at 1999 and 2025.

A one degree field of view for Luyten’s Flare Star.

UV Ceti also has an extremely high proper motion of over 3.35 arc seconds per year, as can be seen in this amazing animation.  At 8.7 light years distant, UV Ceti is the sixth closest stellar system to our own, and its proper motion bears a strong resemblance to the Hyades star cluster. Is Luyten 726-8 an ejected outlier of the open cluster? Food for thought as you watch for the next “Big One” from this turbulent star!


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