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B. E. Schaefer (Yale), J. R. King (STScI), C.P. Deliyannis (Indiana)
Flares are well known on many varieties of exotic stars, but normal main sequence stars are generally thought of as significantly changing brightness only on time scales of millions or billions of years. However, we have identified large-amplitude short-duration flares on nine normal main sequence stars with spectral class from F8 to G8 all of which involve energies from 1033 to 1038 ergs. These nine stars (Kappa Cet, Pi UMa, Omicron Aql, 5 Ser, UU CrB, S For, MT Tau, BD +10 2783, and Gmb 1830) are all normal (i.e., not in close binaries, not rapidly rotating, not pre-main sequence) as judged by lithium abundances, rotational velocities, x-ray luminosities, and calcium lines. The flares (from all but one star) were detected with multiple detectors, or on multiple occasions, or by multiple observers. The flares have been recorded with photometers, spectrometers, imaging x-ray cameras, chain photography, and visually, with amplitudes ranging from 0.09 to 3 mag and durations from minutes to days. Estimates for G stars suggest rates of once every century or so on average, although our Sun cannot have suffered a superflare within the past two millennia. Such flares could have profound consequences for stellar energetics, flare mechanisms, and life on planets around solar-like stars.