Superflare
Since superflares can occur on stars apparently equivalent in every way to the Sun, it is natural to ask if they can occur on the Sun itself. An estimate based on the original Kepler photometric studies suggested a frequency on solar-type stars (early G-type and rotation period more than 10 days) of once every 800 years for an energy of 10 erg and every 5000 years at 10 erg. One-minute sampling provided statistics for less energetic flares and gave a frequency of one flare of energy 10 erg every 5–600 years for a star rotating as slowly as the Sun; this would be rated as X100 on the solar flare scale. This is based on a straightforward comparison of the number of stars studied with the number of flares observed. An extrapolation of the empirical statistics for solar flares to an energy of 10 erg suggests a frequency of one in 10,000 years.
Superflare
Spectroscopic studies of superflares allow their properties to be determined in more detail, in the hope of detecting the cause of the flares. The first studies were made using the high dispersion spectrograph on the Subaru telescope in Hawaii. Some 50 apparently solar-type stars, known from the Kepler observations to show superflare activity, have been examined in detail. Of these, only 16 showed evidence of being visual or spectroscopic binaries; these were excluded since close binaries are frequently active, while in the case of visual binaries there is the chance of activity taking place on the companion. Spectroscopy allows accurate determinations of the effective temperature, the surface gravity and the abundance of elements beyond helium ('metallicity'); most of the 34 single stars proved to be main sequence stars of spectral type G and similar composition to the Sun. Since properties such as temperature and surface gravity change over the lifetime of a star, stellar evolution theory allows an estimate of the age of a star: in most cases the age appeared to be above several hundred million years. This is important since very young stars are known to be much more active. Nine of the stars conformed to the narrower definition of solar-type given above, with temperatures greater than 5600K and rotation periods longer than 10 days; some had periods above 20 or even 30 days. Only five of the 34 could be described as fast rotators.