Comparison between the occurrence rate of superflares on G-type stars and those of solar flares (Shibata et al., 2013). The solid-line histogram shows the frequency distribution of superflares on Sun-like stars. The error bars in the histogram represent the square root of the event number in each bin. This distribution can be fitted by a power-law function with an index of ≈1.5 ± 0.3 (thick solid line). The dashed line, dotted line, and dot-dashed line indicate the power-law distribution of solar flares observed in EUV (Aschwanden et al., 2000), soft X-rays (Shimizu, 1995), and hard X-rays (Crosby et al., 1993), respectively. The red line corresponds to a power-law function with an index of 1.8 (from theoretical estimate by Aschwanden, 2012). Largest solar flare marked by blue oval.