Fractional X-ray luminosity (L_X/L_⋆) as a function of the effective temperature for 1996 stars with measured T_eff, L_⋆, L_X. Filled fuchsia triangles indicate upper limits for 130 stars with d ≤ 17 pc. The horizontal gray band shows the range of the fractional X-ray luminosity for the Sun (P. G. Judge et al. 2003). Adopting the M. J. Pecaut & E. E. Mamajek (2013) T_eff–L_⋆ relation for main-sequence stars, the green line indicates approximate SRG/eROSITA and ROSAT detection limits at distances of 10–20 pc.