Mean X-ray surface brightness FX vs. B-V color for a complete sample of main sequence stars (A-type stars: upper limits; F and G: diamonds; K and M: upward triangles). Clearly the observed solar coronal hole surface flux (two dashed curves) provides a good description of the observed stellar minimum X-ray flux, suggesting the interpretation that the stars of minimal X-ray flux are those completely surrounded by stellar analogs of solar coronal holes.
Jürgen H.M.M. Schmitt, Astron. Astrophys. 318, 215-250 (1997)
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