Stars earlier than about F5 (Teff ~ 8000 K) donŐt have convective envelopes and donŐt have any X-ray
emissionÉ
Except that O and early B stars do have X-ray
emission - they are strong sources of soft X-rays. And they have strong stellar winds.
Wind broadened and
blueshifted UV absorption lines of an O
and a B star.
HST image of h
Car; an extreme example of a hot star wind.