Leo Wagner: Does playing the outside at A5 not work also?
caranthir: No. I suggest you study it yourself, with board or sgf editor if necessary, to make sure you see why it doesn't work. See also d4rkm4tter's response below. The space inside is too large to use any ordinary nakade (result of that is just a seki). Instead, the attacker must make an eye.
chtz: Can't W tenuki? Any approach can be followed by B19, and if B throws in first W captures and can still make a bulky 5 the next move. Or captures in case of self-atari.
d4rkm4tter: No, he has to play. tenuki-B19-A19 and it is seki.
It becomes seki, you can play it out yourself and see that black can't kill after A19.