2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/220154The Vaidya solution describes the gravitational collapse of a finite shell of incoherent radiation falling into flat spacetime and giving rise to a Schwarzschild black hole. There has been a question whether closed trapped surfaces can extend into the flat region (whereas closed outer trapped surfaces certainly can). For the special case of self-similar collapse we show that the answer is yes, if and only if the mass function rises fast enough.14 pages, 4 figures; minor polish added to version 2General Relativity and Quantum Cosmologytext