Subject: Type II supernovae...how can an iron core exceed the Chadrashekar mass...

...or am I missing something.  Silicon burning, which forms the iron core, is fairly short (one book I 
have says it lasts about 24 hours).  Now, if high mass stars, I'd expect that once the iron core exceeds 
1.4 solar masses, that said core shoud collape right then and there.  However, the existence of black 
holes presumes that the collapsed matter is something over 3 solar masses (the uncertain upper limit 
for neutron stars).  So....if we should always get collapse at 1.4 solar masses, how do we get large 
enough cores to form black holes?  The only possibility I can see is that the core collapse can't break up 
the high-mass star fully, so that remnants of its envelopes of lighter elements (Si, Mg, etc) must fall
onto the "iron dwarf" (a form of white dwarf???) or neutron core that resulted from the original collapse.
Yea? Nay?  I've plowed through Arnett's book (Supernovae and Nucleosynthesis) but can't figure this 
out.

Re: Type II supernovae...how can an iron core exceed the Chadrashekar mass...