Expert: Jayendra Upadhye - 2/16/2005

Question
what is black hole?thank you

Answer
Hi Dean,
A black hole is the remnant of a massive star (more than 5-6 times the the sun's mass.
Even at the sun's center, the combined weight of overlying gas layers is enough to squeeze hydrogen into a metallic hydrogen form and the pressure is high enough to start hydrogen fusion reactions. (something like being in the middle of a hydrogen bomb explosion).

In more massive stars as above, the imploding pressure due to gravity is much much greater, but is balanced by outward pressure due to heat of its nuclear reactions. When the star can no more generate heat by virtue of having exhausted all "burnable" fuel (when its starts to "make" iron), this internal pressure is lost.
The resulting collapse is very fast and the turmoil so severe, we get a supernova explosion. (the infalling material hits the ultrahard core and rebounds.)
the remnant then continues to contract indefinately, into a mathematical point called a "singularity".

That in essence is a black hole.
Its gravity is so high light emanating from it cannot escape it. (so the name "black").
Jayen