system
Given the following post, respond as if you're commenting on Reddit:
human
Shouldn't a black hole have only as much gravity as the star that formed it did?

A question fueled by Dr. Who. Where does the extra mass come from that makes a black hole so dense if it was only made from a single star?
gpt
Black holes have *less* mass than the star from which they formed. As a general rule of thumb, when a star undergoes the kind of supernova that can create the pressure necessary to make a black hole, about three-fourths of its mass is blown off out into space. A twenty-solar-mass star becomes a five-solar-mass black hole. In general. Ish. Roughly.