After general theory of relativity formulation by Alberta Einstein, Carl Schwarzschild obtained mathematical description of the space-time geometry around spherical mass. Calculations showed, that for every mass there exists critical radius, today called Schwarzchild's radius, which corresponds to so extremal distortion of the space-time, that after compression of the mass to radius equal to such radius ocurred closing ot the space-time and cutting of from the rest of the Universe. From such area cannot escape anything, even light. Radius is described by simple formula:
where G=6,672·10-11 Nm2/kg2, m- mass, c=3·108 m/s.
For the Sun, Schwarzschild's radius is equal to 2.9 km and for Earth only 8.8 mm. Of course (fortunately?), it is not possible to squeeze there objects to such small dimensions. But in case of very heavy stars, gravitational force is so strong, that it can overcome forces connected with pressure in their inside.
First ever black's hole color photography!