An evocative name. These are bodies with an escape velocity greater than that of light. They swallow up matter- increasing in mass themselves. However if anti matter falls into a black hole then its mass decreases and eventually the black hole fills up and disappears in a burst of radiation.
The diagram
(Figure 1) represents the gravitational fields outside a large star and then a black hole. The
depth of the gravitational vortex is much greater for the black hole than for a heavy star –
the escape velocity being the speed of light.
Actually only stars much heavier than
the Sun will form black holes. The Sun will become a red giant and then shrink away to a
white dwarf and finally a black dwarf.
The escape velocity of a body obviously
increases the closer to the body that you go, for example for the Earth the escape velocity
at the surface of the planet is 11.3 km per second 10 000 km above the surface this will
have fallen to just under 7 km per second. Clearly for a Black Hole there will be a distance
from the centre of the Black hole where its escape velocity will be equal to that of light –
closer you fall in and can never escape, beyond that distance you could theoretically
escape if you have a space ship that could travel fast enough.
Although the Sun is
not sufficiently massive to form a Black Hole the theoretical "radius" of its event horizon would be a little less
than 3 km!