OAS NOVEMBER 20013 ASTRONOMY EZINE VOL 2 | Page 18

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What is a black hole?
Posted in Astronomy, Physics, Science, tagged Black Hole, Neutron Star, White Dwarf on 13/07/2013 | Leave a Comment »

Last weekend was a pretty big sporting weekend. Not only was there the Third and deciding test of
the 2013 Lions’ tour of Australia, but there was also the men’s and women’s finals of Wimbledon,
and the German Grand Prix. As far as I can tell there is no sport going on this weekend. Before
someone comments below that England are playing Australia at cricket in the First Test of the Ashes, I should remind my readers (all 2 of you) that a bunch of overweight men standing around for 5
days not doing much does not constitute sport. So, with this lull in the sporting calendar, I thought
I would write this post about black holes, which I have been meaning to do for a while.
Nearly every one has heard of black holes, but what actually are they? And do they actually exist?
Well, a black hole gets its name because it is an object from which not even light can escape. The
radiation (light and other forms of electromagnetic radiation) which the central part of the black
hole gives off is not able to escape the extreme gravitational field the black hole creates.

An artist’s impression of the black hole in Cygnus, with matter falling into the black hole.

Calculating the escape velocity
Newton’s law of gravity allows us to calculate the force of gravitational attraction between two
bodies of masses

and

. It is simply

Earth, which we are now going to call

. Let us suppose

; and

face, we are going to call this second mass just

is the mass of the

is the mass of an object on the Earth’s sur. In order for the object on the Earth’s surface

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