MadSci Network: Astronomy

Re: How can gravity collapse if it's universal?

Date: Mon Nov 21 16:02:44 2005
Posted By: Bryan Dunne, Grad student, Astronomy, University of Illinois
Area of science: Astronomy
ID: 1132349323.As

Gravitational collapse in astronomy is the sudden inward fall of a body
under the influence of the force of gravity.  It occurs when all other
forces fail to supply enough pressure to counterbalance gravity.  

Think about it like a account balance sheet.  On one side you have gravity,
which is determined by how much stuff the body contains (what scientists
call "mass") and how densely/loosely packed the stuff if (what scientists
call "density").  The more massive and/or denser an object is, the stronger
its force of gravity.  On the other side of the balance sheet you have
forces that work to keep a body from collapsing - magnetic forces, thermal
pressure, etc..  As long as both sides of the balance sheet are equal, the
body is stable and won't collapse, but if we change the balance sheet so
that the gravity side is more than the other side, the body will start to
collapse.  There are many reasons this could happen; the body might cool
off, reducing its thermal pressure, or a nearby exploding star might send a
shock wave through the body, compressing it and increasing its density (and
thereby increasing its gravity).  Gravitational collapse is a common
process in the Universe.  The gravitational collapse of enormous clouds of
gas and dust in space gives birth to stars (and planets!).  

In most bodies, as the object collapses, the balance sheet comes back into
balance.  In stars, it is when the core of the star has become hot and
dense enough from the collapse that nuclear fusion begin in its core,
providing a new source of pressure to balance gravity.  

In some bodies, they are so massive/dense that the sheet can't be balanced.
 These objects collapse into what we call "black holes".  The gravity
near these objects is so strong that nothing, not even light can escape.

The objects 100 to 1000 times brighter than our galaxy that you read about
are called quasars.  They are the distant objects that are being powered by
material falling into supermassive black holes, hence the connection to
gravitational collapse.  As the material falls
towards the black hole, it is compressed and heated to millions of degrees,
causing it to radiate tremendous amounts of energy.

Einstein taught us to think about gravity as a warping of space.  Let's do
a thought experiment:  Imagine a giant rubber sheet.  Now, place a bowling
ball on the sheet; the bowling ball warps the rubber sheet around it,
making a depression in the sheet.  Objects in the
Universe do this to space; the amount of warping depends on an object's
mass and density.  Now, roll a marble towards the bowling ball on the
sheet; the path of the marble curves when it nears the bowling ball.  This
is a mimic for how gravity works.  Interestingly, even light follows the
warps of space caused by massive objects.

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