[net.space] Supernova, Gravity, Etc

knudsen (08/05/82)

HOLD IT!  Basic hih-school physics states that the center of mass
of an object cannot change (or its net momentum change) merely
thru action of forces generated WITHIN that object --
rather, OUTSIDE forces are required.
The example often used is a hand grenade flying thru outer space;
if it explodes, the center-of-gravity/mass of all the fragments
(& gases, etc) continues to move as before.
Substitute a star for the grenade, comes the supernova explosion,
and, presto, the center of mass stays ewhere it was.

I think someone was wondering how much mass of the star would be changed
into ENERGY a la E=mc^2, which *would* cause an instant change in mass.
This change is radically different from the percentage of star matter
that is merely blown away into space -- probably the 50~90% guesses
are close.

PS: If matter and energy are the same thing, and you have a volume of
space with a VERY high energy flux, does it exert gravity
(ie, distort space around it)?
--mike knudsen ihnss!knudsen

rhm (08/06/82)

Re ihnss!knudsen comment.
Again, during a supernova explosion, the change in mass is zero, both
instantaneously and long term. Photons do just as good a job of causing
gravitation as anything else of the same mass.

Wedekind.ES@PARC-MAXC@sri-unix (08/09/82)

	This is in response to Mike Knudsen' comment that a supernova's
center of mass (more precisely, the path that the center of mass follows)
remains unchanged by the explosion.

	This doesn't mean that the gravitational force on external bodies
remains unchanged.

	The gravitational force DOES stay the same in the special case where
the exploding mass retains a spherically symmetrical density, as Newton first
showed.  In particular, it's the same as if all the mass were at the CM.

	But in the general case it can get bigger or smaller, and change
direction too.  You can see all this if you imagine simple cases (where the
star splits in half, for instance, and one half lands on your doorstep!).
The galaxies where we see jets shooting off across the line of sight - they're
not pulling on us quite as hard as they used to.  This isn't surprising, since
CM is linear with position and gravity force isn't.

						cheers,
							Jerry