On Sat, 29 Dec 2001, Ahmed Nadeem Rafi wrote:
> My question is as follows:
> First, I'll quote a statement from an essay about black holes, "When a star 
> reaches the end of its lifetime and exhausts all of its fuel, it begins to 
> collapse under its own gravity".
> This statement implies that a star has either no gravity or too small a 
> gravity to cause collapse of the star during its lifetime. Why would a star 
> not collapse during its lifetime when it essentially has the same gravity as 
> it has after its death? Why does gravity become so strong to cause collapse 
> of the star after its death? We know that gravity is related to mass. As a 
> general rule, gravity would increase as the mass of a body increases. When a 
> star collapses on itself, its total mass does not change. Or does it? 

Ah-hah!  Good question.  There is an other force, however, besides
the gravitational force which must be considered.  Yes, the mass of
the star, and thus its own intrinsic gravitational attraction, stays
about the same throughout its life.  But, the radiation which a star
generates inside its body, through thermonuclear fusion, contributes
a radiation pressure which pushes outward *against* the inward pull of
its own gravity.  Light is made of photons and photons carry momentum.
These photons are created inside the star (which is why the sun and all
other stars shine).  They contribute a pressure to the stars body as
they propagate out to the universe.  This what keeps a star from inward
collapse.  As a star dies, its thermonuclear reactions stop and the
production of photons stops.  Thus, the gravitational force, which has
been pushing in ward all along, is finally able to act...

> Although its density does reaches some sort of infinity. Does density has 
> got anything to do with gravity?

Gravity is generated by mass.  Density is a measure of how much
mass is in a given volume (three grams per cubic centimer, for
instance).  Roughly speaking, a large hunk of matter gives off
the same gravitational field regardless of its density.  So,
the same star mass in different sized packages would give off the
same gravitational attraction.

Scott Armel-Funkhouser
Berkeley