Does the mass of a star change as it collapses into a black hole?

Question

I know (I think!) that when a really big star collapses on itself it creates a black hole.

My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or does it change while collapsing?

This question came to me and my friend while studying Newton's law: $$F=G \frac{m_1 \cdot m_2}{r^2}$$ If the mass of the star doesn't change, then it can't have enough force to "eat" light (unless it has that force in the first place). Does the force change because of the density?

Answer 1

During a supernova, a star blasts away its outer layers; this actually reduces the mass of the star significantly.

Any star or planet has an escape velocity - the slowest an object must be traveling for it to escape the gravitational field of the star/planet. For Earth, this is 11.2 km/s. (Note that this value doesn't account for any atmospheric effects.) For a black hole, however, the escape velocity at the event horizon (the "edge," in some sense) of the black hole is the speed of light, $c = 300,000 \text{ km/s}$. For anything within the event horizon to escape the pull of a black hole, it must exceed the speed of light, a physical impossibility. There's a certain mass-dependent radius - the Schwarzschild radius - to which an object must shrink in order to become a black hole.

Newton's Law of Universal Gravitation, which you stated, doesn't apply in its standard form to light. Rather, you need to use Einstein's general relativity, which considers gravitational forces in a much different light than Newton did. However, Newton's Law of Gravitation can intuitively apply here: when a star collapses, $m$ does decreases. However, $r$ becomes much smaller, so the net effect of these changes is the creating of a stronger gravitational force on the surface of the remaining object.

Answer 2

The formula $F=G \frac{m_1 \cdot m_2}{r^2}$ is valid only for point masses. However, it can be applied to non-point masses if its spherically symmetric. Enter Shell Theorem:

1.A spherically symmetric body affects external objects gravitationally as though all of its mass were concentrated at a point at its centre.

So, when a spherically symmetric massive star attracts an object at its surface, its like its actually attracting that object from a distance equal to its radius.

2.If the body is a spherically symmetric shell (i.e., a hollow ball), no net gravitational force is exerted by the shell on any object inside, regardless of the object's location within the shell.

So, if you put something near center of that star, it can still escape because it is experiencing force due to only mass below it.

But, when that star collapses to a smaller volume, force due to whole mass on surface increases (because its inversely proportional to $r$) which makes escaping tougher (required escape velocity increases). When this radius is decreased to Schwarzschild radius, the escape velocity exceeeds $c$.

Answer 3

I was considering this question as well. Neither matter/energy can be created or destroyed, only converted from one form to the other. Consumption of star stuff by a singularity would convert that star stuff into star energy, but with no other way of expressing that energy there seems no choice but for nature to convert with 100% efficiency that matter into gravitational energy. When I model this in my mind I can envision a neutron star that collides with an object that tips the scales, the straw that breaks the camels back, and the neutrons give up their integrity and the whole thing drops into a singularity, and as this happens an immense gravity wave, a gravity tsunami, propels itself into the universe, leaving an object that has more mass than it did before it collapsed. Exactly how much more is beyond my math skills.