A BLACK HOLE? Why does it need a mammoth star to implode? The gravity is still just as much. if not more.?

Question

Why does it not draw in all light when it is fusing atoms. If light escapes before its dying cry, why afterwards?

I have read the responses so far,,,but it does not answer my question. Light does not care or even know how compact the matter is. It only obeys the law of gravity.. and the gravity may be less after the dying ex- or implotion.

Thank you Charles. You have enlightened me

Answer 1

Having read your comment, let me see if I can re-arrange some of the responses to help clarify the issues. We'll hit the part about "light" at the end. So, here goes...

When I throw a ball in the air, it comes back down. The harder I throw, the higher it goes. Can I throw the ball so hard that it leaves the earth altogether? Yes, I must throw it hard enough to reach escape velocity. The escape velocity for any object is directly dependent on that object's mass. The more massive, the higher the escape velocity. Put another way, the stronger the gravity the higher the escape velocity. Is it conceivable that an object can be so massive that the escape velocity exceeds the speed of light? If nothing can travel faster than the speed of light would we have a Black Hole?

Yes on both counts. Here is the equation:

v = sqrt(GM/r)

where v is the escape velocity, G is the gravitational constant, M is the mass of the object in question, and r is the radius of the object. "sqrt" means square root. If, for example, you plug M and r in for the earth you will get an escape velocity of about 11 Km/sec. Note, too, that given values for any two of the three variables (v, M, or r) you can calculate the third. So, let's make v = speed of light "c" and rearrange the equation to get r.

r = 2GM/c^2

Because c^2 is such a big number, only object that are very massive *and* very small have any hope of becoming a black hole. Could the earth become a black hole. Sure, *PROVIDED* you can squeeze the mass of the earth into a small enough size. How small? Well, crush the earth until it is less than 0.00074 meters in diameter (that's about .03 inches) and you will have an object where the escape velocity exceeds the speed of light.

How are you going to crush the earth into such a small size? Well, a really, really big explosion would do it.

Or you can find a really big and massive star, wait for it to explode, i.e. become a supernova, and see if a black hole is left behind.

But why would light care? It is easy to see that objects with mass could not escape, but why can't light escape. This requires a little understanding of what happens to space and how light travels through space. In open space, light always takes the gravitationally shortest path between two points. This is Einstein's assertion about light and space. In the presence of very massive objects, space gets curved or bent. The greater the mass the greater the curvature. Light always (I mean always) follows the curvature of space.

So, you may ask, is it possible for space to become so curved that it curves back on itself. Yes, is the answer. It is possible for an object to become so massive that the path that light takes curves back to the object. What's the raduis of such an object? Heh, you have already seen it:

r = 2GM/c^2

So, Einstein helps us see that not only can mass not escape a black hole, but light can't either.

So, why don't big stars just collapse into black holes right away? Because the radiation produced during fusion keeps them from getting small enough. The squeeze of gravity that should make them a black hole can't because the radiation pressure (the heat) won't let them contract. Hot things expand, and very hot stars really, really want to expand! In the end, it takes the supernova explosion to overpower that radiation pressure to squeeze the remaining mass of the star small enough to become a black hole.

Make sense?

HTH

Charles

Answer 2

What keeps things like Earth from collapsing is the electromagnetic force between atoms. Gravity is weaker than the electromagnetic forces keeping all the atoms in the Earth from getting too close to each other. What keeps a star from collapsing is the heat from nuclear reactions, because a hot gas expands. When a star has converted all its hydrogen into other elements that cannot support nuclear fusion, then the heat source is gone and gravity takes over and starts collapsing the star. It collapses until some other force stops it. For a small star, the same electromagnetic forces between atoms that keeps Earth from collapsing are enough to keep the star from collapsing, and the result is a black dwarf. If the star is bigger, then its gravity is bigger too and it overcomes the electromagnetic force to pushed all the electrons in all the atoms into the protons, smashing the whole star into one mass of neutrons to make a neutron star. But a big enough star can have gravity so strong that no known atomic force can prevent infinite collapse, and the result is a black hole.

Answer 3

The effective force of gravity has two components, mass and DISTANCE. The center of gravity of a normal star can be hundreds of thousands of miles from its surface, diluting the force of gravity. When that same mass is collapsed into a singularity, the center of mass may be only a few miles from its surface, greatly intensifying the effective force of gravity.

A crude comparison would be a 20-pound plastic ball versus a 20-pound steel ball. The plastic ball is much larger than the steel one, and even though they weigh the same, the plastic ball seems lighter because of its greater volume. This is not a true demonstration of increased gravity but an allegory. The point is, when the source of gravity is closer to the object, the effect of gravity is increased.

Answer 4

It's based on the amount of mass packed into a certain radius and only a very large star can have enough mass to be compacted to the proper density to form a black hole. When the star is active, the radiational pressure inside keeps the star larger than the critical radius.

Answer 5

Stars massive enough to create a black hole will not so long as there is sufficient energy from thermonuclear fusion to counteract the gravitational forces.

Also, there is a critical mass for stars under which a black hole will not be formed because there is not enough gravitational attraction to compact it any further than like a neutron star.

However, if a neutron star, after further accretion of mass, crosses that boundary, it will collapse into a black hole.

Answer 6

The star must FIRST go through expelling itself of the remaining thermonuclear reactions that cause it's properties to expand against it's own gravity. Once those reactions are done, it will cease expanding and it will collapse. The retention of it's own light can't occur unless these reactions stop, causing it's mass to collapse upon itself via its own gravity.
Once its collapsed and central region has imploded into being infinitely small and infinitely dense, it is only THEN a black hole.

Answer 7

There's an escape velocity caclulator at the link, along with the explanatory maths. Try keeping the mass the same but reducing the radius - you'll see that the escape velocity increases. If you make any mass small enough you'll end up with an escape velocity equal to the speed of light.

Answer 8

Apparently, you do not know the laws of gravity (and light) as well as you think you do. Physics 101: it's not quite rocket science, but it will help you sort out the basics.

Answer 9

because the giant star Implodes and the density is so great that it rips a whole in space.