Why does a black whole have more gravity thant the star it formed from?

Question

If a black hole is a collapsed star, and it retains the mass of the star and just shrinks in diameter, then why does a black hole have enough gravity to pull light in, but the star that it formed from doesnt? Should they have the same gravity since they have the same mass? (law of conservation of mass)

Answer 1

A black hole has the same gravity as the star that it came from. It's just that now all the mass is inside a smaller volume so you can get closer to it. As you get closer to it the gravity will get larger than the original gravity of the star because now you're "inside" of where the star used to be and the comparison is no longer valid. This is shown by Newton's law of universal gravity F=GMm/R^2. But the fact still remains that if you "stand" on the surface of a star (not recommended) you will feel a certain amount of gravity pulling on you. Now if the star becomes a black hole and you "stand" on a point where the surface of the star used to be with the black hole below you, you will feel the exact same amount of gravity pulling you downward.

So it all has to do with the volume of the object in question same amount of mass in a smaller object means that you can get closer to the center of the object and will feel a greater gravitational pull.

Answer 2

From a great distance the gravitational attraction of a black hole and the star that it formed from are identical (provided no mass was lost in the collapse of the star as in a supernova).

The inverse square law (see link) demonstrates that the gravitational field (and strength) varies with the distance between the "observer" and the center of gravity. For a star you can not get any closer than the surface of the star. But for a black hole that condenses all matter into a singularity about as large as the period ending this sentence, you can reach the very center of gravity. The escape velocity for a rocket launched from earth is about seven miles per second. If the speed is less the rocket must fall to earth when the fuel is used up. The escape velocity from the surface of a star is much greater than seven miles per second. However, for a black hole, there is an event horizon (a sphere surrounding the black hole) that is close enough to the center of mass that the escape velocity from its"surface" equals or exceeds the speed of light and any photon trying to escape must fall back into the black hole. Of course if no photons escape you can not see the black hole and it looks just like a black hole in space. Scientists know it is there for example because it deflects the light of stars passing by it on the way to earth, etc.

Answer 3

Newton's Law of Universal Gravitation describes the attractive force of gravity as proportional to the product of two objects' mass and inversely proportional to the square of the distance,

F = (G * M * m) / r^2
where G is the universal gravitational constant, M and m are the masses of the two objects, and r is the radial distance between each object.

You can see that as r doubles, the force of gravity is 1/4 the original value. Inversely, if the radial distance is halfed, the force of gravity becomes 4 times stronger.

As the mass of the original star is compressed, it becomes denser and denser, to the point where the acceleration at the surface is so great, the escape velocity is the speed of light, this occurs at the event horizon of a black hole. The event horizon of the black hole is formed well within where the original star's outer boundary was located.
What has essentially happened is that the distance r has shrunk drastically, but M has remained the same, allowing the gravitational acceleration to grow very large.

Answer 4

All the above answers are right. If you go below the surface of the star (assume you can do that without burning up), then gravity gets less until at the very center it is zero. Maximum gravity is at the surface. If you compress all the mass into a tiny space, then you can get closer to the center without going below the surface and gravity continues to double every time your distance to the center is cut in half. If the compressed object is small enough, then you can get close enough to the center without going below the surface to make gravity so strong that the escape velocity is greater that the speed of light. At any distance farther than the old, pre-collapse surface, gravity is unchanged. So a black hole may have strong gravity, but it is really small.

Answer 5

Gravitational acceleration (gravity) is a function of both the mass *and* the distance from the mass. It can be calculated from Ag = MG/r² where M is the mass, G is the 'gravitational constant' (6.6728*10^(-11) Nm²/kg²), and r is the distance to the center of the mass. Gravitational attraction 'acts' as if all of the mass were concentrated at one point (called the 'center of mass') of the object in question.

So.... If the entire mass of the Earth were 'compacted' somehow so that the radius of the Earth was only about 3,189 km (instead of about 6,378 km) the acceleration due to gravity (at the surface) would be about 39,2 m/s² instead of 9.8 m/s². (since it's an inverse square relationship)

In fact, if you could compact the Earth enough, the escape velocity from the surface *could* be made to exceed c and, at that point, the Earth would be a (very small) black hole. (I'll leave you to work out the arithmetic of what the Earths radius would have to be for that to happen. It's v=√(2Agr) so have fun ☺)

Steven Hawking once worked out the minimum amount of mass that could be 'collapsed' into itself and form a black hole. He refered to it as a 'unit black hole' but I don't remember off the top of my head what the number was (the Alzheimers deems to be kicking in again ☺)

In nature, the only time that such huge gravitational fields can be generated is when a massive star has consumed most of the fuel in its core and the pressure of the outward radiation is insufficient to support the mass of the stars outer layers. When they then collapse inwards there is then enough mass concentrated in a small enough volume that a black hole is formed.

Hope that explains it ☺


Doug

Answer 6

Because it's more compact. The event horizon may only be a few miles in diameter for a given mass. So if you can compress a large star with that mass into that diameter, even if it's not a singularity, it can pull in light too. In practice of couse, this is impossible, since in do so, the star will automatically collapse into a singularity.

Answer 7

Hi. It doesn't. The only difference is that now you can get closer to it's center. The gravity stays the same. If the sun turned into a black hole (not likely) the earth's orbit would not change.