Black holes

[Charles D]

This may be me being ignorant, but why is it that a black hole seems to have more mass than the star that created it. Light can escape from the star, but because all the stars mass is concentrated to a point, suddenly an event horizon forms. Where does this extra ability come from?

[insane_alien]

it doesn't have more mass (it has less as the source star sheds a lot of matter in creating a blackhole.)

 

The big difference is a blackhole is incredibly small compared to the source star.

 

for instance, to turn the sun into a blackhole you'd need to squash it into a sphere only 6km in diameter.

 

http://en.wikipedia.org/wiki/Schwarzschild_radius

[Greg H.]

So that leads me to ask a question:

 

If the earth was larger in diameter (yielding a larger volume of matter) but the mass remained the same, what would happen to the surface gravity?

 

My thinking is that it should go down, right?

 

If that is so, then is it true to say that an object's gravitational influence is related to the density of that object in some fashion?

--- Edit

Ok, so a little research and some math later, and it turns out I'm right.

 

Assuming the mass stays constant, the surface gravity increases as the radius of the object decreases.

 

[math]

g = \frac{Gm}{r^2}

[/math]

 

Where G = gravitational constant, m = mass of the object, and r = radius of the object.

 

From Surface Gravity

Edited June 19, 2012 by Greg H.

[zapatos]

So that leads me to ask a question:

 

If the earth was larger in diameter (yielding a larger volume of matter) but the mass remained the same, what would happen to the surface gravity?

 

My thinking is that it should go down, right?

Correct.

 

If that is so, then is it true to say that an object's gravitational influence is related to the density of that object in some fashion?

It is related to density, but more generally gravity decreases with increased distance from the center of mass.

 

The earth exhibits a certain gravity now. If the same amount of mass was in a larger earth the gravity would be less at the surface due to the fact that the surface is now further from the center of mass.

 

Similarly, if you have a black hole the 'surface' is now closer to the center of mass, and gravity will consequently be stronger on the surface.

 

If the sun were to suddenly become a black hole and lose no mass in the process, we would notice no change in gravity from the sun, due to the fact that we remain the same distance from the suns's center of mass.

Edited June 19, 2012 by zapatos

[Charles D]

Oh ya inverse square ratio of the concentrated mass. Thought I was being thick

 

it doesn't have more mass (it has less as the source star sheds a lot of matter in creating a blackhole.)

 

The big difference is a blackhole is incredibly small compared to the source star.

 

for instance, to turn the sun into a blackhole you'd need to squash it into a sphere only 6km in diameter.

 

http://en.wikipedia.org/wiki/Schwarzschild_radius

[/quote

 

Cheers for the link

[jozef]

This may be me being ignorant, but why is it that a black hole seems to have more mass than the star that created it. Light can escape from the star, but because all the stars mass is concentrated to a point, suddenly an event horizon forms. Where does this extra ability come from?

 

 

unbelievable how much wisdom in one issue! Certainly you're not a professional physicist, I am not. Why I will not but you are right, at least to me, but I'm just sewing machine repairman. It is interesting that due to the phenomenon which you took the increase in weight space, note weight not matter! As a result of the collapse of the universe!

[Mynameson]

It has those extra physic properties because the atoms of the star are so close together that there is no space between them.This makes the black-hole the perfect black body with a nearly infinite gravitational force. Now imagine that two atoms in the star have a distance between them e.i d but at the black hole the two atoms have a distance of numerical value 0 so thats how its body is formed and the mass of the star is still there but more dense then ever. Then the event horizon happens cuz time is affected by gravity and a gravity so huge can change the leap of time-space. And at the event horizon the event is easily predicted by Theory of Relativity. But thats just my idea. PS Dont't call it an extra ability cuz it happens all the time but we dont notice it even the weakest gravity interacts with time-space.