Super black holes and stars orbiting them

[littlebanger]

seen a programe about super black holes in which some stars where orbiting them . speeding up as they are slingshoted around them or whatever the proper term for this is . so why if the black hole is strong enough to pull them in the first place why do they not get pulled in completely ... what stops the stars being pulled in . sorry for the stupid question but the programe never said anything about it i think they where making crap up as they went along

 

[zhermes]

The same reason planets don't fall into the stars they're orbiting: they still have too much momentum/kinetic energy to be captured by the super-massive black hole (SMBH). Remember, a BH isn't a vacuum cleaner. If our sun instantaneously turned into a BH of the same mass, none of the orbits would change at all.

 

[littlebanger]

thanks for the answer but it doesn't really explain much i do understand about the kenetic energy thing but really when something is pulled by a BH it should go to the BH not around it unless it has a force of some sort pushing it away when it gets closer . from what i seen they orbit in an oval shape how is that even possible if you know what i mean . i just think if something is pulled by a BH it should go to it or though it not around no matter how big the kentic energy is. wouldn't it always be weaker than the force pulling ? i could understand a circle orbit but not an oval one

thanks again for your comments and sorry if you feel like your beating you head of a wall talking to me

 

[Drakkith]

A black hole doesn't just suck up everything that is affected by its gravity. Many objects can orbit one just as planets do a star if they don't get close enough. If we compressed the sun into a black hole, all the planets could still orbit as they were because the overall gravitational forces did not change. The same mass is still there. The only difference is that you can get very close, (Closer than the surface of the sun is now) and gravity would increase until you reach the event horizon.

 

[94JZA80]

thanks for the answer but it doesn't really explain much i do understand about the kenetic energy thing but really when something is pulled by a BH it should go to the BH not around it unless it has a force of some sort pushing it away when it gets closer . from what i seen they orbit in an oval shape how is that even possible if you know what i mean . i just think if something is pulled by a BH it should go to it or though it not around no matter how big the kentic energy is. wouldn't it always be weaker than the force pulling ? i could understand a circle orbit but not an oval one

not one sinlge planet, asteroid, or comet orbits the sun in a perfect circle. as a matter of fact, they all orbit the sun in the shape of an ellipse. its just a matter of eccentricity (how pronounced the eliipse is). the Earth's orbit is nearly circular, but not perfectly so, and so it orbits the sun in an ellipse of low eccentricity. sometimes your intuition/instincts can mislead you.

back to your original question, if an object were on a direct path (collision course) with a black hole, then yes, it would fall into it. but very few objects in the universe are on direct collision courses with other objects. therefore, most objects just come really close to each other and not directly at each other. in some cases, the objects can come within a close enough range of one another that their mutual gravitation can pull them into orbit around each other. at any rate, it is b/c the velocity of one object relative to another most often has both radial AND angular components (and not just a radial component, implying a direct collision course) that objects typically orbit each other and don't simply collide head-on.

if you still can't picture it, think of orbiting as "falling." when you run off the edge of a tall building, you travel both forward and down before hitting the ground. when you drive a fast car off the edge of the building, again you travel both forward and down before hitting the ground, but this time you will have probably traveled forward much farther than you did when you ran off the edge of the building. ultimately though, you eventually hit the ground. the reason for this is b/c the curvature traced out by the combination of your forward and downward motions is much smaller than the curvature of the surface of the earth. now imagine launching yourself off the edge of the building so fast that the curve traced out by the combination of your forward and downward motions match the curvature of the surface of the earth. then you would continue "falling" around the earth, but you would maintain a constant altitude and never hit the ground. this is called "orbiting." contrast this situation with one in which you are standing atop a tall building that suddenly vanishes beneath your feet. b/c you have no lateral/horizontal (forward/backward) velocity when the building vanishes, you fall directly toward the center of the earth. it is also easy to see that your velocity consists only of a radial component, and has no angular component, which is also why you don't fall in any other direction other than directly toward the center of the earth. you can replace the Earth with a black hole, and the same concepts/physics apply.

now the instances i highlighted above are oversimplified obviously, and nothing orbits something else forever, or vice versa. energy is radiated away in the process of orbiting, and angular momentum is lost over time. hence, eventually orbiting bodies will orbit closer and closer until the collide/merge. a star continuously loses orbital energy as it orbits around a black hole, and will eventually fall in. why don't we ever hear about these events happening in reality? b/c stars take millions or billions of years to sprial into the black hole, depending on both the initial distance between them and their angular velocity with respect to each other. so we're lucky to see such a thing happen in our lifetime, let alone be able to resolve the general vicinity of a black hole with a telescope in enough detail to observe such an event.

hope that helps...

 

[littlebanger]

Thanks well and truly answered.

sorry Zhermes i failed to click on the part where you said ( they STILL have to much energy ) and did answer my question i can be pretty dense myself sometimes .

 

[Darryl]

94JZ that is a nice answer,

I would think the black holes rotation would also have a large effect, 'draging' space/time around the BH, making space a funnel, with a rotating base.

So due to the rotation, the gravitation lines will not be perpendicular to the surface of the BH, So if shot a bullet directly at the black hole, you would see the bullet moving in the direction of rotation of the BH, into a decaying orbit.

Im not sure how fast a BH would rotate, or if it can be measured, but I expect something very small and very dense to rotate quickly. But I don't know, things are confusing in black holes !, (to me they are).. :)

 

[94JZA80]

94JZ that is a nice answer,

I would think the black holes rotation would also have a large effect, 'draging' space/time around the BH, making space a funnel, with a rotating base.

So due to the rotation, the gravitation lines will not be perpendicular to the surface of the BH, So if shot a bullet directly at the black hole, you would see the bullet moving in the direction of rotation of the BH, into a decaying orbit.

Im not sure how fast a BH would rotate, or if it can be measured, but I expect something very small and very dense to rotate quickly. But I don't know, things are confusing in black holes !, (to me they are).. :)

though i don't know for sure myself (i'm definitely no expert), i think you are probably correct about the black hole's rotation also having something to do with an object's tendency to enter into an orbit around it, as opposed to just plunging directly into the black hole perpendicular to its surface, or event horizon - even if the object is initially on a direct collision course with the black hole. i probably should have suggested that the distance between two mutually orbiting bodies, as well as their angular velocities with respect to each other, are probably just some of the factors that cause orbits to decay over time.

as far as a black hole's rotation goes, I'm no expert in this area either. but i have to assume that if pulsars (rapidly rotating neutron stars) are less dense than black holes, and some of them rotate upwards of 600 times per second, then many black holes must rotate this quickly or quicker...don't quote me on that one though lol...

 

[Nabeshin]

though i don't know for sure myself (i'm definitely no expert), i think you are probably correct about the black hole's rotation also having something to do with an object's tendency to enter into an orbit around it, as opposed to just plunging directly into the black hole perpendicular to its surface, or event horizon - even if the object is initially on a direct collision course with the black hole. i probably should have suggested that the distance between two mutually orbiting bodies, as well as their angular velocities with respect to each other, are probably just some of the factors that cause orbits to decay over time.

as far as a black hole's rotation goes, I'm no expert in this area either. but i have to assume that if pulsars (rapidly rotating neutron stars) are less dense than black holes, and some of them rotate upwards of 600 times per second, then many black holes must rotate this quickly or quicker...don't quote me on that one though lol...

Right, so an object plunging into a rotating black hole will reach a certain radius at which it is obliged to start "orbiting" the hole to some extent (i.e some component of its velocity will be in the angular direction).

Orbits in binary black hole systems decay because the system emits gravitational radiation which is related to the orbital frequency (we suspect almost all physical binary black hole systems to be in very nearly circular orbits, simply because the effect of this gravitational radiation over cosmological {or stellar} timescales is to circularize the orbit as the energy leaves the system).

As far as how fast astrophysical black holes rotate, we don't know at this point. Although like you suggest, we have a strong belief that they will be rotating very fast. Just how fast is certainly open for debate, but it has been suggested that they could be very close to approaching the case for a maximally spinning black hole.

 

[JDługosz]

nin some cases, the objects can come within a close enough range of one another that their mutual gravitation can pull them into orbit around each other.

If they don't collide, the approaching objects will recede again. They won't just spontaneously form an elliptical orbit, but will fly apart in a reverse image of the approach. The system needs to lose energy to turn the hyperbola into an ellipse.

 

[94JZA80]

If they don't collide, the approaching objects will recede again. They won't just spontaneously form an elliptical orbit, but will fly apart in a reverse image of the approach. The system needs to lose energy to turn the hyperbola into an ellipse.

in some cases, the objects can come within a close enough range of one another that their mutual gravitation can pull them into orbit around each other.

that's why i said it only happens in some instances. as i stated before, I've left things quite oversimplified intentionally, but just enough so to get the OP's question answered. that said, your instance is also somewhat oversimplified. your example describes only one of two possible resulting trajectories that arise from gravitational encounters between two objects, just like my example only describes one of two types of orbits. the other trajectory taken by some objects in gravitational encounters is the parabola, and the other kind of orbit is obviously the circular orbit. i would imagine that the two objects' mass, initial speeds, and initial trajectories all play into whether one object follows either a hyperbolic course or a parabolic course about the other. likewise, i would think that the same factors would play into whether the two objects fall into either and elliptical or circular orbit about each other. that is to say, if two objects approach each other on near parallel trajectories, and with a sufficient (but not overwhelming) difference in speed, they could possibly fall into orbit about each other if their timing is right and their masses are in the right ranges. and however more unlikely, i would think that even two objects on nearly opposite trajectories could still fall into orbit about each other if their speed relative to one another is very small, among other factors.

i think, given the vastness of space, that your instance is more likely - that two objects approach each other on significantly different trajectories and high speeds relative to one another, causing them to pass by each other following hyperbolic, or perhaps parabolic, trajectories. but that isn't to say that there isn't the occasional "near parallel trajectory" or "extrememly low relative speed" encounter that may end up in an actual orbit of some kind.

 

[JDługosz]

that is to say, if two objects approach each other on near parallel trajectories, and with a sufficient (but not overwhelming) difference in speed, they could possibly fall into orbit about each other if their timing is right and their masses are in the right ranges.

How? Without involving a 3rd body, friction or other effects? If they fell from infinity or had more KE initially, they will pick up enough speed that they will fly apart again. KE + PE is constant, and you need to lower the sum in order to form a closed orbit.

 

[94JZA80]

How? Without involving a 3rd body, friction or other effects? If they fell from infinity or had more KE initially, they will pick up enough speed that they will fly apart again. KE + PE is constant, and you need to lower the sum in order to form a closed orbit.

this much is most certainly true for two objects [that are not influenced by any other external forces/effects] falling more or less toward one another from infinity. i had implied in my previous post that two objects, absent of any forces other than the gravitational forces they exert on each other, don't simply spontaneously fall into orbit about each other. but i guess i didn't make it very obvious. in fact, i only very briefly mentioned the unavoidable influences (such as friction, or the gravity of any other object in close proximity). i do see though that, back in post #5, it seems like I'm implying that all it takes is the mutual gravity between two objects to pull them into orbit...i should have been far more specific with this, b/c without specifics, the statement does look quite misleading...my apologies.

i guess it boils down to the two objects' initial locations (and thus the distance between them) and initial relative velocities (trajectories and speeds relative to one another) in determining whether they will fall into orbit or continue off into infinity after their initial encounter. in other words, if the initial distance between two objects AND their relative velocities are such that they will not have reached the escape velocity of the system by the moment of closest approach, then they will not continue off to infinity, and will therefore be gravitationally bound in an orbit of some kind. i hope that clears things up a bit.

 

[JDługosz]

I think it's true at any point, not just the point of closest approach. The escape velocity is dependent on the current separation. In general, you can look at your KE at any point.

 

[94JZA80]

I think it's true at any point, not just the point of closest approach. The escape velocity is dependent on the current separation. In general, you can look at your KE at any point.

i'm not quite sure i follow you...then again, I'm not sure you're following me either. what I'm trying to convey is that, if object A does has not achieved (or exceeded) the escape velocity of the system by the instant of closest approach to object B, then object A will not have the velocity necessary to escape object B's gravitational attraction. therefore, object A will eventually reach a point of greatest separation with object B, at which point it will begin its journey back toward object B for another instant of closest approach. in other words, object A is now in orbit around object B, or vice versa.

either way, regardless if one notion is right and the other is wrong, even the incorrect notion (if it is incorrect) is enough to answer the OP's question of why objects very rarely plunge directly into a black hole.

 

[twofish-quant]

I would think the black holes rotation would also have a large effect, 'draging' space/time around the BH, making space a funnel, with a rotating base.

It will, but you don't see those sorts of effects unless you are really, really close to the black hole (i.e. a few kilometers) or moving really fast (i.e. close to the speed of light). For the stars we can see moving around massive black holes, you can ignore "weird physics" because you can show that things aren't moving fast enough, and the gravity field isn't strong enough to cause "weird physics" effects.

Im not sure how fast a BH would rotate, or if it can be measured, but I expect something very small and very dense to rotate quickly. But I don't know, things are confusing in black holes !, (to me they are).. :)

Part of the way to get less confused is to demonstrate that you can ignore the confusing parts of the problem. It turns out that for stars revolving around a super-giant black hole, you can ignore the weird and confusing physics and just use Newtonian mechanics to describe what is going on.

 

[stevebd1]

Looking at objects in orbit, you might also want to take a look at the http://en.wikipedia.org/wiki/Vis-viva_equation" which takes into account the conservation of angular momentum-

v^2=G(M+m)\left(\frac{2}{r}-\frac{1}{a}\right)

where v is the relative speed of the two bodies, r is the distance between the two bodies, a is the semi-major axis, G is the gravitational constant and M,m are the masses of the two bodies.

Using the above equation it's apparent that v increases the smaller r gets (which is in accordance with the conservation of angular momentum where L=vmr where v and r are variable). The more v increases, the more centripetal acceleration increases (a_c=v^2/r where the units are the same as gravity, m/s²) until the point where a_c is greater than the gravitational pull, now the orbiting object will begin to pull out of orbit. As r increases then v decreases and eventually, a_c drops below the gravitational pull and the object is pulled back to the larger body. This process pretty much establishes major and minor axes which can eventually become similar over time (as in the case for Earth around the Sun).

Regarding orbits around rotating black holes, frame dragging will contribute to the velocity of an object in close prograde orbit and boost the object, the energy being taken from the rotational energy of the BH. This would also explain why orbits close to a rotating black hole can be wildly elliptical.