Collision of massive star with black hole

[Johninch]

I saw somewhere a thread about a cow falling into a black hole. I don’t know why a cow, perhaps the OP was thinking about the Milky Way. With the cow example it is not clear to me what we are supposed to see when an object falls towards the event horizon. If time slows to infinity, does the object remain viewable? I don’t understand what is meant in this example, partly because a cow does not emit light. I prefer the following example:

A star with 100 solar masses approaches a BH with 10 solar masses, from behind, at high velocity, as seen from the Earth with optical telescopes. What do we see?
My guess is that we see the following:

- a halo exists round the star (due to lensing)
- then a dark point appears in the center of the star
- the point grows into a small black disk
- the star changes to red and starts to fade
- suddenly the black disk disappears and the star becomes as bright as before

I am assuming that the velocity of approach is high enough that only a small accretion disk has time to form round the BH.

Please tell me what’s wrong with my guesswork. Basically I want to know, can a BH be consumed by a star, so that it is no longer a BH, and what do we see when this happens? What do the relative masses of the star and the BH have to be, in order that the star consumes the BH and not the other way round?

Also I would like to understand what we mean by the singularity at the center of a BH. If nearly all the mass of the BH is concentrated in the singularity, presumably we get a gigantic explosion when the singularity expands into ordinary matter? Or if the BH’s mass is relatively small, does the BH just fall into the star pretty quick like a comet into Jupiter?

 

[DaveC426913]

I saw somewhere a thread about a cow falling into a black hole. I don’t know why a cow, perhaps the OP was thinking about the Milky Way. With the cow example it is not clear to me what we are supposed to see when an object falls towards the event horizon. If time slows to infinity, does the object remain viewable?

Yes. It gets redder and dimmer, until you can't distinguish it from the rest of the event horizon.

I don’t understand what is meant in this example, partly because a cow does not emit light.

It reflects light.

A star with 100 solar masses approaches a BH with 10 solar masses, from behind, at high velocity, as seen from the Earth with optical telescopes. What do we see?
My guess is that we see the following:

- a halo exists round the star (due to lensing)
- then a dark point appears in the center of the star
- the point grows into a small black disk
- the star changes to red and starts to fade
- suddenly the black disk disappears and the star becomes as bright as before

No. The star starts to break apart as it nears the BH. Matter is stripped from its surface and forms an accretion disc around the BH.

Please tell me what’s wrong with my guesswork. Basically I want to know, can a BH be consumed by a star, so that it is no longer a BH,

No.

and what do we see when this happens? What do the relative masses of the star and the BH have to be, in order that the star consumes the BH and not the other way round?

Relative masses have nothing to do with it.

The star will never consume the BH. Sure, the star may envelope the BH, but the BH will simply continue in the core of the star, consuming it from within.

The collision has two possible outcomes.
1] The star is moving so fast that much of its mass continues past the collision. In what state this mass continue is debatable, but it will certainly not be a stable spherical star.
2] The star is not moving fast enough to escape the BH. It gets torn apart into an accretion disc of matter around the BH.
Either way, the BH grows.

Also I would like to understand what we mean by the singularity at the center of a BH. If nearly all the mass of the BH is concentrated in the singularity, presumably we get a gigantic explosion when the singularity expands into ordinary matter? Or if the BH’s mass is relatively small, does the BH just fall into the star pretty quick like a comet into Jupiter?

The star will never consume the BH. The BH will always survive the collision. It will certainly never expand to ordinary matter. At best, the BH will orbit the core of the star, drawing the star's matter into it until the star is gone, leaving only a larger BH.

There is a conceptual error that is causing you to believe the BH can be destroyed. What makes you think so?

 

[Johninch]

I don't follow. The star will never consume the BH. The BH will always survive the collision. It will certainly never expands to ordinary matter.
What makes you tihnk it would?

So to take an extreme example, you are saying that a BH of 5 solar masses would consume a star of 1000 solar masses in a head-on collision? In other words the collision would form a BH of approximately 1005 solar masses, irrespective of the dynamics of the collision.

If this is the case, then I was not understanding the stability of mass in the form of a singularity i.e. a singulartiy cannot revert to something else, whatever the force applied and iirespective of the mass of the BH containing the singularity. Therefore BHs cannot get smaller, except thru Hawking radiation. Have I got it right now?

 

[DaveC426913]

So to take an extreme example, you are saying that a BH of 5 solar masses would consume a star of 1000 solar masses in a head-on collision?
In other words the collision would form a BH of approximately 1005 solar masses, irrespective of the dynamics of the collision.

The actual full consumption would be difficult to achieve. Most of the star would either escape or become an accretion disc.
But it he simplified, ideal scenario, the BH would eventually mass 1005 sols, yes.

If this is the case, then I was not understanding the stability of mass in the form of a singularity i.e. a singulartiy cannot revert to something else, whatever the force applied and iirespective of the mass of the BH containing the singularity. Therefore BHs cannot get smaller, except thru Hawking radiation. Have I got it right now?

Yup.

 

[Johninch]

Yup.

Thanks. May I add a supplementary question?

Does current theory say that a BH can consume the surrounding Dark Matter?

And, what is the level of probability that a massive central BH could consume a whole galaxy (or most of it)?

I guess that it could, if the BH can accrete Dark Matter. Otherwise, I suppose that the distribution of stars could prevent it. Is there a model which we can use to extrapolate the reach of the BH's gravity-well in order to answer this question?

 

[Chronos]

The only thing we can say with certainty about a black hole is all its mass resides inside its Schwarzschild radius [event horizon]. The notion that it is condensed into an infinitesimal point at the center of its event horizon is mere speculation. Black holes are not believed to consume much dark matter. As dark matter is essentially collisionless, it has no way to shed kinetic energy and inspiral into a black hole in the manner ordinary matter does.

 

[Johninch]

As dark matter is essentially collisionless, it has no way to shed kinetic energy and inspiral into a black hole in the manner ordinary matter does.

Yes, but DM has mass, so it should be captured by a gravity well. So are we saying that DM is attracted by the BH but passes thru it and out again?

 

[phinds]

Yes, but DM has mass, so it should be captured by a gravity well. So are we saying that DM is attracted by the BH but passes thru it and out again?

If dark matter is headed straight for a black hole, it will fall in just like regular matter. Most matter enters black holes though an accretion disk (google them if you are not familiar with their properties) but dark matter will not interact with an accretion disk so if it's going fast enough to bypass the BH it will do so, unlike normal matter which will collide with the accretion disk and be slowed down enough to eventually fall in.

 

[phinds]

And, what is the level of probability that a massive central BH could consume a whole galaxy (or most of it)?

zero. The galaxy outside the immediate region of the BH cares not a whit that the large mass in the center is a BH, it's just a large mass and the stuff in the galaxy orbits it because it's at the center of mass of the entire galaxy.

 

[DaveC426913]

Thanks. May I add a supplementary question?

You may ask all the questions you wish! That's what PF is all about!

Does current theory say that a BH can consume the surrounding Dark Matter?

While DM is affected by gravity, not a lot will fall in ,as others have pointed out.

Consider the solar system. In its early days, lots of dust and gas was floating every which way, and, when it collided, a lot of it would fall into the sun. Nowadays, there's little left to collide, so our planets, though they are affected the the sun's gravity, do not fall in. There is nothing left to knock them. Likewise, any DM near a BH will virtually always have some angular velocity, so it won't tend to fall straight in, it will loop around in an elliptical orbit - you'd need to have an extremely eccentric orbit for the DM to come near an object that is so small.

And, what is the level of probability that a massive central BH could consume a whole galaxy (or most of it)?

I wouldn't have said zero. But BHs aren't the cosmic vacuum cleaners many think they are. A BH massing 10 million sols has the same gravity at a distance as does 10 million suns. Either one could live at the centre of our galaxy. Stars farther out will orbit a BH exactly the same as if they were orbiting a cluster of 10 million suns, so the BH won't do any more consuming than will the sun cluster.
A BH could consume a galaxy, but the circumstances balancing stellar density would have to be juuuust right.

 

[Johninch]

BHs aren't the cosmic vacuum cleaners many think they are

Thank you and Phinds and Chronos for clarifying this. I have read that most galaxies were initially seeded with a BH which has since grown, and also that DM is expected to be densest near the center of a galaxy, which would help it grow. However, the mass of central BHs which have been identified is never more than 10 percent of the mass of the total galaxy and is mostly a lot less. Thus I conclude that the growth of the central BH is limited by the available matter, including DM, within the range of its gravitational attraction (which depends on mass, not size).

A proviso which I would add is that we have only identified about 30 galaxies with a BH at their center, which is not a representative sample. A galaxy where a BH had consumed 90% of its mass would be difficult to detect, unless it is relatively near. I suppose that in such a case there could be difficulties in measuring its size, distance and in evaluating the red shift.

 

[phinds]

... and also that DM is expected to be densest near the center of a galaxy

This is incorrect. Dark Matter exists primarily in the "halo" at the outer edges of the galaxy. This is a direct consequence of their lack of interaction with anything other than gravitationally. Normal matter is slowed by collisions and then attracted to individual solar systems and then to the formation of stars/planets/etc. Dark Matter just fall in towards the center of the galaxy but isn't slowed down so follows a sine wave of oscillation from halo to center to other side and back again. If you look at the implcations of that motion its easy to see that most of the time is spent as slow-moving objects (I hesitate to say "particles" because we have not yet even proved conclusively that DM is particles) in the halo and less time as fast moving objects passing through the center of the galaxy.

 

[Johninch]

Dark Matter exists primarily in the "halo" at the outer edges of the galaxy.

I based my remark on the Wikipedia article on WIMPs, in the paragraph on Experimental Detection, which states:

"Indirect detection efforts typically focus on locations where WIMP dark matter is thought to accumulate the most: in the centers of galaxies and galaxy clusters, as well as in the smaller satellite galaxies of the Milky Way."

 

[Chronos]

What you are describing is known as the core cusp problem. Simulations suggest dark matter density should peak in the center of galaxies. This profile lacks observational support which suggests DM is relatively lacking in galactic cores I agree with Phinds, there is no apparent reason it should dwell longer in the core than the halo given it has no means of shedding kinetic energy and achieves its highest velocity as it approaches galactic cores. On that count it makes more sense DM density should peak in the halo region where it is traveling more slowly. It appears simulation models inaccurately model DM distribution.