Another black hole question - black hole vs star, who wins??

Factual Questions
1

Okay, I enjoyed the other black hole thread currently up on the board, and I remember having one a while back on what would happen if you dropped a tiny black hole on the surface of a planet… but I don’t believe this question has come up recently. Oooh (remembers to check search.)

This is inspired by a book I’ve read recently. In it… well, there’s a small black hole orbiting a star, and a couple of different factions are using it at one point as a point in a power struggle. Side A adjusts the orbit of the hole so that it’s permanently between the sun and planet X, where side B lives, so that they get no sunlight for solar power and gradually become an ice world.

The leader of side B gets the bright idea that to stop that sort of thing once and for all, they need to drown the black hole inside the sun, and does so. Now, at this point, I was thinking “What the hell?? Black holes don’t drown, and even the small ones can absorb practically anything fluid - like, say, solar matter, a little bit at a time.” If you send any size black hole into just about any conventional fusion star, I would think you’d eventually end up with a black hole posessing the combined mass of both bodies. Am I right??

Anyone with more experience in this stuff wiling to venture a guess what would happen if a black hole collided with a star? Would the mass of the black hole or the type of star make much difference?? What if the closest approach wasn’t ‘head on’ but within the radius of the star??

I’ll add the pertinent astronomical facts mentioned in the book.
Star class: S6, “off main sequence” approximate age from start of fusion 5 billion years. Not much is mentioned about the hole other than that it’s “micro”

PS: As it happens, the next step in the power struggle is that side A extinguishes the sun involved totally, so maybe they were able to just accelerate the effect of the black hole, though it wasn’t really mentioned clearly by the author. Things get kinda confusing at that point.

Would appreciate any input, even if it’s just “yeah I wondered about this too” :slight_smile:

2

You are correct - it sounds like your author didn’t have much of a clue how black holes (or much else in physics) work.

First problem, blocking light with a black hole. In order for the black hole to be between the sun and the planet all the time, it has to be places in the planet’s L1 point. (Read up on lagrange points here: http://www.physics.montana.edu/faculty/cornish/lagrange.html ). First problem is that this is an unstable point - the black hole will tend to wander out of it over time. Second problem is that the black hole won’t block light! The black hole itself is tiny - in order to actually seriously effect light from the star to the planet it would have to be so massive that it would be severely disrupting the orbits of everything in the solar system anyway.

Second problem, “drowning” a black hole. A black hole will quite happily accept any matter you throw into it, adding it to its own. So your suspicious is correct - eventually you’ll end up with a solar-mass black hole where the star used to be. The only exception is if the black hole was so small that it was radiating mass away as Hawking radiation faster than it can draw it in from the star - and if that’s the case, it was going to evaporate anyway.

3

If the black hole was small enough (particle sized), it could travel to the center of the star without really interacting with matter in it. Any matter (particles only) it touched would be consumed, but the rate of consumption would not be enough to hurt the star in either planet A or B’s lifetimes. It may even evaporate during the process. If it were the size of a golf ball or a measurable object, it would interact with enough matter to keep feeding on the star and would also grow; thereby feeding faster. Bye bye star and life for both planets.

Planet B: Whoops… Our bad.

In either case it would not block light from the star if it were inbetween the planet and the star. If it were large enough to do that, it would have already eaten the planet too.

4

I’m not sure what you mean there by “particles only”. Could you elaborate?

chrisk, the book you were reading was assuredly fiction, but I would not call it science fiction. Aside from the fact that there indeed exists a class of objects called “black holes”, the author doesn’t appear to have the first semblance of a clue about anything.

Incidentally, while it’s true that a sufficiently-small black hole could pass through a star without absorbing much, it’s going to absorb some. Unless it’s in a hyperbolic orbit (in which case there would be no point in sending it through the star at all; just send it far away), it’ll pass through the star repeatedly, picking up a little more mass each time. Any mass it absorbs will both slow it down, thereby increasing the time it spends in the star on the next pass, and enlarge it, thereby making it more efficient at scooping up matter. Eventually, it would eat the whole star, even if it takes a while.

5

By ‘particles only’ I don’t mean that that’s all the BH would consume. I just mean that the only way to have a BH consumed inside the star would be to have a particle sized BH that did not interact with the matter in the star readily. Perhaps it would evaporate, perhaps it would eat enough of the star to gain enough mass to actually begin feeding on a full sized atom. However at first what type of matter could cross this exceptionally small event-horizon? Only particles, the BH’s schwartzchild radius could very well be smaller than an atom.

6

I think the key points here are mass, event horizon, orbits, and “evaporation”.

As someone already pointed out, even if you had a large planet or something and tried to place it between the evil planet and its star, it wouldn’t stay there. Objects in different orbits travel at different angular velocities. If it were in one of the evil planet’s lagrange points, then by definition it would be in the same orbit as the evil planet and would never get between it and the star.

As someone else already pointed out, if the black hole is massive enough to have a significant event horizon, it is going to be so massive as to mess up the orbital dynamics of the system anyway. If it isn’t that massive, there wouldn’t be any way for it to eclipse the star, even if it could stay between the star and the evil planet, which it can’t.

Very small black holes “evaporate” via Hawking radiation at a pretty good rate. That’s why the super-tiny black holes that might be created in the Large Hadron Collider won’t be any problem – they’ll only last a tiny fraction of a second (cite: Scientific American, May 2005, “Quantum Black Holes”). Even if you aimed one at a star, it would dissipate faster than it could absorb mass.

So on several counts, the story is full of holes. :smiley:

And, people (especially CanTak3), don’t go around worrying about “particle-sized black holes.”

7

I think it was in a Sci-Am article a while ago…kind of a fun article, really, about, literally, when stars collide. One example discussed what happens if a white dwarf hits a Sun-like star pretty much head-on.

If I remember correctly, the white dwarf plows right through pretty much unscathed, carries off a nice outer shell of incandescent gas (which might even reignite fusion reactions near its surface for a while, giving it new life), and the poor Sun-like star it collided with gets blown apart in a giant thermonuclear explosion, caused by the enormous increase in energy and temperature produced within the star by the shock wave of the white dwarf passing through it.

Now replace the white dwarf with an “average”-sized stellar black hole, and say it’s moving along relative to the star at a pretty good clip, but nothing outlandish. It would probably take the black hole many minutes to pass through a star even at tens-of-thousands of KPH. What happens? Does it stir up the inside of the star so much that whatever it doesn’t accrete gets blown apart?

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Here you go! (Scientific American, November 2002)

It says a white dwarf would take about an hour to tear through the star.

I think the problem with “it’s gonna eat the whole thing” is the speed of the impact. But it would depend on the mass of the black hole. A black hole with the mass of a white dwarf, I assume, would have quite a smaller event horizon than the white dwarf. Conversely, a black hole with an event horizon the size of a white dwarf would have much more mass and so much more gravity that it might drag the entire star with it until all the bits of the star spiraled down to their deaths.

9

What about gravitational lensing? Could you put a black hole between a planet and a star and burn people out like ants?

Generally, speaking, though, if you did place a black hole between a planet and it’s star, what sort of effects would it have on the light the planet recieved?
If we replaced Venus with a Venus-sized black hole, what will the transit of formerly-Venus look like?

On the “black hole smaller than an atom”… As a sub-atomic black hole passes through the “empty space” between nuclei… does it absorb the atoms’ electrons or not? How much of an atom’s electron shell “cloud” has to pass the event horizon before it’s caught?

10

If we replaced Venus with a Venus-sized black hole, the sun would start orbiting around it. A black hole with the mass of the sun would only be about six kilometres across, so a black hole that size would be many times more massive than the entire rest of the solar system.

11

Can someone help me understand how it’s possible for black holes to radiate anything away from it? I thought nothing could exit a black hole’s event horizon, not even EM radiation. Not that I’m disputing the conclusions of my betters… I’m just trying to understand the basis for this paradox.

12

Hawking Radiation. Note that due to the requirement of particle frequencies higher than inverse Planck time in the formulation of Hawkings solutions, the validity of the evaporation concept is questionable (though largely currently accepted by the physics community, I think.) The actual basic theory is beyond my current understanding of the topic, so I can’t offer a more detailed explaination.

Stranger

13

That’s only for the final moments of the evaporation, and I don’t think anyone even claims to know what happens there. But evaporation of larger black holes is as well-understood as anything involving quantum mechanics, and can be derived via a wide variety of theoretical frameworks, so it’s pretty well accepted (at least, by those who accept black holes in the first place).

LivingInThePast, there are five Lagrange points in a two-body system, only two of which share the planet’s orbit. There is indeed a Lagrange point in between a planet and the Sun, and if you put an object there, it can indeed stay in between the planet and the Sun indefinitely. But it’s unstable, so you’d better have a very steady hand when placing your object, or it’ll eventually wander off.

And 1010011010, you could approximate gravitational lensing as an ordinary lens a few times larger than the black hole’s event horizon. If we’re talking about a microscopic black hole, then your magnifying glass is going to be much too small to fry ants, much less people.

14

To be fair, I do think he was asking about a Venus-sized black hole. Does Venus ever even pass between the earth and sun? (guess that’s irrelevant because if she did ever magically transform into a black hole, we’d have bigger problems to deal with)

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Yes it does, just under twice per century. You just missed a chance to see it happen, since the most recent such transit of Venus was last June. There’s another in June 2012. Miss that and you’ll have to hope to be still around in 2117 to catch the one after that.

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I think it’s important here to make the distinction between “size” and “mass”. As Sofis said, a Venus-sized black hole is very massive indeed, and it would become the centre of our solar system.

However, a black hole with the same mass as Venus, if it were to magically replace Venus, would carry on in the same orbit that Venus is in now. The sun would be unaffected, we wouldn’t go spiralling out of our orbit etc.

The transit of black-hole-Venus would be unspectacular, as it would be very small indeed and therefore we wouldn’t see it’s disc. As I understand gravitational lensing, you’d get the same amount of lensing from BHV as you currently do from Venus, ie very very little.

What I’m wondering is - if Planet A has something that can move a black hole into a specific orbit, why didn’t they use it to move Planet B into a different orbit? A much more elliptical one, maybe, so they got frozen for 6 months, then roasted for 6 months (assuming the rotational period of the planet was 1 earth year).

17

And to answer the OP… the “black hole inside the sun” problem depends on how big this black hole is. If it just sat in the centre of the sun, being quite small, with relatively low mass, it might gobble up a few particles now and again, but not really get in the way of the thermonuclear reactions. It might speed up the death of the sun a bit, by hastening it’s collapse.

I think the bigger problem is not the black hole in the sun per se, but more getting it there - as has been mentioned about the white dwarf, the sun would suffer from the black hole passing through it. Probably it would disturb the convection patterns within the sun’s various layers, so reactions would slow down, or speed up, where the black hole was. The sun would therefore be a bit off balance in terms of power output, energy keeping it at it’s current radius etc. Could get messy.

18

The “size” I was referring to was “mass” not “volume”, though I certainly can’t fault you for not noticing my rather unconventional use of the word “size”.

But it looks like the Venus-massed blackhole question got answered eventually anyway… and the answer seens to be “it would be really small and you wouldn’t notice it doing anything interesting to the light during transit”.

What about the subatomic black hole passing through an “electron cloud” around a nucleus, though? If no one is there to observe whether an electron’s position is inside the event horizon… is it ever inside the event horizon?

19

This is a good question - I like this question a lot… this is exactly why I like the SDMB. Now I’m going to wait around for a real scientist to tell us because I haven’t got a clue.
Well… while I wait around for a real scientist, I’m going to throw something into the mix - I would imagine you’d have a sort of probability thing… if you worked out the probability of the electon’s orbit intersecting the BH to be 50%, say, and repeated the experiment over and over again, you’d find that the electron we gobbled up 50% of the time. Or something.

I hope a real scientist turns up quickly! :stuck_out_tongue:

20

Yeah, I would think this would turn into a sort of quantum experiment… as I understand it, quantum mechanics doesn’t say that you can never say where an electron is ever, just that you cannot say where it is without disturbing it… you cannot isolate both its position and its speed, or something like that.

If the black hole swallows it, you know (at least generally) where its position is, but not its speed, and now you can never tell that because it’s been gobbled up. Does that make sense??