# A Hole Lot of Trouble?



## Ursa major (Jun 13, 2013)

> Twenty-six new black hole candidates have been discovered in the neighbouring Andromeda galaxy. According to the astronomers involved, these could be just the tip of the iceberg. Details of the find will be published in the 20 June issue of _The Astrophysical Journal_.
> 
> The discoveries are the culmination of 13 years of observation. Researchers used Nasa's Chandra and the European Space Agency's XMM-Newton satellites. Both record the X-ray light emitted by celestial objects.


From http://www.guardian.co.uk/science/a...13/jun/13/black-hole-bonanza-andromeda-galaxy.


Here's a worrying thought: given that black holes could, potentially, be weapons of mass destruction rolleyes, is a plan being hatched, at this very moment, to invade the Andromeda galaxy?


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## AMB (Jun 13, 2013)

I'm just wondering what part of them being there the astronomers were involved in.


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## Velocius quam lucem (Jun 14, 2013)

My God, It's full of black holes!

I'll be impressed when they can count the number of black holes we _can't_ see - be it Andromeda or here.


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## Gordian Knot (Jun 14, 2013)

Well considering they have already summed up all the missing matter in the universe we can't see, how hard could it be to count the black holes we can't see.....


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## Velocius quam lucem (Jun 15, 2013)

Gordian Knot said:


> Well considering they have already summed up all the missing matter in the universe we can't see, how hard could it be to count the black holes we can't see.....



It's rather ironic actually... Black holes "eat" dark matter too according to this article:

Black holes gobble dark matter

Maybe that's where dark energy get's it's calories. (Just kidding).


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## Vertigo (Jun 15, 2013)

Idle query: I wonder what the chances are of a star system getting a 'close pass' by a black hole and also how close it would have to be to be damaging to that system. Obviously dpendent on size, and I'm not sure of my maths here, but if a black hole say 100 times the mass of the sun came close to the solar system, how close would it need to be for it's gravity to effect, say, the orbits of the planets?


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## Velocius quam lucem (Jun 15, 2013)

Okay, here's the Newtonian formula. The Einstein field equations are  more accurate, but way more complex. M1 and M2 are the masses of the two  objects - the black hole and our sun (the majority of visible mass in  our solar system). See below for math.








So M1  = 100xM2. I will use this as the mass of the sun (1.98892 x 10^30 kg). So we have G*(1.98892 x 10^30)*(1.98892 x 10^32)/  r2 where r is the distance between the two masses. Now if we choose a  gravitational force that is strong enough to pull on the solar system it  would be in Newtons. Let's say for a rough starting point we choose 1  billion Kilo Newtons. (Probably barely enough to cause a slight bow  shock on the solar system.) Then we rearrange and solve for r2. (r2 =  (G*M1*M2/F) = ((6.67 x10-11)*(1.98892 x 10^30)*(1.98892 x 10^32))/1 x 10^12 N = 2.64 x 10^40. Now since that is r2, we take the square root, then r is 1.624 x 10^20 meters, or  1.624  x 10^17  Km. Light travels at 3.0 x 10^5 Km/s, then it would be (1.624  x 10^17  Km/ 3.0 x 10^5 Km/s) or about (5.413 x 10^11 light seconds)/(86400 sec/day) = (6.265 x 10^6 light days)/(365 days/year) = 17,165 light years away. (Please correct any mistakes if I've made some.)

The  Milky way is 100,000 light years across. So there could easily be black  holes in that range (17,165 light years away), but then if black holes  and dark matter are both smattered somewhat evenly about the galaxy, the  forces could kind of balance out, and we could be gently tugged at from  all directions. Besides - 1 billion Kilo Newtons would be so small at the radius of the Kuiper belt I've probably used too small a force. 

Perhaps I'll try again later with a force the size of the sun at the edge of the solar system, and then move that out to the correct radius based on the 100X mass black hole. the gravity at the surface of the sun is 28 times Earth gravity, but way out here (1.47 x 10^8 Km) the sun's pull on us is only 0.0006 Earth G.

Edit: Come to think of it, we could just scale up the sun's pull by 100 times and get 0.06 Earth G for the pull of a black hole with 100x the sun's mass at the same distance from us as the sun. Another problem with this is momentum. Everything in this system is moving, and it would depend a lot on force vectors, etc. Plus 100X the sun's mass is fairly small as black holes go from what I understand. I don't know. Maybe I should've just kept my fingers away from the keyboard.


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## Vertigo (Jun 15, 2013)

That's brilliant VQL, thanks! I've not taken the time to go through the maths but will do later and whilst I knew Netwon's gravitation formula I had absolutely no idea of the sort of force needed to cause an effect. So that is excellent!

And a little scary; so, without going into huge detail, a black hole of that sort of mass passing within say 10 light years is likely to have quite significant impact on the orbits of the planets. Okay I know the odss are probably tiny but still...

Mmmm little cogs turning in my head here...


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## Mirannan (Jun 16, 2013)

The most typical mass for a black hole is somewhere between the Chandrasekhar limit (sp?) which is just under 3 solar masses, and around 10 solar masses. Said limit being the theoretical maximum for a neutron star, incidentally. Bigger black holes are rather rare, at least that's what is thought, because really big stars are rare. And there is a big mass gap between black holes that are stellar remnants and the next group, thought to be those at the centre of globular clusters at maybe 50,000 times Sol mass.

The real point is that, unless you get really close, there is no difference between the effects of a black hole and any other similarly massive object. Ballpark figure for serious gravitational effects on Earth's orbit for a minimum-mass BH might be 30-40 AU, around the distance of Neptune. Although an object considerably beyond that might well throw quite a lot of Kuiper belt and/or Oort cloud objects in our general direction.


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## paranoid marvin (Jun 16, 2013)

Ah, I didn't quite appreciate the gravity of the situation...




What I could never understand was how a hole could be 'black'??


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## Vertigo (Jun 16, 2013)

Mirannan said:


> The most typical mass for a black hole is somewhere between the Chandrasekhar limit (sp?) which is just under 3 solar masses, and around 10 solar masses. Said limit being the theoretical maximum for a neutron star, incidentally. Bigger black holes are rather rare, at least that's what is thought, because really big stars are rare. And there is a big mass gap between black holes that are stellar remnants and the next group, thought to be those at the centre of globular clusters at maybe 50,000 times Sol mass.
> 
> The real point is that, unless you get really close, there is no difference between the effects of a black hole and any other similarly massive object. Ballpark figure for serious gravitational effects on Earth's orbit for a minimum-mass BH might be 30-40 AU, around the distance of Neptune. Although an object considerably beyond that might well throw quite a lot of Kuiper belt and/or Oort cloud objects in our general direction.


 
Yeah done a bit more research since last night Mirannan. Seems the biggest Stellar black holes are around 20 solar masses so not that big really. Even with that 100x mass one that I suggested it would have to be within 72 light days to exert just 1% of the gravitational force of the Sun on the Earth, so that is pretty darn close. Haven't figured it for a 20x black hole but it's obviously going to be a lot closer.

I did wonder if stirring up the Kuiper belt and Oort cloud would be the most significant effect.


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## Gordian Knot (Jun 16, 2013)

Which brings up a good point Vertigo. Just what are we meaning when we say solar system. Because obviously the eight planets is just a tiny piece of the solar system. There is the Kuiper belt, and the supermassive, super far away Oort cloud. As I understand it the boundary of our Sun's gravitational influence is the outer edge of the Oort Cloud.

At least according to The Scale of the Universe as shown on The Astronomy Picture of the Day. (3/12/12).


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## Vertigo (Jun 16, 2013)

Yes that is my understanding and, as Mirannan pointed out, disturbances out there could have later (much later) consequences farther in.


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## Velocius quam lucem (Jun 17, 2013)

Thanks for clarifying the average size of black holes Mirannan. It seems that their mass is trifling during any colloquy of the subject. I had somehow fallen into the (obviously) false impression that most of them were large, possibly because they could potentially come from stars that were 1000x the mass of the sun, but then those blue giants must also be rare. 

As far as the "edge" of the solar system, there isn't any current exact consensus among astronomers that I've seen. Here is an excerpt from an article on Universe Today:

_"One _way to look at the diameter of the Solar System is to assume  that it ends at the edge of the heliosphere. The heliosphere is often  described as a bubble where the solar wind pushes against the  interstellar medium and edge of where the Sun’s gravitational forces are  stronger than those of other stars. The heliopause is the term given as  the edge of that influence, where the solar wind is stopped and the  gravitational force of our Sun fades. That occurs at about 90 AU, giving the Solar System a diameter of 180 AU. 

Looking at the aphelion(according to NASA figures) of the orbit of the  farthest acknowledged planet, Neptune, the Solar System would have a  radius of 4.545 billion km and a 9.09 billion km diameter (about 60 AU)."

From: Solar system diameter

It seems that there is also some contention as to the exact location of the Oort cloud as well: 

"Some believe that it begins at 2000 or 5000 astronomical units and ends at 50,000 AU, which is almost a light-year. Others think  that it may extend to over 100,000 AU." 

From: Oort Cloud diameter

​


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## Mirannan (Jun 17, 2013)

Theoretical maximum mass of a stable star is around 90-100 solar masses, depending on composition. From what I've read (I'm no astrophysicist) larger stars would likely blow up immediately, and also the cloud that a larger star might be born from would most likely fragment into smaller pieces before stellar ignition anyway.

In addition to that, even in really big stars, when they go supernova quite a lot of the mass is lost to interstellar space.

None of this applies to the conjectural population III stars, which were the first ones to form, because their composition would be very different from today's stars. In particular, they would be almost pure hydrogen/helium mix. For reasons which, again, I don't understand this would mean that the first stars would all be really huge and also very short-lived. How much of their mass would be left behind in the black hole, I don't know. I'm not sure anyone does.

For a bit of scariness: Rather MORE dangerous than a black hole would be a newly hatched pulsar or magnetar that happened to be close and happened to have its beam going our way. This might mean a beam of luminosity a significant fraction of the Sun's radiation (or even several times that depending on distance) coming our way. The scary part is that it would be mostly in gamma rays and hard X-rays.

We would know it's about to happen though, because the warning would be a close supernova; a difficult event to miss. There would, unfortunately, be feck all we could do about it.


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## chrispenycate (Jun 18, 2013)

paranoid marvin said:


> Ah, I didn't quite appreciate the gravity of the situation...
> 
> 
> 
> ...



If the escape velocity (the speed {"velocity" is inexact. Unless motion id directed to the body, the quantity is scalar, not vector} at which something has to be going to get totally free of its gravitational and, if relevant electromagnetic fields) is as great as, or greater than, the speed of light it stands to reason that electromagnetic radiation can't get out of it, either. So it is a perfect absorber; all energy is drawn into it, none is emitted, much blacker than mere black body radiation. 

More worrying is the terminology "hole". Hole through what, into what, relative to what? Since it consists of mass (albeit mass with a certain limitation on ways to express itself) the hole has been filled, if not plugged.

And Hawking assures us black holes are not actually quite black. Big ones are very cold, but tiny ones (masswise – I'm not sure any black hole has a physical size) of a few gigatons are quite hot (and the heat emitting region has a linear size, the size of the event horizon. He's better at the equations than me, so I'll believe him, however irrational it might seem.


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