Black Hole at Center of Milky Way

kwertii writes: "The Washington Post reports new evidence that there is a black hole with the mass of 2.6 million suns at the center of our galaxy. The Chandra X-Ray Observatory happened to be looking at the presumed site of the hole at the moment it absorbed a comet, blasting x-rays off into space as a byproduct. The implication is that the Milky Way is slowly spiraling down into a giant galactic drain..."

Comments

Haiku

[quintessent]

At the candy shop--
Dark, terrible, he requests:
"One Milky Way, please"

Re:Haiku

[geomcbay]

Offtopic? +5 Funny/Genius.

Re:Haiku

[Luyseyal]

Nitpick: You forgot the seasonal reference in the second line. :-)

-l

No fear, the galaxy's safe.

[Jodrell]

A mass of 3 million Suns may seem a lot, but it isn't when you remember that the Galaxy is quite a bit bigger than that. It's unlikely that this Black Hole could "swallow" the galaxy, in fact it's probably the only reason our galaxy exists.

Incidentally, the BBC article is here.

Re:No fear, the galaxy's safe.

[gazbo]

Possibly due to the name, everybody seems to treat a black hole as if it is exactly that - a hole. Really it is just a massive gravity source due to phenomenal mass and density.

The implication in this case is that the black hole provides a central gravity source large enough that the entire galaxy slowly circles it, in a gradually degrading orbit. In this aspect you are right - without such an object the Milky Way could not exist as it does now, as there would be nothing stronger than the attraction between solar systems to hold it together.

However, that does not mean that the black hole is incapable of "swallowing" the galaxy. The fact that the Milky Way is a spiral demonstrates that the orbit is degrading. As more objects are drawn in to the black hole, it can only serve to increase the size and mass and make an even more powerful gravity well.

Re:No fear, the galaxy's safe.

[CheshireCatCO]

n this aspect you are right - without such an object the Milky Way could not exist as it does now, as there would be nothing stronger than the attraction between solar systems to hold it together.

The attraction between stars would be quite enough to hold the galaxy together. For decades, galactic researchers didn't have any reason to think that there was a black hole at the center of our galaxy. They never needed it to hold things together; after all, the black holes looks just like 3 million solar mass stars in the galactic nucleus to our Sun.

By way of analogy, globular clusters are hold themselves together without black holes in their cores (N-body simulations indicate that they are, in fact, dynamically stable). And there is at least one case of a galaxy that probably does not have a black hole in its nucleas. All tests have come up negative for it.

The fact that the Milky Way is a spiral demonstrates that the orbit is degrading.

Not really, no. The spiral structure of galaxies has nothing to do with "spriralling down the hole." It's probably some time of density wave phenomenon, stable and self-perpetuating. The orbits of individual stars and gas clouds are basically stable, Keplerian orbits.

Re:No fear, the galaxy's safe.

[Cheeko]

actually the black hole will continue to grow as it swallows more matter. That is to say that the event horizon will continue to expand. This is the result of the increase in gravity as it becomes denser and denser. In effect it swallows some matter, expands, thus making it stronger and able to swallow more matter, and so on. In theory as long as there is matter flowing into the black hole then it could continue expanding to swallow the whole galaxy in time.

Steven Hawking explains this concept pretty well in his Brief History of Time

Re:No fear, the galaxy's safe.

[Neil Rubin]

However, that does not mean that the black hole is incapable of "swallowing" the galaxy. The fact that the Milky Way is a spiral demonstrates that the orbit is degrading. As more objects are drawn in to the black hole, it can only serve to increase the size and mass and make an even more powerful gravity well.

Actually, it is impossible to for a black hole to ever swallow all of the matter orbiting it, unless some outside force (not gravity) starts literally pushing it in. This is a simple consequence of the conservation of energy.

For any object in a bound orbit in a gravitation field with a 1/r^2 force (true for a black hole except when you get extremely close), the average kinetic energy of the orbit <T> and the average potential energy <U> obey <U> = -2 <T>. This is due to the famous Virial Theorem. As a result, the average total energy is always negative and equal to half of the average potential energy.

Now, as the average radius of an orbit decreases, the potential energy will become more negative, and so will the total energy. If this were to happen to all of the matter orbiting the black hole, the total energy of the system would decrease--impossible!

What actually happens is that the particles in orbit constantly bounce off each other, some gaining energy and some losing. Those that lose enough, fall into the black hole. Those that gain enough, escape never to be seen again.

This is exactly what is observed to happen with the clouds of dust that collect to form stars. It all bounces around, and some of it ends up in the star while the rest of it flies off into the great beyond. Of course, some of the extra energy in the black hole case is lost from the X-rays originating from the extremely hot region just outside the horizon. That, however, can't explain how something the size of a galaxy could all end up that close to the horizon of the black hole to begin with. A very large fraction of the matter must escape long before then.

Re:No fear, the galaxy's safe.

[Dyolf Knip]

The orbits of individual stars and gas clouds are basically stable, Keplerian orbits

Yeah, really fscking long ones; plus they've got this vertical oscillation. I think Sol's orbit is 200 MYears and the oscillation is 26 MYears. It's thought there may be some correlation between mass extinctions here on earth and when we go through the thickest part of galaxy.

Re:No fear, the galaxy's safe.

[Luyseyal]

You're forgetting Hawking radiation... black holes do give off energy, albeit slowly compared to, uh, fusion in Sol, but they still lose energy over time. Unless the consumed energy is greater than that energy radiated, it will die off like the rest of us.

As Levar Burton would say "But don't take my word for it". Look up Hawking radiation. I'd do an awful job describing it.

-l

Other links

[Joao]

Here are a few more links on this:

Official website

Official press release

Story on CNN

Vindicated at last

[Rogerborg]

• The implication is that the Milky Way is slowly spiraling down into a giant galactic drain...

They all laughed when I built my Y2k bunker and bought all that Spam(tm). Well, who's laughing now?

Shouln't this exact moment not last for ever??

[Deag]

I'm probably completely wrong here, but as you go near a black hole doesn't time not slow down, so to us this comet going into hole should last forever... or something???

Re:Shouln't this exact moment not last for ever??

[krlynch]

You are correct ... and you are also wrong.

Let me explain :-)

To an observer outside the event horizon of the black hole, the object never appears to actually cross the horizon, just to approach it more and more slowly as time goes on. In other words, the clock of an infalling observer will appear to run slower than the clock of an observer that does not approach the horizon. More generally, to a distant observer a clock in a strong gravity field will run slower than a clock he carries around with him.

Meanwhile, for the poor observer entering the black hole, as he approaches the horizon, the clock HE carries appears to continue ticking away at its usual rate, while his view of the universe slowly gets distorted, so that it looks like he is travelling down a tunnel towards the hole's surface. In a finite amount of time, he crosses the event horizon, and the "tunnel vision" he has of the rest of the universe shrinks to zero size. He doesn't notice his clock slowing down, and he eventually will hit the "bottom" of the hole.

Interesting fact: if he tries to fight the hole to prolong the time before he hits the bottom, he'll actually hit the bottom sooner than if he didn't fight.... of course, when you've already been ripped apart by the tidal forces, you wouldn't notice, but let's consider just and "ideal observer" :-)

This "strange" (some would incorrectly say "paradoxical") behavior of the same set of events appearing differently to two observers is one of the hallmarks of the "Theory of Relativity" ... but results like this where two people disagree qualitatively on the outcome can only occur when the two can never again communicate with each other. Otherwise, they will only disagree quantitatively on the outcome of an "experiment".

Re:Shouln't this exact moment not last for ever??

[Sebastopol]

Yes, you're right. But...

The philosophical argument about the astronaut falling into a black hole is just filler material for science journalists who need an article ASAP. Don't waste your brain cells. There's no proof black holes exist. The philosophy arises from the singularity in the gravitation equation and lorentz's time dilation equation. (radii decrease, gravitiy increases, time slows down... oops...)

I'm can't wait for the day when we do away with this time-stopping/zeno's paradox thing and find a different model for gravity, other than the centuries old newtonian model.

my $0.02

Feeding the mozillaquest troll...

[simpleguy]

" The implication is that the Milky Way is slowly spiraling down into a giant galactic drain... "

Do we get to see Mozilla 1.0 before that happens ?

Heh.

more nitpicking...

[radja]

>By a stroke of good fortune, NASA's Chandra X-ray Observatory satellite happened to be looking when the presumed black hole enjoyed a quick snack of gas and dust.

give or take a few million years... light does take a little while to reach us from the middle of the galaxy you know :)

//rdj

Re:more nitpicking...

[Dyolf Knip]

30,000 years. Not a few million.

Not necessarily spiralling into it

[1984]

I saw a BBC "Horizon" about this the other day on a flight. They talked a lot about "feeding" of apparent suppermassive black holes that they think live in (probably all) galactic centres.

Apparently they stop "feeding" after a while because the mass of the surounding matter in the galaxy means it won't fall in. The attraction from the black hole is balanced, so the matter orbits the hole. Anything itinerant -- like a comet say -- that passed near the hole slowly or closely enough would still get swallowed, but most of the galaxy should stay intact.

Of course, that's iff nothing else intereferes. The Andromeda Galaxy is heading our way, so in some (distant) future time matter in it will become a significant gravitational influence on matter in our own Milky Way. That should upset the balance, and researchers are hypothesising the disruption setting off feeding of the black holes at the centre of both galaxies, which will go on to swallow up large portions of each galaxy.

Should be quite a show.

Re:Not necessarily spiralling into it

[Dyolf Knip]

Andromeda Galaxy is heading our way

Hmmmmmm, estmated speeds of 200 to 1000 kps, figure 600 kps average, 9.4 trillion km per l.y., 2 million l.y. to Andromeda, (9.4e12 * 2e6) / (600 * 31,536,000 sec/year) = close to 1 billion years away. Don't hold your breath.

Should be quite a show

No doubt. Get tickets early, cuz the theater's bound to be packed.

Re:Not necessarily spiralling into it

[JetJaguar]

You've got the first bit right, it's generally accepted that the difference between quasars, active galactic nuclei (less powerful quasars), and "normal" galaxies has to do with the amount of material falling into the galactic core, in quiet galaxies all the available material near the supermassive black hole has been sucked in and there's nothing left to cause a ruckus, whereas in quasars and AGN there's still stuff falling in.

As for Andromeda colliding with the Milky Way... Sigh. This is only hypothetical at best. Andromeda does have a negative radial velocity, but we do not know what the tangential velocity is. Before we can say, definitively, that Andromeda will collide with us, we MUST know the tangential velocity...we do not know what it is, and there isn't any easy way to measure it.

Anyone modeling Milky Way-Andromeda collisions are just satisfying their own intellectual curiosity. There's nothing wrong with that and I fully support it, but it's disengenuous to say that these models predict with any accuracy what will happen in the future.

Re:Not necessarily spiralling into it

[Scoria]

And that Slashdot story is here.

(I'm adding this to my comment to please the lameness filter. Apparently, the Slashcode doesn't love me and has given me the lameness filter twice.)

Re:Minor nitpicking...

[Guppy06]

"This hipothesis of a giant blach hole at the center of the galaxy is really interesting... too bad we won't have the galactic center exploding, as in Larry Niven's "Known Universe" books."

You mean the center of the universe. And we can't look at the "center" of the universe all that easily because that would require pointing our telescpes in a direction that is perpendicular to everything.

which way

[Hooya]

is it 'draining' clockwise or counter-clockwise?

Re:..Its not really suprising..

[Hater's Leaving, The]

According to the BBC article, the size is 108 times that of the diameter of the sun, and the mass is 2 million times the mass.
108^3=1.25 million
=> the density is 8/5 that of our own sun.

Anyone else think these figures sound like they've been pulled out of someone's arse? Or am I just a cynic?

THL

The Real Story is that they found Sha Ka Ree

[jea6]

SYBOK Our destination is the planet Sha Ka Ree, which lies beyond the Great Barrier at the center of the galaxy.
KIRK (alarmed) The center of the galaxy?
SPOCK There Sha Ka Ree is fabled to exist.
KIRK But the center of the galaxy can't be reached. No ship has ever gone into the Great Barrier. No probe has ever returned.
SPOCK Sybok possessed the keenest intellect I have ever known.
KIRK Spock! My only concern is getting the ship back. When that's done and Sybok is in here then you can debate Sha Ka Ree until you're green in the face. Until then, you're either with me or you're not.
SPOCK (as if it's obvious) I am here, Captain.

News for nerds, indeed.

density of a black hole is infinite.

[moller]

Or approximately infinite.

Density is defined as d = m/v (m is mass, v is volume.)

The volume of a singularity (the object at the center of a black hole) is effectively zero, so the density of the singularity is undefined (though commonly said to be infinite).

When the diameter of a black hole is referred to, they are most often talking about the Event Horizon, the boundary around the singularity from which nothing can escape, not even light.

Note that the distance of the event horizon from the singularity is determined by the mass of the black hole, not the density or volume (since density and volume for ALL singularities are effectively equal). Gravity is still dependent on mass, and the event horizon is simply the region of space where the escape velocity from the singularity's gravitational pull exceeds the speed of light.

(on a side note, since the only real requirement for a black hole is to have zero volume, anything could become a black hole if compressed enough.)

~Moller

Re:density of a black hole is infinite.

[Dyolf Knip]

the only real requirement for a black hole is to have zero volume, anything could become a black hole if compressed enough

Indeed. Problem is, the smaller a black hole, the faster it evaporates due to Hawking radiation*. So while you could theoretically turn my cat into a black hole, neither he nor it will last very long (provided you do it somewhere far away from some mass is can eat up). And you'd owe me a new cat.

* Hawking radiation: he hypothesized that all the time all over the universe, pairs of virtual particles pop up. They are anti-particles to each other, so they annihilate each other as soon as they appear and nobody is the wiser. But, should a pair appear right on the event horizon, one particle gets sucked in and the other goes free and to balance the energy books, the black hole loses a very, very small amount of energy. Needless to say, it takes a while. This big monster of a hole will probably evaporate around 10^100 A.D.

Re:density of a black hole is infinite.

[dragons_flight]

Hawking didn't hypothesize the virtual particles, they are a neccesary feature in some aspects of quatum mechanics, especially QED (quantum electrodynamics), whose predictive validity has been established with incredible accuracy.

Hawking's contribution was thinking about how they might interact with black holes. Interestingly his theory was incomplete in that it rested on a major assumption that was not proved (it's math so "proved" is the right word) until quite recently. To be honest though we won't be certain about Hawking radiation till we have a good understanding of quantum gravity. Until then it's just a good hack trying to apply both quantum mechanics and general relativity to a problem, despite the fact that they are inherently incompatible theories.

As far as compressing things down, many physicists believe a black hole can't swallow anything whose de Broglie wavelength is greater than the diameter of the event horizion. De Broglie wavelength is a quantum mechanical property that in this context can roughly be thought of as a measure of something's intrinsic size. Once something gets pulled in, it would get compressed far smaller, but the black hole has to be able to catch it first. Electrons have a wavelength on the order of 10^-10 m, where as nuclear particles are about 10^-15 m. Schwarzschild radius is given by 2*G*M/c^2, which implies that a hole of 10^-15 m has about 6.7e11 kg of matter in it.

Thus you can't make a black hole out of a cat because a cat doesn't have enough mass to generate an event horizon that would encompass it's atoms. Besides we already wondering whether the cat is dead or alive, why subject him to anything else.

One final note, some of the plans for quantum gravity would replace the singularity with a highly compact structure of miniscule but non-zero volume. IIRC something with radius on the order of 10^-30 to 10^-34 m.

Say hi to Nigel Walmsley...

[dpilot]

when you get to the Wedge.

I've heard before that galaxies don't rotate right, that the core and outer velocities are not "correct" with respect to each other. This has been mostly in connection with dark matter and missing mass. I wonder how supermassive black holes affect this apparent mismatch. (for better, or worse)

Re:..Its not really suprising..

[tolan's my name]

They may have been, however a large [diameter] black-hole [and by that I just mean large diameter of event horizon] does not have to be very dense.

Basically if we take an object [well a sphere] of density d with a mass m then as we increase the diameter x [in a linear manner] the volume increases as x^3. So since g~m/x^2 the effective gravity on the perimeter increaes linearly.

In otherwords [in newtonian terms anyway] a large enougth object of any density would become a blackhole.

Interestingly as such an object would not necessarily be particuarly different from our world [ie if our universe is big enough and is evenly distributed etc then light is bounded, bounded universe ~ black-hole]

Care and feeding of black holes

[hodeleri]

From my understanding, a black hole will dissipate energy in the form of gamma/X-rays throughout its life. If the hole is not actively 'feeding' it will eventually dissipate (conservation of energy and all that). (I doubt a hole the size of this one will dissipate in any reasonable amount of time though.)

Of course, this is from my reading of Earth by David Brin, so I may be totally off kilter.

Re:Care and feeding of black holes

[quintessent]

Whew! I was starting to worry that Slashdot's servers had been sucked in. Good to have you back, Slashdot.

Re:Care and feeding of black holes

[krlynch]

Yes and no

Yes, theoretically a black hole will lose energy (evaporate) by "Hawking radiation" in a time proportional to some power of its mass (I forget which power, but it doesn't matter that much). What happens is that, in effect, black holes act as perfect black body radiators with the surface temperature determined by its mass. However, the bigger the hole, the smaller the effective temperature

Now, here comes the NO part of my response). For stellar sized black holes, the temperature of the black hole is LOWER than the ambient temperature of even the cosmic microwave background radiation not to mention the possible higher temperature of the stellar neighborhood around the black hole. So if you are considering the quantum effects (i.e. the Hawking radiation) tearing down the black hole, you also have to consider the radiation impinging on the surface from the CMBR. Since the CMBR will be "hotter" than any stellar black hole for a long long long long long long long long time (many orders of magnitude longer than the current age of the universe), it will be nearly forever before any stellar black hole even starts to lose mass to evaporation.

By then, the universe will be so old, there (likely) won't be any free energy left to power any type of other process (i.e. the entropy of the universe will be approaching its maximal value, and no "useful work" will be extractable from it). It will (assuming protons decay....) be a very very cold, very very very old, very very very very boring place with some photons, neutrinos, electrons and positrons floating about, and not much else around.

But all of us will be long gone and forgotten way before that happens, so don't let it trouble your sleep :-)

Re:Care and feeding of black holes

[krlynch]

Sigh... I can't believe I'm even responding to this troll ...

I love how people with a slight physics back ground love making statements about black holes.

Well, if by slight you mean that I've only spent ten years studying physics and the past 6 researching for a Ph.D. in the field, then I've only have a "slight" physics background. I'd be willing to wager that I have forgotten far more about this subject than you have ever learned.

The sad thruth is that most of these people are merely making *guesses* about the bahavior outside the event horizon.

Well, in a word, no. You clearly don't know what you are talking about. By saying "black hole" physicists have a very very clear definition of what we are talking about, and very very clear calculations of what will and won't occur in certain regions of spacetime. If the behavior predicted for observed objects is NOT correct, then those objects are by definition not black holes. They are something else that we don't understand and can't yet make predictions for, but they are not the "black holes" of general relativity. So, I am not "guessing" as you so idiotically put it. I've done the calculations, I know what they say, I've read the experimental literature, I know that what I have calculated is in agreement with those experiments, so I have strong reason to believe that GR is currently the most correct description of the universe that we have. So when I say that such and such is what is going on inside a black hole, I mean that I've calculated it, and have confidence that I know what I am talking about, because every other prediction made by the theory has been shown to be right. Could the predictions be wrong? Certainly, but then the theory is wrong, and the object is not, strictly speaking, a black hole. And nothing that YOU say will be correct in that case either... so I'd prefer to believe my own calculations to your proclamations.

*Anything* can happen inside the event horizon. Our physical laws don't applly there.

You are quite far from being correct. Anything CAN'T happen inside the event horizon... the physical laws of the universe govern the behavior of matter and energy everywhere in the universe, including inside the event horizon of a black hole. There is no such thing as "our" physical laws ... there are THE physical laws of the universe, and GR is currently believed to describe some of those laws, because its predictions match observations everywhere they have been made.

To the extent that the mathematical structure of General Relativity describes the laws of physics (and, in every observation to date, that has been the case), then yes, we can explain what is happening inside the horizon ... but we know that we CAN'T describe what is happening at the "singularity", because the mathematics tells us that it can't tell us anything, AT THAT ONE POINT. And the theory predicts its own breakdown at that point, but it does not predict that there is any real issue at or even anywhere near the event horizon.

One of the great things about science, as opposed to your incorrect statement that "anything can happen", is that you, or the poster down the street, or your mailman, or an African Bushman, can sit down, learn the theory, compare it to the experiment, and make predictions for other events. General Relativity is just a specific example of a theory for which that can be done: it is relatively easy to understand, highly predictive, and immensely observationally successful scientific theory. Your donut suggestion is none of the above. So I ask, who should people listen to: someone telling them something that they can go out and calculate themselves, or a condescending, rude individual who suggests that they can make solar mass objects out of powdered donuts, in violation of every observation about the behavior of the universe ever done by man? I know where my bet lies, and it certainly isn't with the donut man....

Can't all go down the drain

[Michael Woodhams]

Only stuff that goes very close to the center of the galaxy can get sucked into the black hole - i.e. only matter with a very low angular momentum (relative to galactic center.) The total angular momentum of the system is conserved, so there is only so much you can feed the hole before you 'run out' of 'low angular momentum'. This is a likely reason why such black holes cause quasars in young galaxies, but they aren't being fed fast enough to do this in older galaxies.

I expect that if you could let the galaxy run for long enough (ignoring collisions with Andromeda, exhaustion of fuel for stars, proton decay, evaporation of black holes etc.) you would end up with some fraction of the mass eaten by the hole and the rest in circular orbits in a flat disk - as this is the minumum energy configuration for a given amount of angular momentum.

Actually, if you're prepared to wait a really long time, the angular momentum will be shed by gravitational radiation and the black hole wins after all. (Or would, if it hasn't evaporated.)

Re:..Its not really suprising..

[Dyolf Knip]

Ok, if sphere A is twice as wide as sphere B, it's 2^3 = 8 times as big. If it's 107.8 times as wide, it's therefore 107.8^3 = 1252726 times as big. Volume is a third degree relationship.

If it's 2.6e6 times as massive, then its density is 2.6e6/1252726 = 2.08 times as dense.

But really, this is all moot. A black hole does not have density in any sense of the word. Its gravity is so large that it consumes itself and neatly exits the universe. It is a perfect geometric point, having mass but no volume, and since density = mass/volume, its density is a division by zero and requires a universal exception handler. The 93 million miles refers to the diameter of the event horizon, which is the point at which light itself can no longer escape.

Of course, nobody knows what it actually looks like inside the event horizon, so it's possible that the black hole consists of 2.6 million Solar masses worth of chocolate bars or something.

A small piece of basic physics

[PD]

I've noticed that some people have a bit of confusion here about exactly what the effects of a black hole are. Here's are examples:

Q: What would happen to the orbit of the earth if all the matter in the sun were suddenly compacted into a black hole?

A: Absolutely nothing. A black hole which contains the mass of the sun would still also have the same gravity as the sun. The earth would continue to orbit as it always has.

Q: The galaxies stars orbit around the black hole.

A: This isn't proven. Some galaxies don't have any evidence of a black hole, yet theirorbit around a center of mass. In any case, the black hole at the center of our galaxy is 2.6million solar masses. This is NOTHING compared to the billions of stars in the galaxy, so the effect of the black hole of the actual shape and orbit of the stars is not significant.

Q: Doesn't it sound like someone has pulled the stats on this black hole out of their arse?

A: Not really, the size of this black hole has been measured in several ways, including observing very high velocity stars near the black hole. The motion of these stars betrays the existence and size of the massive object at the galaxy's center.

Q: Aren't black holes required for the formation of galaxies?

A: We don't know for sure yet. There are galaxies without black holes, so it might not be required. Of course, we might just not be detecting the black holes that are in those galaxies.

Re:A small piece of basic physics

[PD]

The sun could not turn itself into a black hole, that is true, but black holes much smaller than the sun exist.

All you need to do is increase the density by compression and you'd have a black hole. It is not unthinkable that a far future hypothetical civilization could compress a small star into a black hole if they wanted to.

Re:A small piece of basic physics

[PD]

I didn't mean that the black hole would be compressed, I meant that a star could be compressed enough so that it forms a black hole. If you compress our sun enough, you'll end up with a 1 solar mass black hole.

I'll leave the engineering details to somebody else...

Spiral != degrading orbit

[JetJaguar]

Galactic dynamics is a very interesting subject. One of the first things you discover are orbital resonances. Currently there are several theories that attempt to explain the propogation of the spiral structure in terms of resonances with the orbit of the aspherical central bulge.... And in the case of barred spirals, there has even been some success in modeling this (the bar structure is easier to model). None of the current theories ascribe the spiral structure to simple orbital decay (in fact, galaxies would look a lot different from spirals if significant orbital decay were present).

Nature article

[mgarraha]

Here are the articles the popular press cited (very awkward registration required). The "news and views" piece explains that astronomers previously thought the galactic core might be as much as 1500 times as wide as the event horizon of an equally massive black hole. The new observation indicates that it's no more than 20 times as wide.

Re:A black hole can be as dense as water

[dragons_flight]

I've never personally encountered this result and don't know for sure how it is derived, but I think I can take a stab at it classically. I'm not doing GR and won't get the same equations but the behavior is the same so this is probably along the right line to consider.

Escape velocity is dictated by having enough kinetic energy to "escape" the gravitational potential well. This comes to v = Sqrt(2*G*M/R) in the classical regime. Surface gravity on the other hand goes as g=G*M/R^2. This means that you could have an escape velocity v = the speed of light, c, so long as M/R = (c^2)/(2*G). This is the black hole conditon that not even light is moving fast enoguh to get out. By making M really large and proportionally R also really large you can end up with g small because g depends on 1/R^2 instead of 1/R.

As I said this is a crude hack but it does suggest why this might be true without delving into GR.

eh?

[child_of_mercy]

a large enougth object of any density would become a blackhole

what? like the universe?

I think u've skipped a step somewhere

I suppose that given that everything is made up of everything else, and there are some blackholes in this universe, then you could argue we are part of a huge black hole, just outside of it's event horizon?

Really tho it's the event horizon that counts.

Re:eh?

[Stephen Samuel]

No. Even the universe may be a black hole. That's actually one of the big questions of cosmology.. Whether there's enough mass/energy density in the universe to make it a black hole. We're reasonably close to that point. At the moment, the error estimation in most calculations tends to be less than the difference between the calculated mass of the universe, and the minimal black-hole mass for the universe.

I did some calculations around black holes and gravity, many moons ago.. I came up with some interesting conclusions. In this case, my definition of the 'size' of a black hole is it's schwartzchild radius (it's event horizon).

The gravity differential of a black hole is very dependant on it's size. At normal sizes, that gravity differential is enough to rip most material objects to pieces. If you could get a black hole small enough, I'm sure that you could rip single attoms into their sub-atomic particles, but I'm not good enough at physics to figure out how high the differential has to be to do that.
I think that the tidal effect is part of what leads to Hawkings radiation.

The 'surface' gravity of a black hole decreases as it's size increases. You can actually have a black hole with a surface gravity of 1 Earth Gravity, but it would have a 3 light-month radius. (our solar system is less than one light-day in radius, and the galactic core black hole would fit comfortably inside of mars's orbit.

The 1 G black hole would have an average density less than water. (i.e. it would float -- presuming that it were solid, and you could find a big enough ocean to float it in.

Re:..Its not really suprising..

[Dyolf Knip]

Don't confuse black holes with the singularities inside them. Singularities are perfect geometric points; black holes are regions bounded by an event horizon that contain a singularity.

Uh, hello? Finish reading my post.

"But really, this is all moot. A black hole does not have density in any sense of the word. Its gravity is so large that it consumes itself and neatly exits the universe. It is a perfect geometric point, having mass but no volume, and since density = mass/volume, its density is a division by zero and requires a universal exception handler. The 93 million miles refers to the diameter of the event horizon, which is the point at which light itself can no longer escape."

Our Destiny is Density? Computation & Black Ho

[Singularian]

It seems that our universe is constructed such that any intelligent organisms will quickly gain powerful proof that black holes are real and (eventually)unavoidable entities, and thus that there are multi-local places where matter leaves this universe, on a one way trip. Supermassives are the easiest to see, but it is my expectation that we'll gain much more evidence for the multi-local presence of ordinary black holes in years to come. The most interesting question seems to be not whether or not we will *ultimately* end up in a black hole, but rather *how quickly* we are headed there.

I write a monthly newsletter on accelerating change, Signs of the Singularity, available at my website:

http://www.SingularityWatch.com

If you've heard of the singularity, or ever thought carefully about accelerating change from a cosmological or developmental perspective, I'd suggest you check it out.

Major Speculation Warning:
As many of my readers know, I see black holes (the garden variety ones, not the rare and easily observable supermassives) as the most reasonable candidates for the transcension of complex civilizations. This scenario very nicely explains why we haven't been colonized by robotic Von Neumann probes from other clearly ubiquitous civilizations in our galaxy, even though the galactic core is many billions of years older than us, and we are a mere 30,000 light years away from it. If Eric Chaisson, Seth Lloyd, and others are right, the developmental computational destiny of all complex systems appears to be the exponential approximation of black hole density with our computational architecture (ie, macro, meso, micro, nano, femto, black hole computational substrates). It's a short leap from this to realize that the whole universal system may be built for accelerating computational transcension, with black holes as the most likely multi-local endpoints and portals. As I argue in my forthcoming book, Destiny of Species, we may be perhaps twenty or thirty years away from theoretically (and eventually, experimentally!) proving a black hole destiny for all complex systems in the universe, as they head off to some even more complex environment within the multiverse. Keep your eyes open. Whatever we find, it's guaranteed to be a fascinating story...