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"

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.

[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

Other links

[Joao]

Here are a few more links on this:

Official website

Official press release

Story on CNN

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".

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

[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.

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.

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.

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.