Sagittarius A*

December 13, 2008 12:02 AM

A sixteen year long astronomical study, led by Dr. Reinhard Genzel of the Max-Planck-Institute for Extraterrestrial Physics, has provided what is considered to be the best empirical evidence yet of the existence of supermassive black holes, specifically one a relatively cozy 27,000 light years away.... "The stellar orbits [QT] in the Galactic Centre [QT] show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt."

A video describing the study; more photos and video here.

posted by Kronos_to_Earth (43 comments total)

One of those videos is a time-lapse film of stars at the center of the galaxy, and one particular one is in orbit around the black hole. They managed to catch a complete orbit. It's in a highly elliptical orbit, and when it gets near it really whips around. That particular star's behavior is the single best piece of evidence they collected; from its orbit and behavior they were able to make a really good guess as to the mass of the object it was orbiting.

Just awesome to watch, too.
posted by Class Goat at 12:12 AM on December 13, 2008

I've always wondered what happens inside a black hole and am always amazed that we will probably never have a way to find out. Such a mysterious idea.
posted by Blazecock Pileon at 12:12 AM on December 13, 2008

I've always been fascinated by black holes ever since learning about them in junior high. One thing that scares me: Is the entire universe (including us on earth) hurtling toward the nearest black hole? If not, why not?
posted by amyms at 12:22 AM on December 13, 2008

16 light-years long study? That's a big study!
posted by blue_beetle at 12:26 AM on December 13, 2008

If not, why not?

Because nothing particularly strange occurs in the vicinity of a black hole outside of the event horizon. It's just really, really massive for its small size.

I've always wondered what happens inside a black hole

A whole lot of squishing.
posted by Durn Bronzefist at 12:49 AM on December 13, 2008 [2 favorites]

Did our cosmos exist before the big bang?
posted by homunculus at 12:50 AM on December 13, 2008 [2 favorites]

Because nothing particularly strange occurs in the vicinity of a black hole outside of the event horizon.

But, what prevents things (including us) from hurtling toward the event horizon?
posted by amyms at 12:54 AM on December 13, 2008

Is the entire universe (including us on earth) hurtling toward the nearest black hole? If not, why not?

Two reasons.

You know how it's possible for a rocket on Earth to reach escape velocity? And how once it reaches that velocity, even with Earth's gravity pulling on it, it's going to get away? If you're outside a black hole's event horizon, you can do the same thing — achieve escape velocity, moving so quickly away from the black hole that its gravitational pull can't suck you back in.

And as a matter of fact, everything is moving very quickly away from everything else, on account of the fact that the universe is expanding. You can think of the whole universe as having reached escape velocity — everything is hurtling outwards so quickly that it's (probably*) never going to fall back in.

That brings us to the other reason: gravitational attraction is much weaker over long distances. Nearby small objects have much more pull on us than large faraway ones. F'rinstance, we feel the Earth's gravity much more strongly than we feel the sun's, even though the sun is far more massive than the Earth. Again, it's the same way with a black hole — if you're far enough away from it, it just doesn't affect you very much at all, massive though it may be. The Earth, the moon and the sun, tiny though they are in the grand scheme of things, have a much stronger gravitational effect on us than any black hole.
posted by nebulawindphone at 1:00 AM on December 13, 2008 [3 favorites]

*Whoops, forgot my footnote there. There are cosmologists who think the universe is going to collapse eventually. But for the moment, it's still all rushing outwards. If we're not a rocket at escape velocity, we're at least pop fly that hasn't begun to level out yet.
posted by nebulawindphone at 1:02 AM on December 13, 2008

The same thing that prevents you from hurtling toward the nearest basketball -- competing gravitational forces, and at distance, they're weak.

Consider the nature of orbits. The moon orbiting the earth; the earth orbiting the sun; the galaxy rotating around a supermassive black hole. You seem to be picturing sink drains and vacuum cleaner hoses. These are not good analogies.

Added, to which, with the universe expanding, nothing is hurtling toward anything else.

on preview: thanks, nebulawindphone, for the better explanation.
posted by Durn Bronzefist at 1:03 AM on December 13, 2008 [1 favorite]

Honestly, I like mystery, too. But we're talking about dense stars. Why aren't we hurtling toward Rigel? Or Spica? Or a billion other stars? Let me repeat what I said before: there is nothing particularly strange going on in the vicinity of a black hole outside of its event horizon. Sorry.
posted by Durn Bronzefist at 1:08 AM on December 13, 2008

amyms: From a distance, a black hole acts just like any other object of similar mass. So, yes, we're being pulled towards the nearest black hole, just like we're being pulled towards the sun; but we're in orbit around the Sun, so we don't actually hit it, and likewise the solar system is in orbit around the center of the galaxy, so we don't actually reach the black hole.

A kind of counterintuitive aspect of orbital mechanics is that, due to conservation of angular momentum, it's about as hard to get to a closer orbit about something (including hitting it) as it is to get to a more-distant orbit around it (including escaping it entirely).

Within a few radii of a black hole, things do start to get a bit strange, but 'way out here we can pretend everything is Newtonian.
posted by hattifattener at 1:10 AM on December 13, 2008 [3 favorites]

A kind of counterintuitive aspect of orbital mechanics is that, due to conservation of angular momentum, it's about as hard to get to a closer orbit about something (including hitting it) as it is to get to a more-distant orbit around it (including escaping it entirely).

Ha! That is, yeah, nicely counterintuitive. Something tells me I'm gonna be spending my walk to work for the next few days trying to get my head around it.
posted by nebulawindphone at 1:14 AM on December 13, 2008

Isn't the simplest way to achieve a closer orbit to apply thrust opposite to direction of motion and reduce orbital velocity? If you're a ship and not a planet, that is.
posted by Durn Bronzefist at 1:19 AM on December 13, 2008

[aw, I just has to]

16 light-years long study? That's a big study!

If you think that's impressive, you should see the Powerpoint they made.


Draft version of the paper [PDF] | group's home page
posted by mandal at 1:25 AM on December 13, 2008 [2 favorites]

Collapsing, exploding, whatever: either way it means no more General Products hulls.
posted by orthogonality at 1:36 AM on December 13, 2008 [1 favorite]

I've always wondered what happens inside a black hole....

A possible view before the plunge, anyway...

No more General Products hulls? Dang. Those were pretty neat. Bye Bye....*
posted by Kronos_to_Earth at 2:17 AM on December 13, 2008

Just when I thought bailing out the auto industry was bad, I learn of supermassive black holes.
posted by twoleftfeet at 3:42 AM on December 13, 2008

also known as The Great Attractor at the center of our Galaxy
posted by infini at 4:18 AM on December 13, 2008

Max Planck sure has a lot of shit named after him.
posted by synaesthetichaze at 7:34 AM on December 13, 2008 [1 favorite]

I haven't read the article, but based on skimming the comments, we're all doomed, right? We're going to get sucked into this...this thing before Obama has a chance to save us? Buy gold, Google Ron Paul, and all that?
posted by infinitywaltz at 8:20 AM on December 13, 2008

There are cosmologists who think the universe is going to collapse eventually.

Right now, consensus is not, but it's close. Ω₀ is currently 0.94 +.06 -.09 (a range of 1.0 to .83.) Ω₀=1 is a flat universe, Ω₀<1 is a hyperbolic universe (parallel lines diverge over distance), both are open.

Ω₀>1 is a spherical (parallel lines converge over distance) universe, and is closed.

The 5 year data from WMAP is pretty compelling -- that the universe is open, and that we are *not* in a matter dominated universe, dark energy comprises almost three quarters of the total mass-energy of the universe, and is actually accelerating the expansion of the universe.
posted by eriko at 8:35 AM on December 13, 2008

The universe is expanding, but is every galaxy expanding? I think if you have a massive enough black hole at the center of your galaxy, it will eventually collapse.

My understanding was that the galaxies are getting further apart, like the raisins in a raisin bun, as it bakes. The raisins aren't necessarily getting any bigger. At the same time, the galaxies can either shrink into their black holes, collide with other galaxies, or in some cases a cluster of supernovae can blow them apart into a nebula.
posted by autodidact at 9:17 AM on December 13, 2008

So the "open" universe expands infinitely until the stars burn out and then ... nothing forevermore? We only get one shot at stars and planets and galaxies, just a few billion years of light and then darkness for all eternity? Matter too dilute to get up to anything interesting, just floating around doing nothing until the end of time?
posted by Quietgal at 10:09 AM on December 13, 2008

A kind of counterintuitive aspect of orbital mechanics is that, due to conservation of angular momentum, it's about as hard to get to a closer orbit about something (including hitting it) as it is to get to a more-distant orbit around it (including escaping it entirely).

I dimly remember solving a homework problem where the energy required to drop something into the sun from an earthlike orbit was exactly twice the energy required to eject the same thing from the solar system. I forget whether it was an exact result for all kinds of orbits, or whether there were strings attached.
posted by fantabulous timewaster at 10:09 AM on December 13, 2008

So the "open" universe expands infinitely until the stars burn out and then ... nothing forevermore?

Quite possibly, yes. See Future of An Expanding Universe on Wikipedia. In the end, in an open universe, we're probably looking at a maximum entropy Heat Death.
posted by eriko at 11:58 AM on December 13, 2008

Isn't the simplest way to achieve a closer orbit to apply thrust opposite to direction of motion and reduce orbital velocity?

It's just the opposite. If you do that you get a higher orbit not a lower one. That is why geosynchronous orbit is approximately 22,300 miles out whereas the ISS/shuttle and imaging satellites are about 200 miles out (and make a pass every 90 minutes or so.) You can always go back to the mental image of twirling a tether ball (or any object/rope combination) above your head. If you take in the rope, you have to twirl faster at the same time to keep it spinning.
posted by Rhomboid at 12:10 PM on December 13, 2008

Rhomboid, you're half right; it's true that higher orbits are slower. The counterintuitive thing, though, is that thrusting prograde (in the direction of motion) momentarily speeds you up, but in the long run it actually raises your orbit and slows down your average speed. Thrusting retrograde lowers your orbit and speeds it up.

To "drop" an object in circular orbit into its central body, you have to slow its orbital velocity to almost zero. When measured in terms of delta-v, this requires 1+√2≈2.4 times as much thrust as boosting it up to escape velocity.

Digression: 90% of what I know about orbital mechanics I learned from Orbiter, the excellent spaceflight simulator. Go download it. Now.
posted by teraflop at 1:34 PM on December 13, 2008 [1 favorite]

A simple way to think about it is that higher orbits are faster in km/h but have longer periods.
posted by ROU_Xenophobe at 1:36 PM on December 13, 2008

A sixteen year long astronomical study, led by Dr. Reinhard Genzel of the Max-Planck-Institute for Extraterrestrial Physics, has provided what is considered to be the best empirical evidence yet of the existence of supermassive black holes

Uh, the best evidence since the nine year study released in 2004 that there exists a black hole at the center of the galaxy?
posted by euphorb at 1:37 PM on December 13, 2008

Everything is turning into iron!

"I've always wondered what happens inside a black hole"

As I understand it, nothing "happens" inside a black hole. Happening is a temporal event, and I don't think time exists within a black hole. Wikipedia does cover what happens inside the event horizon, though.

"hurtling toward the nearest black hole? If not, why not?"

http://en.wikipedia.org/wiki/Hill_sphere
posted by Eideteker at 1:52 PM on December 13, 2008

(The Hill Sphere also explains why the "so fat things orbit you" joke doesn't work in reality.)
posted by Eideteker at 1:56 PM on December 13, 2008

The universe is expanding, but is every galaxy expanding? I think if you have a massive enough black hole at the center of your galaxy, it will eventually collapse.

Here's where Durn and hattifattener's points about orbits come into it. In galaxies with a black hole at their center, everything is orbiting that black hole at a (nearly) constant distance, rather than falling straight into it. Unless something happens to those orbiting stars to make them lose momentum — and remember, there's no friction in the vacuum out there — they'll just keep going around and around and around.

In the very long run, things can happen to make bodies in orbit gain or lose momentum: there are tidal forces, collisions and gravitational interactions with other bodies, and so on. But these have a small, slow effect over a long time frame. So stars aren't plunging towards the galactic center — they're either going in circles (well, ellipses) around it or they're spiralling towards or away from it very gradually.

(For what it's worth, water circling a perfectly frictionless drain would do the same thing: just go around and around forever, without ever going down the drain, until some external force came along to slow it down. Fun, innit?)
posted by nebulawindphone at 3:15 PM on December 13, 2008

Eriko, thanks for the link, I think (the holidays are depressing enough without pondering the ultimate heat death of the universe).

Forgive me for treading the thin line between cosmology and philosophy, or possibly religion, but is there any layman-friendly explanation for why the universe has this one-way fate? (Bang, expand, die) Where did the "stuff that went bang" come from and why can't it form again to create another universe?

And another question: does time have an end? If the universe dies, does time stop? (Apart from the fact that there won't be anybody around to measure it.)
posted by Quietgal at 3:32 PM on December 13, 2008

Happening is a temporal event, and I don't think time exists within a black hole.

Good point!

Also: I'm not sure why heat death should be considered a "win".
posted by Durn Bronzefist at 3:58 PM on December 13, 2008

The paper as accepted; not yet in print. The authors have come up with well-described orbits for a couple dozen stars, all of which are consistent with going around the same "point mass" some four million times heavier than the sun.

The precision in this study is really impressive: they claim a long-term accuracy of 300 microarcseconds. If you hold a hair from your head at arm's length, it's between five and fifty arcseconds across. At that same arm's length, 300 microarcseconds is about a nanometer, the size of a cluster of atoms. But viewing the center of the galaxy from here, this angle corresponds to a distance of about 2.5 AU, about the diameter of Earth's orbit around the sun.

It looks like the star "S2" which has made a complete 15-year orbit during these observations was, at its closest approach in 2002, only about 125 AU from the black hole. There is one other star "S14", with a 50-year period, which will get marginally closer, about 75 AU; this is about twice the sun-Neptune distance.

If the invisible four-million-solar-mass object is, in fact, a black hole, its event horizon has a radius of about 0.08 AU --- it would fit well within the orbit of Mercury. If it were not a black hole, but a supermassive object with the density of a white dwarf (impossible) or a neutron star (unlikely), it'd have a somewhat larger radius --- but not enormously larger, probably still well within one AU. So while these stellar orbits argue very strongly for the presence of an enormously massive, very compact, dark object in the radio source at the center of the galaxy, I don't see evidence presented here for any of the unique and shocking predictions of general relativity: no event horizon, nothing redshifting away to invisibility, and so on. And the event horizon of a supermassive black hole like this one is a much less dynamic place than the event horizon of a stellar-mass black hole, which is in turn much less dynamic than the sort of micro-blackholes people are hoping will turn up at LHC.

There's some discussion in the paper that data from the closest approach of S2 to the galactic center, in 2002, are somehow confusing, and a discussion that the confusion may be instrumental or may come from some new phenomenon happening 100 AU from the galactic black hole. I guess that S2 will have another closest approach in 2017, and there are half a dozen stars with similar closest approaches and fifty-year periods. More interesting things will come from observations of this system in the future.
posted by fantabulous timewaster at 5:11 PM on December 13, 2008 [2 favorites]

"And another question: does time have an end? If the universe dies, does time stop?"

Fun fact: some folks believe that time is a function of the increase in entropy, not vice versa. Which leaves us with two scenarios.

1. Expanding, heat-death universe: Perception of time slows. If time is the sequence in which events happen, the fewer events that happen = slower progression of time. It's like watching a pot boil vs. riding a rollercoaster. When you're being bombarded by a non-stop chain of events, time goes quickly. When you're waiting for a single event, it seems interminable. It's not directly analogous, but it's a similar principle. So time peters out as the universe does.

2. Contracting universe: As the universe contracts, entropy decreases. The perception of time reverses. This means: Wherever we are (whenever?) we perceive ourselves as going away from the Big Bang/Crunch. And because the expansion is slowing down right before it reverses, time slows. The effect is similar to the heat-death universe, because either way, where the entropy flow is 0, the perception of time stops.

So we may not ever know, and even if we do, it might not matter.
posted by Eideteker at 5:39 PM on December 13, 2008

Wikipedia does cover what happens inside the event horizon, though.

If you know what happens at the singularity, be sure to post it to Wikipedia. I'd be curious to hear your theories.
posted by Blazecock Pileon at 9:08 PM on December 13, 2008

Blazecock Pileon, that's not quite what he said. Relativity has the quirk that nothing, not even the location of an event horizon, is defined absolutely; it all depends on which inertial frame an observer finds himself in. Someone outside a black hole would have a very different impression of what happens near the event horizon than someone falling in. This distinction is actually important to Hawking's argument that black holes radiate by make electron-positron pairs near the event horizon. There's a region inside a black hole's horizon where it makes some sense to ask what an infalling observer could notice. For a supermassive black hole, this "drab-but-inescapable" region is actually very large.
posted by fantabulous timewaster at 10:38 PM on December 13, 2008

Another group presented a supermassive black hole video, inescapable (ha!) in 2006 or so, though a follow-up video from the same group was, in my opinion, was denser with information & more compelling.
posted by Pronoiac at 12:57 AM on December 15, 2008

Quietgal writes "Where did the 'stuff that went bang' come from and why can't it form again to create another universe? "

My understanding, probably grossly ignorant, is what was happening before the big bang is as unknowable to us as the state of the flame before the match would be to hypothetical intelligences living within a lit candle.
posted by Mitheral at 7:31 PM on December 21, 2008

Fantabulous: AIUI, no, the location of the event horizon is independent of the observer's frame of reference. It's a lightlike surface.
posted by hattifattener at 3:05 PM on December 23, 2008

hattifattener, hmmm. I was dimly remembering this drawing, but I had some details different.

I also dimly remember that there is at least some way to write the metric around a black hole (perhaps Eddington-Finkelstein coordinates, but the wikipedia entry sucks right now) where the event horizon is somehow "less special" than in more ordinary coordinates.

I'm pretty sure a free-falling observer could correctly compute the location of the horizon, but I don't think anything would magically happen at the horizon that such an observer could detect. I tend to take an operational view of things: if you can't tell whether you've passed the horizon or not, what exactly does it mean to say you know where the horizon is?
posted by fantabulous timewaster at 8:20 PM on December 23, 2008