The Milky Way's supermassive black hole

Aug 08, 2011
The milky way's supermassive black hole
A Chandra X-ray Observatory image of the region around the supermassive black hole at our Galaxy's center. (More than two thousand other X-ray sources were detected around the central black hole, making this one of the richest fields ever observed.) Credit: Chandra X-ray Observatory

Supermassive black holes - objects with masses of millions or even billions of suns - are found at the nuclei of galaxies. Our Milky Way galaxy, for example, has a massive black hole at its core, albeit one that is relatively quiescent. Despite their reputation for being implacable sinks for matter and energy, both radiation and matter can be ejected from the vicinity of a black hole, often in powerful jets, when it is ringed by a disk of matter and material accretes onto it.

Thus in dramatic cases, like quasars, nuclear are responsible for some of the most spectacular phenomena in the cosmos.

A black hole is so simple that it can be completely described by only three parameters: its mass, its spin, and its electric charge. The at the center of the Milky Way, because it is example closest to us, has been a prime target for study by scientists trying to measure these parameters and understand how they influence the behavior of the region. For example, why it is that our black hole is so quiescent, while bright jets and luminous emission are seen in more dramatic cases?

CfA astronomer Avi Loeb, his recent student Avery Broderick, and two colleagues have modeled the nature of the Milky Way's black hole. The mass has been estimated by other scientists who, by watching the motions of stars in its vicinity, find that it is about 4.3 million solar-masses. (The charge of the black hole remains unknown.)

The CfA team used results from ultra-high spatial resolution millimeter wavelength astronomy to measure the angular size of the weak emission around the black hole; the equipment has an angular precision roughly equivalent to seeing a dime on the moon. Based on their analysis of the data, they find that the Milky Way's central black hole spin is at best modest - although whether this explains why the emission is weak is still uncertain. They also conclude that the millimeter emission is slightly extended - it has an axis ratio of about 2.3, but the orientation does not appear to align with any other known features.

Not least, their models strongly suggest that the comes from some kind of an accretion process. The new result is just the opening salvo that indicates astronomy has entered an epoch in which it will be able to study black hole physics directly using high spatial resolution techniques.

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jsdarkdestruction
5 / 5 (1) Aug 08, 2011
Awesome, I cant wait to learn more.
Pete1983
not rated yet Aug 08, 2011
Would love to learn more myself. I can barely visualise anything related to black holes. Hawking radiation isn't so bad if you accept QM, but I still don't understand how spin comes into it.

Sure, any accreting body will maintain it's angular momentum, thus speeding up it's rotation, but if the radius of the matter is 0 (as it seemingly is), does this not make rotational speed infinite? Yet if spin is measurable, then can we not determine the radius of the degenerate matter inside the black hole itself?

I guess the spin measurement in this case must be frame dragging or something and not related to classical spin...
Objectivist
not rated yet Aug 09, 2011
Would love to learn more myself. I can barely visualise anything related to black holes. Hawking radiation isn't so bad if you accept QM, but I still don't understand how spin comes into it.

Sure, any accreting body will maintain it's angular momentum, thus speeding up it's rotation, but if the radius of the matter is 0 (as it seemingly is), does this not make rotational speed infinite? Yet if spin is measurable, then can we not determine the radius of the degenerate matter inside the black hole itself?

I guess the spin measurement in this case must be frame dragging or something and not related to classical spin...
I believe the radius of a black hole is infinitesimal and not 0.
LKD
not rated yet Aug 10, 2011
I believe the radius of a black hole is infinitesimal and not 0.


It should be the radius of the event horizon. Unless something has changed.
Objectivist
not rated yet Aug 10, 2011
It should be the radius of the event horizon. Unless something has changed.
The event horizon does not mark the edge of the object itself, it only marks the point where gravity pull exceeds the speed of light. The object itself is much, much smaller than the event horizon.
Callippo
1 / 5 (2) Aug 10, 2011
IMO black hole is just a dense blob of vacuum, a dense extension of space-time. It may exhibit solid surface bellow event horizon, but it will be only mildly different from the rest of space-time inside. You can imagine it like the hot & fuzzy surface of supercritical fluid with no sharp boundary. IMO black holes, in which the surface of matter (which they're formed) reaches the event horizon, manifest itself like brightly shinning quasars and they evaporating excessive matter at the free space.

Why we cannot see the solid surface of black hole? Because it's density is comparable with the density of the tiniest and most dense particles in our body: quarks and neutrinos. We cannot observe/interacts with higher gradients of energy density, than the gradients which are forming observable bodies. Such gradient of energy would dissolve us into radiation and we would observe anything.
lomed
5 / 5 (2) Aug 10, 2011
Sure, any accreting body will maintain it's angular momentum, thus speeding up it's rotation, but if the radius of the matter is 0 (as it seemingly is), does this not make rotational speed infinite? Yet if spin is measurable, then can we not determine the radius of the degenerate matter inside the black hole itself?
It turns out that the singularity in a black hole with non-zero angular momentum is in the form of a ring rather than a point (at least for the Kerr solution), so it has non-zero extension in one (angular) direction.
jsdarkdestruction
not rated yet Aug 10, 2011
Well, thats a great opinion i guess, but it's still just that, an opinion. Thanks for sharing though.
LKD
5 / 5 (1) Aug 11, 2011
The event horizon does not mark the edge of the object itself


It does, and is the determination for the size of a black hole. The properties within that area are anyone's theoretical guess. That delves into an area of physics that no one can predict yet.
yyz
not rated yet Aug 11, 2011
"It turns out that the singularity in a black hole with non-zero angular momentum is in the form of a ring rather than a point (at least for the Kerr solution), so it has non-zero extension in one (angular) direction."

IOW a ring singularity: http://en.wikiped...gularity
Pete1983
5 / 5 (2) Aug 11, 2011
Thanks for the responses guys. So I guess this is territory that is pretty speculative at the moment (a bit like strange stars and such). Thanks for clearing up that the singularity itself may not be a point of 0 radius. The ring concept of the Kerr solution that you mentioned lomed is very interesting... and seemingly quite sensible from a spacetime perspective.

Although I must admit I still like the idea of this causing a tension on spacetime large enough to actually "crack through" spacetime itself. What the implications of this are I have no idea, or even if this occurs in any models, but regardless, the concept is cool.
Pete1983
5 / 5 (1) Aug 11, 2011
Oh and yyz thank you very much for that link...

Extremely fun stuff. Wormholes ftw.
Objectivist
not rated yet Aug 12, 2011

It does, and is the determination for the size of a black hole. The properties within that area are anyone's theoretical guess. That delves into an area of physics that no one can predict yet.

Correct me if I'm wrong, but I believe it marks the edge of the black hole; where light cannot return, i.e. the point where gravity pull exceeds the speed of light. The object itself, the singularity, is much smaller than the event horizon.

I don't think we disagree. We're just answering two different questions. One being the size of a black hole and one being the size of the singularity.
ccr5Delta32
not rated yet Aug 14, 2011

Correct me if I'm wrong, but I believe it marks the edge of the black hole; where light cannot return, i.e. the point where gravity pull exceeds the speed of light. The object itself, the singularity, is much smaller than the event horizon.

Presumably if we imagine space inside the event horizon as familiar to what we commonly understand as 3D space ,we could have two objects or more .The only qualitative data we have are the orbits of S2 and the other stars ,witch appear to orbit a point mass,It could be a just so coincidence or the idea of space inside EH is faulty