This New York Times article below from a few days ago is about star S2, which closely orbits the Milky Way central black hole. The observation is that S2’s orbit processes in agreement with relativistic calculations.
The article quoted astronomer Reinhard Genzel, then continued, “The observation, he said, strengthened the case that Sagittarius A* is a black hole.”
Is that statement actually true? What if Sagittarius A* is not a black hole but a huge, same-mass sphere of unobtanium? Would that difference affect the orbit S2?
We know how massive the object is, and we know how close stars can get to it. Nothing else with that mass, so far as we know, can be that small.
We can’t rule out the possibility of “gravatars”, objects that are just barely too big to be black holes, supported by… step 3, profit. But they require magical physics for their step 2.
But AFAIK we knew the mass of A* and the orbit of stars around it before this new research, and the new research doesn’t add to that. The article is about measuring the precession of the orbits around the star. Orbital precession in GR obviously doesn’t require a black hole (Mercury’s orbit precesses measurably). So independent of the previously known limits on the size and mass, is there something about the precession that is only explainable if A* is a black hole?
I think it’s more that this particular star gets really close, and because of that, its precession is noticeable, and also because of that, we can use it to get better bounds on the size of the central object.
This quote, I think, is the most important one.
“Additional masses in the GC would lead to Newtonian perturbations of the S2 orbit. An extended mass component (e.g., composed of stars or remnants, but also of other particles) would result in a retrograde precession. The presence of a second massive object would lead to short excursions in the smooth orbit figure. Our data place tight constraints on both, which we detail in Appendix D, where we discuss several important astrophysical implications of our measurements.”
In other words, the precise precession observed is inconsistent with the idea that SagA is a non-point object - and inconsistent with the idea that SagA is 2 or more not-quite black holes orbiting each other. Therefore - SagA is probably a blackhole itself
It had already been proven that Sagittarius-A* is a black hole, in the sense that there are no other models that fit all observations. This is yet another observation that is consistent with that model, so yes, it acts as additional proof for that model. Any alternative explanation would need to explain this observation as well as all others.
I was reading too much into the quote. I thought it was saying: a black hole has a different gravitational effect than a non-black-hole of the the same mass - and the orbit of S2 supports the black-hole model.
As opposed to what? There is no observation that can rule out your “unobtanium” model unless you make some testable predictions about how unobtanium behaves differently from a black hole. There are already many observations that provide the upper limit of the size of the GC (see diagram in appendix D of this paper).
I have not Googled it but wouldn’t a supposed “gravatar” be either a neutron star or (possibly) a strange star (aka a quark star)?
After that seems to me you have a black hole.
Not to mention, you’d get no physical object that has “almost” black hole mass of galactic center massive black holes to make whole stars orbit like they are.
A neutron (or quark) star is supported by the degeneracy pressure of the particles that make it up, in accordance with known (or at least somewhat-known) physics of the forces between those particles. They have a maximum possible mass (probably around twice the mass of the Sun), beyond which they collapse further, and are several times larger than a black hole of the same mass.
Gravitars, if they exist, stop just short of being black holes, so short that an outside observer couldn’t hope to tell the difference, and are supported by mumble mumble quantum gravity LOOK OVER THERE A SQUIRREL mumble mumble, and can be any size, because obviously the Universe wouldn’t allow the existence of black holes, because obviously.
Isn’t a neutron star or (maybe) a quark star the definition of just before collapsing in to a black hole?
Presumably we have a neutron/quark star which is super-duper dense…barely holding itself up from complete collapse. If we keep adding matter to them I would think there is a straw/camel back moment when you add one particle too many and poof…black hole.
Yes, in the sense that the neutron star is the most dense form of matter that we know of. We think the upper limit of neutron stars is about 2.16 solar masses - if it’s any more massive, the pressure at the core will overcome the neutron degeneracy pressure and collapse. If there is some other type of pressure that can support an even more massive compact star, we don’t know of it.
The mass of Galactic Center is 4.26 million solar masses, by the way.
A neutron (or maybe quark) star is the closest thing to a black hole that we know of. A gravatar (if they exist) is much closer, but they’re not things we know of. The idea behind gravatars is that there’s something about the laws of physics that somehow prohibits black holes from forming, and that there therefore must miraculously be some other form of pressure that arises just before they would form to prevent that final collapse.
I don’t think you can - but that’s not what the question is (I think); they are distinguishing between a rotating ellipsoidal object from a spinning point mass
You don’t. I believe what the study established, AIUI, is that SagA* does not consist of multiple objects, and that at least the vast majority of its mass is deep within its center. This doesn’t prove that a point singularity exists, but it does prove that a single object exists whose mass density is greater than can be explained by anything other than a black hole.