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Have astronomers found a new class of tiny black hole?
Black holes are the cosmic champions of hide-and-seek. Einstein predicted they existed in 1916, but it took over 100 years before a telescope as wide as the world snapped the first picture of a black hole. They're elusive beasts, avoiding detection because they swallow up light. Even so, astronomers can see the tell-tale signs of black holes in the universe by studying different forms of radiation, like X-rays. So far, that's worked -- and a huge number of black holes have been discovered by looking for these signs.
However, an entirely new detection method, pioneered by researchers at The Ohio State University, suggests there may be a whole population of black holes we've been missing.
The findings, published in the journal Science on Nov. 1, detail the discovery of a black hole orbiting the giant star 2MASS J05215658+4359220 (J05215658, for short) using data from Earth-based telescopes and Gaia satellite observations. The team shows J05215658 is being orbited by a massive unseen companion -- and they suspect it might be an entirely new class of black holes.
A noninteracting low-mass black hole–giant star binary system, Science (DOI: 10.1126/science.aau4005)
(Score: 3, Interesting) by bzipitidoo on Monday November 04 2019, @04:28PM (4 children)
I have read that there's a gap in our knowledge between the largest known neutron stars, at approximately 2.5 to 3 solar masses, and the smallest known black holes, at approximately 5 solar masses. What kind of object is in between? A black hole, or a neutron star? Or could it be that there's some instability at that particular mass and density, and ther eare no such objects?
So this may be it, these extremely compact objects of about 4 solar masses do exist, and they are black holes, not neutron stars. I would also guess theory predicted it. It also makes sense that it is a black hole, because they are so difficult to observe, and that's why we haven't seen anything. Of course, until an example is found, one can never be completely sure. If the headline is an exaggeration, it's not by much.
(Score: 3, Informative) by HiThere on Monday November 04 2019, @05:20PM
Neutron stars are also quite difficult to observe, when they don't have an axis that causes their radio signals to periodically sweep across our observation. And that means almost all of them. And the older ones are expected to be pretty quiet anyway.
So the difficulty of observation doesn't allow one to choose between black hole and neutron star. The choice has to have been either on theoretical grounds, or on "what will grab the headlines". (I do notice that even in the summary they qualify with the term "might".)
My wild guess would be neutron stars, but perhaps they've got grounds to thing a neutron star couldn't be that heavy.
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(Score: 3, Informative) by Immerman on Monday November 04 2019, @09:05PM (2 children)
I believe the "2.5 solar mass limit" on neutron stars is a theoretical limit - we know roughly how dense neutronium must be, and we know how strong the weak nuclear forces is. Combine the two, and you get an upper mass limit before the weak nuclear force is no longer strong enough to keep a body from collapsing under its own gravity.
There might be some possibility for quark stars above that limit though - the theory is still pretty crude as to how exactly the physics would work on a star-sized quantum object.
Of course, we're still not 100% certain that black holes actually exist either. As one example, a slight reformulation of Relativity to treat the energy in gravitational fields the same as all other energy fields (which generate their own gravitational field based on the field's energy density) would mean that gravity well would asymptotically approach a maximum "depth", leveling out a little before an event horizon could form due to the gravitational "pullback" from the intense energy stored in the surrounding gravitational field, no matter how much material you put in one place. In which case it wouldn't matter that there are no known forces strong enough to resist gravity, because gravity almost disappears when very close to an ultra-dense object. And you'd have some sort of degenerate-matter soup sloshing around on the almost-flat bottom of an extremely deep gravity well. If the resulting "dark hole" were massive enough it might even be possible for exotic "normal" matter to exist at the bottom*
*One of the curious properties of a black hole, is that its effective density (mass divided by the volume enclosed within the event horizon) shrinks rapidly with mass - for a mass M the density is about (1.8x10^16 g/cm3) x (Msun / M)^2 (according to a quick search). A huge number for "normal" black holes, but something massive like Saggitarius A* at the center of our galaxy, at 2.6 million solar masses that falls to about 2.6kg/cm^3. Still about 200x denser than lead, but 10^14x less dense than neutronium. Assuming that gravity actually plateaus at least a little outside where the event horizon would form, the average density of the material at the bottom would be no greater than that.
(Score: 2) by FatPhil on Monday November 04 2019, @11:39PM (1 child)
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 3, Interesting) by Immerman on Tuesday November 05 2019, @12:47AM
That is true. Personally I suspect the gulf has something to do with more massive black holes having an easier time stealing material from passing star systems. If we can only see them when they're eating, and have no reason to believe they eat regularly, then it would stand to reason that the black holes we can see will overwhelmingly be the ones that are big enough to have an easy time capturing material from other star systems when they pass nearby. And even then it'll only be captured in orbit - it probably needs several planets to stir things up enough for debris to end up on a collision course with the tiny pinprick of a black hole a few miles across so that it can be devoured.