back to article Astroboffins rethink black hole theory after spotting tiny example with its own star buddy

Astrophysicists may have discovered the smallest black hole yet – just 3.3 times the mass of our Sun – according to a new paper published in Science. The object, described as a “massive unseen companion,” is locked in a binary system with a giant star codenamed 2MASS J05215658+4359220. A large team of academics discovered it …

  1. Chris Miller

    If this object is a neutron star, we would need a new physical theory to explain how it can prevent collapse against the force of its own gravity. If it's a black hole formed as the stellar remnant of a supernova explosion, we need a new theory about how supernova explosions proceed; but a simpler explanation would be its formation by accretion onto a smaller neutron star.

    1. Pascal Monett Silver badge

      In this case it couldn't be accretion, because there is no reason why would the neutron start stop accreting when it turned into a black hole.

      1. Bite my finger

        > "...there is no reason why would the neutron start stop accreting when it turned into a black hole."

        How about, "It ran out of accretable material."

        1. Unicornpiss Silver badge

          Ah, neutron stars..

          ..the lint rollers of the universe..

    2. Anonymous Coward
      Anonymous Coward

      We wouldn't necessarily need a new theory, it might be proof of an existing theory - i.e. a quark star.

      1. Scroticus Canis

        Maybe it's a pair of neutron stars ...

        ... in a tight mutual orbit and the low mass red giant is just orbiting them; gotta be a low mass red as it's orbiting something that's just 3.3⊙ and far enough away that it's not being bled of gas.

    3. Tessier-Ashpool


      Maybe it’s a neutron star that’s spinning really really quickly, like multiple thousands of times per second. Wouldn’t that hold it up?

      1. Saruman the White Silver badge

        Re: Spinning?

        If a neutron star is rotating too fast it will fly apart, just like everything else in the universe (caveat: black holes excluded from this). Also there is an absolute limiting factor: the angular velocity at the surface of the neutron star cannot exceed the speed of light.

        However you are right in one respect; a rotating neutron star can get larger than a non-rotating one before it collapses into a black hole.

        The real problem is that we have very little idea what the Equations of State are for a neutron star. These would describe how the matter in a neutron star behaves under all possible conditions, and physicists still cannot define them (its complicated because, under the conditions in the interior of a neutron star, both relativity and quantum mechanics play a role). I suspect that we could find the upper mass limit for a neutron star changing once we can define the Equations of State (or even get a reasonable approximation).

  2. Blockchain commentard

    Why do I want to call it Baby Groot?

    1. Danny 2 Silver badge

      The Littlest Hobo?

  3. Michael Hoffmann

    4 Msol limit?

    I'm hightly confused. The article stated 4 solar masses as lower limit for a black hole.

    Yet I had learned - and a quick glance at Wikipedia confirms - that the Chandrasekhar limit is 1.4 solar masses, which would first lead to a neutron star and at around 2.3 even neutrons couldn't resist and would collapse to a black hole (though centrifugal force by rotation could forestall this for a while).

    A finding of a 3 solar mass black hole wouldn't contradict that and I couldn't immediately find anything (it's Friday afternoon beer o'clock so there's limits) that this has changed.

    Did I miss something?

    1. Francis Boyle Silver badge

      Re: 4 Msol limit?

      The Chandrasekhar limit is the mass at which electron degeneracy pressure will(eventually) be overcome and an ordinary matter white dwarf will collapse into something more exotic. There's a several other degeneracy pressures that need to be overcome before a black hole can be formed. (Neuton, proton, quark). Maybe we've just found one more. (Though my bet is on something to do with quark degeneracy. I don't think there's been a confirmed or even suspected quark star so it would be nice if this were one.)

      1. Bronek Kozicki

        Re: 4 Msol limit?

        Also, this object might not be a black hole.

        1. Anonymous Coward
          Anonymous Coward

          Re: 4 Msol limit?

          It may be a small leak.

    2. Danny 2 Silver badge

      Re: 4 Msol limit?

      You missed the latest episode of Young Sheldon when he fact checks The Lord of The Rings, while trying not to think about science in case it drove him crazy.

    3. Anonymous Coward
      Anonymous Coward

      Re: 4 Msol limit?

      Yes, if it is true that a 2.5 solar mass neutron star would collapse into a black hole, wouldn't that create a black hole with a mass of 2.5 solar masses? Or less, if the collapse wasn't orderly (since a lot of these types of events tend to throw off a tremendous amount of energy)

      Worst case, if you had a "2.49 solar mass" (i.e. just under whatever the exact limit is) and it was stealing mass from a companion that had inflated into a red giant, it would eventually exceed the limit and become a black hole.

      So not really seeing why black holes in this mass range can't happen, even if 4 solar masses was the limit for "direct" conversion from star to black hole.

    4. Brewster's Angle Grinder Silver badge

      Re: 4 Msol limit?

      The best way to make a small fortune? Start with a big fortune. Black holes are the same.

      Getting to a black hole requires a supernova from a star with a mass many times that of the final black hole. AIUI the smallest stars that will go supernova and still produce a black hole, produce a black hole "weighing" in at at least 4 solar masses. So getting a black hole this small is an engineering problem - not a theoretical one. (AIUI.)

      That said, if I had to put money on it, I'd say they've underestimated the mass of the companion - their error bars go above the mass gap.

      1. Saruman the White Silver badge

        Re: 4 Msol limit?

        Black holes can also be created when two neutron stars merge. LIGO detected just that sort of event on 17th August 2017.

        1. Brewster's Angle Grinder Silver badge

          Re: 4 Msol limit?

          s/companion/black hole/ in the above - I'd just been reading the paper where the black hole is the companion to the observed star.

          Fair point about neutron star mergers. My point was (AIUI) the 4 solar mass limit is down to process rather than a hard theoretical barrier. And, yeah, given that that neutron stars can't be that heavy to start with, it's entirely possible they could produce low mass black holes. So maybe that's what we're seeing. (But I'd still bet it's just the black hole is heavier than they think!)

    5. diodesign (Written by Reg staff) Silver badge

      Re: 4 Msol limit?

      From the paper:

      "the lowest well-measured black hole masses [5 to 6 M☉ (4, 5)]. Whereas some models of black hole formation indicate a lower mass limit of ∼4 M☉"

      The Chandrasekhar limit is about white dwarf stars, which can eventually collapse into black holes, though it needs to take on mass to do this. The limit of 1.4 is the maximum limit for a stable white dwarf. If you want a black hole out of one, it needs more matter, it seems.



    6. Cuddles Silver badge

      Re: 4 Msol limit?

      "Did I miss something?"

      The important thing is the difference between what mass it's possible for a black hole to have, and figuring out a way for a black hole of any particular size to actually form. In fact, it's possible for a black hole to have any mass - compress enough mass into a small enough volume and you get a black hole. The question then becomes what processes can do that. As it turns out, there only seem to be such process to form three broad classes of black hole - supermassive ones, stellar mass ones, and microscopic ones.

      The latter form through high energy particle collisions, which means they don't get above a certain size due to the difficulty accelerating particles to such high energies - even extreme processes in supernovae and quasars can't produce energies high enough to form black holes as big as something like a grain of sand. Meanwhile supermassive black holes aren't understood as well, but appear to be the result of things happening at the galactic scale, and not simply from the merger of lots of smaller black holes (one of the big open questions is whether the black hole or the galaxy forms first). Then you have the stellar mass ones discussed in this article, which oddly enough have masses similar to stars and mostly form as the result of stellar processes.

      What you might notice here is that there are pretty big gaps. There are no black holes with masses falling between atoms and stars, and none falling between stars and galaxies. That's not because such black holes can't exist, but simply because there isn't anything happening at those scales that actually forms them in practice. Finally getting to the actual question, the same is true around the edges of stellar-mass black holes. Yes, a black hole could have 3 solar masses, but we don't actually have a theory of how it could form. Models of things like supernovae predict 4 and up, and we've observed them to actually exist, but smaller ones just don't seem to happen. Without going into too much detail, supernovae are pretty chaotic things that involve throwing a lot of material off into space, and if you're that close to the lower limit of forming a black hole, the models predict that too much material will be lost to actually form one in the end.

      In addition, simply taking something that has too little mass to form a black hole and letting it accrete more material may not actually work. A white dwarf that gains too much mass doesn't turn into a neutron star, it explodes in type Ia supernova. Depending on how much mass is thrown off, it may actually end up as a neutron star afterwards, but ironically one with lower mass than the white dwarf it formed from. Exactly what happens to a neutron star when something similar happens isn't really clear, but it's entirely possible that if you simply take a heavy neutron star and add more material, whatever happens to it could end up still not having enough mass to actually form a black hole.

      tl;dr, It's not that black hole such as described in the article is impossible and its discovery will overturn decades of physics. It's simply that models and observations up to this point suggest such black holes don't actually form in practice, so finding one will need to lead to some revisions to the model, or entirely new theories, to explain where this one came from.

  4. Bronek Kozicki

    So that where ...

    ... all these single socks went to!

    1. Lazlo Woodbine

      Re: So that where ...

      And Biros (other makes of ball point pens are available etc)

      1. Francis Boyle Silver badge


        It's given up on pens. It's now into microsd cards.

      2. Kane Silver badge

        Re: So that where ...

        Breaking the four set rule, are we?

    2. Kane Silver badge

      Re: So that where ...


      1. James Rome

        Re: So that where ...

        Might these tiny black holes be scattered throughout the Universe and account for the missing mass?

  5. John Smith 19 Gold badge
    Thumb Up

    Interesting stuff often occurs on the boundaries of things

    Like the neutron star/black hole boundary.

    I think it's less a case of a new class of black hole as just one that's smaller (quite a lot smaller) than anyone they've seen before.

    Now the question is how common are these? Quite rare (so there aren't a lot of them to see) or quite common but not noticed as they keep themselves to themselves and don't drain any nearby stars material?

    It's always good when new observations show new things. I'm not sure we should scrap existing theory just yet though.

    1. Pascal Monett Silver badge

      I'm pretty sure nobody is going to scrap existing theory. We'll amend it to fit the new findings.

  6. Anonymous Coward
    Anonymous Coward

    It's in the process of forming, the telescopes can't resolve all the black monoliths.

  7. steelpillow Silver badge

    Quark soup?

    Some theoreticians have suggested that a neutron star might undergo an intermediate collapse to a "quark soup" before ultimately getting to the black hole stage. This might help resolve the absurdity in the article that a neutron star of 2.5 solar masses will collapse into a "black hole" yet the "theoretical minimum" for said black hole is 4 solar masses. OTOH maybe vultures are just bad at astrophysics.

  8. Anonymous Coward
    Anonymous Coward

    Fabled quiescent black holes

    #1 Can you find me TWO of these "quiescent black holes", that are interacting strongly (black hole to black hole), and yet have very weak interaction with surrounding matter?

    Which would suggest "gravity" as TWO separate strengths. Having a weak interaction between matter and black-hole just could be a mis-measurement of the black hole size, or some other effect. Having distinct behaviors between black holes to black holes, and black holes to matter would be much clearer.

    i.e. the 2F resonant electric black hole model proof I'm seeking.

    #2 The other thing that would be nice is if you can find lots of spiral galaxies with old stars at the outer edge and young proto stars near the inner black hole. They should be the majority in this universe, perhaps all of them.

    #3 I wonder what a small 2F black hole would look like. It would be driving an electric current in the matter around it. It would have very limited pulling power to that matter ("quiescent"). If its small enough, matter could clump around it. It would be spinning. (Looks down between feet for no particular reason).

    1. Anonymous Coward
      Anonymous Coward

      Re: Which would suggest "gravity" as TWO separate strengths.

      I can only keep up with scientific headlines, but isn't the bottom line still we haven't the faintest idea what gravity really is ?

      There could be a constellation of varying strengths, or combinations of energy being part "gravity" and part something else in varying ratios. Just we've not found the right conditions.


      1. Rich 11 Silver badge

        Re: Which would suggest "gravity" as TWO separate strengths.

        but isn't the bottom line still we haven't the faintest idea what gravity really is ?

        No. We do have a pretty good idea, tested by observation of its predictions and its utility in the development of technologies such as GPS. Have you heard of a bloke called Einstein?

        1. Anonymous Coward
          Anonymous Coward

          Re: We do have a pretty good idea,

          No we don't. Ask any quantum physicist.

          We know what gravity does. But why ? Still searching.

          If we did know, we'd be much closer to the GUT. Which we're not.

          Although admittedly we've a fairly good idea of what we don't know. Which helps.

          1. Tom 38 Silver badge

            Re: We do have a pretty good idea,


            'Then...' Lobsang nodded at the little volcano, which was gently smoking, 'how does that work? It's on a saucer!'.

            Lu-Tze stared straight ahead, his lips moving. 'Page seventy-six, I think,' he said.

            Lobsang turned to the page. ' “Because”, he read.”

            "Just 'Because' Sweeper? No reason?

            "Reason? What reason can a mountain have? And, as you accumulate years, you will learn that most answers boil down, eventually, to "Because""

          2. BuckeyeB

            Re: We do have a pretty good idea,

            Known unknowns are so much better than unknown unknowns.

        2. Anonymous Coward
          Anonymous Coward

          Re: Which would suggest "gravity" as TWO separate strengths.

          To be balanced we know very well what general relativity is... but as for quantum mechanics... we have been show to not know what gravity is.

          (Or we know what gravity does not what or why it does... we assuned we did, then spotted the last chapter had been torn out the book)

      2. Charlie van Becelaere

        Re: Which would suggest "gravity" as TWO separate strengths.

        " isn't the bottom line still we haven't the faintest idea what gravity really is ?"

        There is no bottom line. It's turtles all the way down.

      3. Spherical Cow Silver badge

        Re: Which would suggest "gravity" as TWO separate strengths.

        Obligatory xkcd (the mouse-over text is essential)

    2. Rich 11 Silver badge

      Re: Fabled quiescent black holes

      #1 Can you find me TWO of these "quiescent black holes", that are interacting strongly (black hole to black hole), and yet have very weak interaction with surrounding matter?

      Unless and until you can find that your hypothesis remains unsupported. What observations did you use to generate your hypothesis?

      Actually, don't bother answering that. I've dealt with arse-about-face Electric Universe nutters before. They never give a straight answer.

    3. Paul Shirley

      Re: Fabled quiescent black holes

      The object is not showing 'weak interaction', it's not showing the expected effects of actively consuming matter - not showing signs of interacting with surrounding matter. That could be as simple as there being nothing left close enough to capture and being too light to disturb anything further out in any detectable amount.

      1. HelpfulJohn

        Re: Fabled quiescent black holes

        "... not showing signs of interacting with surrounding matter. ..."

        Hmmm. That's odd. I wonder, could this thing be a *Dark* *Matter* collapsed object?

    4. Anonymous Coward
      Anonymous Coward

      Black to black hole.

      We have LIGO for that already. Guess what. It matched the understanding of General Relativity. So we are still left with cosmological scale (dark energy) and wuantum scale (quantum gravity) for the gap in our understanding.

      We may not know everything... but we are making progress forwards!!!

  9. Jan K.

    If it came in like tennis ball size, it would be the ultimate waste bin!

    After just finishing a large collection of Halton Arp Red Shift papers, this is highly ineresting! Thanks for the heads up!

    1. Anonymous Coward
      Anonymous Coward

      Have you considered how far away from it you would need to be for safety when chucking things into it, and then how accurately you would need to throw?

  10. imanidiot Silver badge

    How would that form?

    How would a neutron star in a binary system collapse into a black hole, but not blow it's companion to bits in the process? I thought black hole formation involved some rather energetic processes (not always supernova levels energetic, but "you don't want to be anywhere in the same star system" levels at least)?

    1. A.P. Veening Silver badge

      Re: How would that form?

      How would a neutron star in a binary system collapse into a black hole, but not blow it's companion to bits in the process?

      Alternate sequence of events: The neutron start first collapsed into a black hole and acquired a companion afterward, thus creating the situation we perceive at present.

    2. Rich 11 Silver badge

      Re: How would that form?

      Binary stars can be as far as a light year apart. Our nearest neighbour, Alpha Centauri, is a trinary system consisting of two close binaries (varying between 11-25 AU apart) with a wide binary (Proxima Centauri) orbiting about 130,000 AU away from them.

      1. HelpfulJohn

        Re: How would that form?

        "... with a wide binary (Proxima Centauri) orbiting about 130,000 AU away from them."

        Are we *sure* about that? Yes, Proxy *looks* vaguely like it may be related to the Alphacents but have we watched it orbit long enough to know that it's not a close visitor? Do we know the orbit isn't hyperbolic in shape? Red Dwarves are common and having one pass between the Alphacents and us wouldn't be that strange a coincidence though the timing is interesting.

        I often have the same doubts when people insist that M31 is coming to Town and not just making a close pass, or even *ending* a close pass before retrating. Indeed, I sometimes doubt M31 is a part of our galaxy's local group and not it's own group's master.

        I'm patient, I can wait for the conclusion to see who's right.

        On the tiniest singularity thing, maybe it's simply the runt of the litter. Some critters just are. They can be the nicest of the bunch.

  11. Anonymous Coward
    Anonymous Coward

    "The researchers aren’t sure if the object is indeed a low-mass quiescent non-interacting black hole, we must stress, and there’s a chance it may be a massive neutron star."

    Maybe it's made out of Dark Matter.

  12. David Roberts Silver badge

    low mass quiescent non-interacting

    Could it just be shy?

  13. Chris Clawson

    "If the object in question is indeed a black hole then the researchers may have stumbled on a new type of black hole that hasn’t been seen before."

    What, no rimshot? No snarky comment from the editor?

  14. Anonymous Coward
    Anonymous Coward

    This is great stuff.

    Very honest of them to also consider this may be a neutron star instead and that more data and research will let us know either way.

    The fantastic thing about these discoveries and extra data is that we will get to find out which QM model is more accurate by finding what sizes of black hole (and other objects such as neutron stars) exist and which ones don't exist.

    I do wonder though... after fibding so many black hole mergers from LIGO... is it possible to throw off a black hole in a failed merger? Like when your pint take a vacation if you turn too fast in your chair.

    1. HelpfulJohn

      Re: This is great stuff.

      Is it possible to smash two objects together to produce shards of neutron star or black hole? Sort of like smashing two "crystal" balls together only with more exotic physics, more violence and much, much more "don't do this in a system you're fond of"-ness?

      I'm thinking of maybe having a teentsy neutron "moon" or black hole continent.

  15. herman Silver badge

    Too much or too little matter

    Black holes matter.

POST COMMENT House rules

Not a member of The Register? Create a new account here.

  • Enter your comment

  • Add an icon

Anonymous cowards cannot choose their icon

Biting the hand that feeds IT © 1998–2021