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Featured I How close is Earth to the closest black hole?

  1. Feb 16, 2018 #1

    Buzz Bloom

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    Is it astronomically known what the closest distance is between Earth and a black hole? (I was not able to locate an answer to this question searching the Internet.) If not, the article
    says
    The Milky Way teems with black holes — about 100 million of them.​
    Based on this estimate, and taking into account the relative density of matter based the distance from the center of the Milky Way, what would be a reasonably accurate estimate of the distance between Earth and the closest black hole?
     
  2. jcsd
  3. Feb 16, 2018 #2
    A quick Google search I found - " The closest black hole we know of is V616 Monocerotis, also known as V616 Mon. It's located about 3,000 light years away, and has between 9-13 times the mass of the Sun. We know it's there because it's located in a binary system with a star with about half the mass of the Sun.

    Also a Wikipedia article: A0620-00
     
  4. Feb 16, 2018 #3

    Buzz Bloom

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    Hi JCMacaw:

    Thank you very much for your post. I confess that my searching skills could definitely use some improvement. Would you please post the text you searched on?

    Regards,
    Buzz
     
  5. Feb 16, 2018 #4

    Drakkith

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    I quickly found it using "closest black hole" in my google search.
     
  6. Feb 16, 2018 #5

    Buzz Bloom

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    Hi @JCMacaw and @Drakkith:

    Thank you both for your posts. I have been thinking about this question since I first came across the 100 million black holes paper I cited in my post #1.

    Wikipedia gives the radius of the Milky way as 50-90 kly.

    Assuming a conservative order of magnitude estimate of 100 kly, the area of a circular disk of this radius is approximately
    3 × 1010 ly2.​
    This gives a black hole density of
    1 black hole per 300 ly2.​
    If I am calculating this correctly, assuming a more-or-less uniform population density of black holes, this means that the average distance from an arbitrary point to its nearest black hole would be approximately
    7 ly.​

    Therefore the distance of 3300 ly from Earth to the cited "nearest" black hole, 1A 0620-00, is about 470 times the approximated average distance from an arbitrary point to its nearest black hole. This suggests that there should be a closer black hole to Earth than 1A 0620-0. One plausible explanation for this discrepancy is the a black hole that is not in a binary system is so difficult to find that astronomers have not yet found many. However, the article
    describes a method (QPO) for finding black holes and estimating its mass that (if I understand it correctly) does not require a binary system. However, I suppose that if this method requires a lot more work than looking for binary systems with a black hole, that ratio of discovered black holes not in a binary system to those that are might be very small. Apparently the QPO method does not give a distance estimate to the black hole.

    Another plausible explanation is that the distribution of black holes is very far from uniform. I found what seems to be a useful source for exploring the distribution of stars
    but I need some time to study it.

    Comments on the above would be much appreciated.

    Regards,
    Buzz
     
    Last edited: Feb 16, 2018
  7. Feb 16, 2018 #6

    Grinkle

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    I tried to find a good reference to the number of high confidence actual black holes have been mapped in the galaxy, and I didn't find an easy answer, but I think the number is around a dozen or so, and there is some judgement involved in whether even some of these objects are definitely all black holes.

    https://www.quora.com/How-many-black-holes-have-been-located-in-the-Milky-Way


    If the above page is more or less accurate, I don't think its significant that we haven't found any closer than 3300 LY to earth given how few we are able to suspect we observe at all.
     
  8. Feb 16, 2018 #7

    jim mcnamara

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  9. Feb 16, 2018 #8

    ohwilleke

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    I think that a better estimate as a starting point would be that black hole population density is proportionate to mass density, although even this would be an underestimate because where matter density is higher stellar collisions are likely to be more frequent, while while matter density is lower, stellar collisions are less common and further, more generally, the amount of matter to feed black holes is less abundant so massive objects should grow to the critical mass threshold less often.

    So, black holes should really be present at a right higher than a frequency proportionate to total mass density in high mass density parts of the galaxy and should be present at a rate less than the frequency proportionate to total mass density in low mass density parts of the galaxy.

    Since mass density is relatively low at the fringes of the galaxy where we live, we would expect the number of black holes per square light year of the plane of the Milky Way to be much lower in our vicinity than near the core of the Milky Way.

    Also, while black holes in binary systems are particularly easy to find, occulting other stars, and gravitational lensing effects, should make black holes that are reasonably close to Earth discernible with existing technologies.

    Another point to consider is that Earth midway along a spiral arm of the galaxy. So, black holes are much more likely to be present further out or further in along the river of stars that make up that arm, than they are to be very far out in directions at right angles to that arm. So, we are really mostly looking for nearby black holes in two directions that make up a pretty modest share of the total celestial sphere around the solar system. We don't have to look in every direction to have a pretty reliable gauge of what might be out there.

    So, while it is certainly possible that there is a black hole closer to Earth than the closest one located so far, (and 1 kpc is a very long way considering that we are about 8 kpc from the galactic center), the closer you get to Earth the less likely it is that astronomers could miss it.

    If I were to give a gut guess using back of napkin level precision inputs, I'd suspect that the closest black hole is probably at least on the order of hundreds of light years away.
     
    Last edited: Feb 16, 2018
  10. Feb 17, 2018 #9

    Buzz Bloom

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    Hi ohwilleke:

    Thank you for your post.

    Since the nearest currently found black hole to Earth, which is in a binary system, 1A 0620-00, is roughly 3000 ly away, and based on your rough estimate that a non-binary system black hole might, as a reasonable rough approximation, be say 300 ly away, then is it plausible to estimate that in the region of the Milky Way near the Earth, the population density of non-binary black holes is about 1000 times greater than the population of binary system black holes? This conclusion seems to contradict the generally accepted concepts that (1) there are definitely more sun sized or larger stars in binary systems than not in binary systems, and (2) that black holes are the end states of sufficiently large stars.

    https://www.space.com/1995-astronomers-wrong-stars-single.html
    "If you go out and look at the all visible stars in the night sky and ask, 'How many of those are binary?' the answer is, 'Most of them,'" said study author Charles Lada of the Harvard-Smithsonian Center for Astrophysics (CfA). "The assumption was that because most bright stars were binaries, all stars would tend to be binaries."​
    (Underlining is mine to highlight the point I want to discuss.) The article then says
    The catch, however, is that most stars in the Milky Way are not bright stars like our Sun, but dim, low-mass stars called red dwarfs.​
    However, red dwarfs are much to small to ever become black holes.

    I am still planning to study the article I cited in post #5 about the distribution of matter. Perhaps that will show that the apparently anomaly I discussed above vanishes when a proper analysis of density is taken into account.

    Regards,
    Buzz
     
    Last edited: Feb 17, 2018
  11. Feb 17, 2018 #10

    stefan r

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    Your calculation assumes disc was 1 light year thick. The
    thin disc is closer to 1000 light years.

    You could say that black holes form in super novas and in black hole mergers. Either would decrease the number of black holes in binaries. The mergers were binary now they are not. Supernova remnants can lurch in the supernova. Large stars also shed a lot of mass. The binaries usually formed in the same cloud. I would expect a large star to have a large companion. A lot of the mass lost by a giant star can end up in the companion star which makes the companion age faster.
     
  12. Feb 17, 2018 #11

    JMz

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    A much simpler estimate (given that the original question didn't ask only about known BH's): From the 100M estimate for the total number in the Milky Way, we can say that this is roughly 1/1000 of the total number of stars [or a little less]. So the distance to the closest one should be ~ 10 [or a little more] times the distance to the closest star, Alpha Centauri, or 10*4.3 LY = 43 LY.

    A little more careful estimate, based on a few more stars (just so we don't stumble if Alpha Centauri is much farther or closer than average): There are 10 stars within 10.3 LY as it happens, so a typical distance among stars in our neighborhood, from a typical one to its closest neighbor, is 10.3/10^(1/3) = 4.8 LY. So a typical distance to a BH would be ~ 48 LY [or, again, a little more]. (Conclusion: Alpha Cen is not much farther or closer than average.)

    If you wanted to use an even larger sample, over a larger region, there are ~ 2000 stars within 50 LY. So you'll get a similar distance. (Try it yourself.)
     
  13. Feb 17, 2018 #12

    stefan r

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    Here is the free arxive article.

    I do not think you can make that comparison. Most of the black holes are coming from population II and population III. We do not see population III stars anymore. The Milky Way's population II stars tend to be spread out in the bulge or halo. The disc formed several billion years after the big bang. Population I stars tend to be smaller which means they live longer. Population I stars have a harder time forming large black holes. We see a lot of population I stars in the disc of spiral galaxies. If we take the total number of stars that ever existed the frequency of large blue stars increases. The average location of all stars that ever existed would be much further from the disc's plain than the average location of stars we can see today.
     
  14. Feb 17, 2018 #13

    JMz

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    Not much disagreement with your statements -- and I wouldn't feel an extra factor of 3 was unreasonable. However, unless the estimate of 100M BHs given in the OP depends sensitively on all of those specifics, then it seems like overkill to analyze their progenitors so closely.

    I didn't interpret the OP as a question asking for much precision. Specifically, it mentioned basing the estimate on density, not on history, so the disk gets more attention than the halo. So I gave a back-of-the-envelope answer that Buzz Bloom could do without any special modeling.
     
  15. Feb 18, 2018 #14

    Buzz Bloom

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    Hi stefan:

    My intent was to ignore the thickness, since it is much smaller than the radius, and I visualized a projection of the position of all stars (including black holes) onto the central plane of the Milky Way. The per kly2 calculation was the result of this projection.

    Regards,
    Buzz
     
  16. Feb 18, 2018 #15

    ohwilleke

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    You aren't wrong, although as a first order estimate, the black hole per star figure is a pretty decent place to start. The point someone made on binary starts being more likely to have black holes is also relevant. Given those considerations though, I still think an order of magnitude estimate for the nearest undetected BH being in the hundreds of light years isn't a bad one.
     
  17. Feb 18, 2018 #16

    stefan r

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    The paper had this to say:


    So about half of massive stars are in binary fractions. At least that is what the same authors who said there were 100 million black holes around the Milky Way believe.

    They also use unit like density per cubic gigaparsec.

    I have no problem with projections. Just have to be careful because statements based on projections can be taken out of context: Unless I did a geometry error the galaxies of the coma cluster are closer to us than Andromeda and Triangulum galaxies. Also earth is flat but parts of it flip upside down twice a day. Other parts of earth follow ellipses in 24 hour cycles and the center of the ellipse is not the center of Earth's gravity except for the equator.
     
  18. Feb 18, 2018 #17

    JMz

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    If it's 100s of LY (say, 400 to be concrete), then the BH density is 1000x lower in our neighborhood than the galactic average. That seems like a large enough discrepancy that there should be a very clear, primary reason. (Circumstances in which lots of 2nd-order effects all push an estimated result up, but each only by a little, are uncommon in practice.) What would that primary reason be?
     
  19. Feb 19, 2018 #18

    Drakkith

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    I don't know of the "primary" reason, if there is one, but I'd venture a guess and say that the lower density of stellar objects in this area of the galaxy (compared to other regions like the core and globular clusters) and the difficulty in finding black holes are probably two main reasons.
     
  20. Feb 19, 2018 #19

    JMz

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    Both of those certainly affect the proximity of known BH's. But the OP asked for an estimate to the nearest, not only a value for the nearest known one. And the local star density affects the proximity of stars in just the same way as the proximity of BHs (apart from "2nd-order" effects), so I believe the estimate still stands.
     
  21. Feb 19, 2018 #20

    Drakkith

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    Those factors should affect the density (and thus proximity) of unknown black holes as well if I'm not mistaken.
     
  22. Feb 19, 2018 #21

    JMz

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    Not sure how the lower density of stars here (than in the galactic center, say) should affect the relative density of BH's -- relative to stars, that is. My back-of-the-envelope simply assumed that that ratio in our neighborhood is about equal to the galactic average. A simple statistic that we could estimate right from the OP, given just the total number of stars in the MW.
     
  23. Feb 20, 2018 #22

    Drakkith

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    My apologies, I didn't realize you were talking about the black-hole-to-star ratio.
     
  24. Feb 20, 2018 #23

    JMz

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    Ah! Yes, just a tactic to get a simple (fairly accurate?) distance estimate.
     
  25. Feb 20, 2018 #24

    Chronos

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    Considering the nearest known neutron star [RX J1856.5-3854] is about 400 ly distant and that neutron stars are probably more common than black holes, it's no surprise the nearest known black hole suspect is a few thousand light years off. Both star types are difficult to detect, which makes their actual abundance uncertain, A black hole is potentially easier to detect because a] they can really stand out in a crowd with proper feeding. b] they are vulnerable to exposure via gravitational lensing in photo surveys.
     
  26. Feb 20, 2018 #25
    Because black holes require density of matter the presence of black holes would be exponentially higher towards the center of a galaxy.
     
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