How do you weigh a supermassive black hole?

By Phil Plait | July 16, 2008 5:34 pm

Black holes are cool. They’re also scary, and weird, and exciting. But if there’s one word I’d use to describe them (besides, well, "black"), it’s mysterious.

They twist our minds, and they push our math and science to the limit. After all, they’re like punctures in the fabric of space and time, and that has some inherent effects on our common sense.

But maybe maddening would be a good word, too. Astronomers love black holes for the same reasons you do, but we also hate them, because they’re so frackin’ resistant to study. They’re black. They don’t give off any light, so that makes them pretty difficult to observe.

However, they do affect their environment, and that can be measured. And, as you’d expect, the more massive the black hole, the bigger its effect. In a very real sense, they throw their weight around.

Chandra X-ray image of NGC 4649

At the center of this image is a very big black hole. Don’t bother squinting: on this scale, even as big as the hole is, it appears microscopic. But the picture itself tells the tale of a galaxy, a black hole, and the gas its slowly consuming.

In the center of every big galaxy, we think, lies a supermassive black hole. And I mean supermassive: they can be millions or even billions of times the mass of the Sun. We think that the very formation of the galaxy itself is affected by the black hole in its heart. Not because the black hole has a large fraction of the galaxy’s mass: in fact, these black holes only have a fraction of a percent of the galaxy’s mass (our own in the core of the Milky Way has about 4 million solar masses, compared to the 200 billion solar masses in the entire galaxy: a ratio of just 0.002%). We think that matter falling into the black hole creates huge winds that blow out, curtailing star formation in the galaxy at large.

We’d love to know more about this, and in the end it all ties into one thing: the black hole’s mass. The thing is, that’s hard to measure. One way is to observe how fast stars are orbiting the center of the galaxy very close in to the black hole (the more massive the black hole, the faster the stars move). On average — that is, if you do this for a lot of galaxies — this method ain’t bad. But for specific galaxies it has uncomfortably large error bars. So what we need are more methods to weigh these black holes, and use them as a check on the other methods. That makes us more confident we know what we’re doing.

So astronomers came up with a clever idea: galaxies have lots of gas floating around, and this gas will tend to fall to the center of the galaxy. As it does, two things happen: it piles up in the center, like water swirling in a drain, and it also heats up, getting to a pretty high temperature. A hot gas tries to expand, and that expansion is balanced by the gas falling in. The whole shebang reaches a kind of balance called hydrostatic equilibrium.

Slowly, the gas in the center cools, shrinks, and falls into the black hole. It cools by radiating away heat in the form of X-rays — the higher the temperature the gas, the more energetic the X-rays it emits. By measuring these X-rays, astronomers take the temperature of the gas. And, it turns out, the bigger (more massive) the black hole, the hotter the gas gets as it falls to the galaxy’s center.

So if you measure the temperature of the gas, you can infer the mass of the black hole using relatively simple gas physics. Cool!

Well, hot, really. The gas gets pretty agitated as it falls to the its inevitable death, heating up to millions of degrees. The means it emits X-rays, and that’s what the Chandra Observatory sees. So astronomers used Chandra to observe NGC 4649, an elliptical galaxy about 50 million light years away (pretty close, as galaxies go). In the picture above, which is a combination of Hubble and Chandra images, the smooth purple stuff is the gas, and foreground stars and background galaxies are bluish-white. The gas is extremely smooth and featureless, which is good: that makes it easier to measure.

The astronomers took the temperature of the gas, and cranked through the equations. They found that the black hole in the center of NGC 4649 is a whopping 3.4 billion times the mass of the Sun. That’s the biggest black hole I have heard of for which we have a good measure of its mass. Wow. Supermassive indeed.

That number agrees pretty well with estimates made be measuring the way stars orbit the galactic center, which is reassuring. What’s also nifty is that this is totally independent of that method, which means that we can use it on bunches of different galaxies, and get good statistics on these monsters. That in turn means we have better numbers to wield when trying to figure out how black holes and galaxies form, and what happens as they grow old together.

And its just in time, too: with the launch of the gamma-ray observatory GLAST in June, we’ll be finding thousands of previously unseen black holes in the centers of galaxies. This new method probably won’t work for those galaxies — we need quiet black holes for it to work, and the ones GLAST finds will be anything but — but the more black holes we see, the better our understanding gets.

And given how scary, weird, and exciting — I mean mysterious — black holes are, the more we know, and the cooler they get.

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Comments (54)

  1. I really want to see a blackhole that has 200 billion solar masses. Hopefully some exist in the universe now, although we can observe it due the amount of time it takes light to reach us. We may see things on that scale as we travel at super-luminal speeds across the cosmos.

    We are truely living in the golden age of astronomy!

  2. How do you weigh a supermassive black hole? Is that a trick question?

    Objects in free-fall have mass, but not weight, right? Weight is something that varies depending on the gravity field, isn’t it?

  3. …although we can’t* observe it due to the amount of time it takes light to reach us. We may see things on that scale as we will* travel at super-luminal speeds across the cosmos in the future.

    @ZorkFox:
    By weigh, he means to find out the mass not the weight. And you are right about the weight as it is relative.

  4. PooprScooper

    Not to change the subject too much, but has anyone heard or know more about the stuff on the phoenix’s leg? http://wanderingspace.net/2008/06/what-is-on-phoenix%E2%80%99s-leg/

  5. How do theoretical wormholes differ from Black Holes? Or, are they the same?

  6. My first paper (well, so far only paper in press) was on how to weigh active black holes 😀

  7. Wow that is frakkin big. I wonder if it’s been looked at for megamasers? Then you could really get a precise measure of the BH mass.

  8. Quiet Desperation

    Ask it.

    Unless it’s female.

    Yeah, I went there.

  9. baryogenesis

    I saw Andrea Ghiz (UCLA) in March give a presentation on the Milky Way Black Hole. She came armed with some spectacular graphic movies made from years of accumulated data of stars very close in to Sag A* esp using Keck, and was a wonderful speaker on the subject. Not afraid to say “We don’t know”. http://www.astro.ucla.edu/~ghezgroup/gc/pictures/index.shtml

  10. Jose

    How do theoretical wormholes differ from Black Holes?

    If I remember correctly, wormholes open gateways to hell (is that what Event Horizon was about?) or a more purple version of earth where your Dad is still alive. Black holes, on the other hand attract evil robots and Norman Bates.

  11. Mark Hansen

    Silly BA. With a hypermassive set of scales of course.

  12. Tom Marking

    I wonder what the evidence is that the standard blackhole theory is correct instead of the gravastar theory. They differ in that the standard blackhole theory predicts a point singularity at the center of the event horizon whereas the gravastar theory predicts a spherical shell of Einstein-Bose condensate just inside the event horizon.

    3.4 billion solar masses squeezed down into a mathematical point. I’m not sure I believe that one – in fact I’m pretty sure I don’t believe that one. At the singularity the density becomes infinite and the gravitational field become infinite. Infinity is not a very good number to predict if you’re a physics theory. So I’m sure string theory or whatever comes next will blow the doors off the singularity concept and prevent these infinities from piling up.

  13. Patrick

    I was going to say carefully but your explanation works too

  14. Gene Miller

    With a really, really big scale which you don’t mind having vanish. And you have to read it fast! Don’t get too close now. . .

  15. Jose

    How do theoretical wormholes differ from Black Holes?

    OK, I’ll try and answer it for real.

    In order to create and maintain a usable wormhole, scientists and sci-fi writers have to fudge a little. One way is through the use of negative energy, which may not exist. The other way is to use a black hole and ignore the obvious problems that presents. So as far as sci-fi is concerned, a wormhole is a hole in space-time through which we can travel, while the extreme physics of black holes are a place where a wormhole might exist.

    From a purely scientific standpoint, even if we assume negative energy exists, no one has yet thought of a way create a usable, stable wormhole that adds up.

  16. @Jose, but I was just listening to Alex Collier and he says…. (Major UFO nut case)

    My head is going to burst at the woo I just subjected myself too… Your first response makes more sense after watching that

  17. baryogenesis

    Tom Marking- I can sort of grok the B-E cond., but I’m not sure it satisfies any more than standard black hole concepts. That’s why I prefer to look at what we can study in our backyard and possibly this can be applied on a more massive scale. It’s a start.

  18. According to my quick calculation, the Schwarzschild radius of that black hole is 3.5 . 10^4 AU!

  19. madge

    Q: Where do you go to weigh a pie?
    A: Somewhere over the rainbow
    😀

  20. Pieter,

    3.5e4 AU is half a year-light. It’s very insignificant in galactic scale but a huge number in star scale.

  21. Edward

    If light can’t escape from a black hole, how do these huge winds do it?

  22. NoAstronomer

    Edward, The gas isn’t *in* the black hole just near it.

    My turn for a question then:

    What methods have been used to determine the mass of the black hole at the center of the Milky Way?

  23. Frecklessboy

    Nice post! I have to prepare a project on the future of our solar system. I have tried to saerch it on the internet as much as posssible but only got information on the expansion of the sun and the possibility of Earth being taken in by the sun or the escape of the Earth’s atmosphere in the space and the evaporation of the seas. If anyone can help then PLZ PLZ mail me at frecklessboy@gmail.com.

  24. rob

    if you want to know about the black hole at the center of the milky way, just go and check it’s myspace page.

    apparently it likes pina coladas, rain, the ocean and champagne.

  25. By the way, when two different methods of measurement (e.g. measuring the gas temperature and measuring the orbiting speed of stars around the black hole) yield similar results, we call that convergent validity. It is like someone else replicating your experiment. It is powerful support for your initial observations.

  26. And hey Rob, you just know that super-massive black hole is just….

    http://www.youtube.com/watch?v=KS2f4Xzod0w

    hehehehe

  27. Derek

    I must confess, I’m not entirely sure why it’s so difficult to get an accurate measure of the mass of a black hole.

    I mean, can’t you just measure the effect of their gravity on nearby stars, and infer what the mass must be from that?

  28. occam's comic

    “So if you measure the temperature of the gas, you can infer the mass of the black hole using relatively simple gas physics. Cool!
    Well, hot, really. The gas gets pretty agitated as it falls to its inevitable death, heating up to millions of degrees. The means it emits X-rays,” -BA
    Once again, it seems to me, that astronomers are making a basic mistake, if you heat a gas to millions of degrees, so hot it is emitting X-rays, it is not a gas anymore. It is plasma and therefore it does not follow simple gas laws anymore.
    This constant confusion by astronomers calling plasma a gas is astounding to me it would be like biologist constantly calling fungi plants. I just don’t get why astronomers keep making the same basic mistake over and over again.

  29. !AstralProjectile

    Tom Marking (July 16th, 2008 at 9:30 pm ) and everyone:

    Question: Two properties of black holes are mass and angular momentum. (I don’t believe they can be charged, regardless of what Niven says.) So presumably the singularity must be spinning infinitly fast for the BH to display even one “Plank Unit” of angular momentum. So if has more than that, is the singularity spinning infinitly faster and so on? (I’m aware that infinities are quanitized, as is angular momentum)

    Comments? Thanks in advance.

  30. occam’s comic: because it’s not a mistake. A plasma behaves differently than a gas in the presence of magnetic fields, for example, but in this case we’re talking about the effects of gas pressure and hydrostatics. So calling it a gas is not a big deal.

    Or are you an Electric Universe person? If so, forget what I’ve said, and assume that I’m part of the Big Conspiracy trying to deceive the public about blah blah blah. :-)

  31. occam's comic

    “A plasma behaves differently than a gas in the presence of magnetic fields,” exactly my point, are you saying there are not magnetic feilds in that galaxie?

  32. Phil said:
    “Or are you an Electric Universe person? If so, forget what I’ve said, and assume that I’m part of the Big Conspiracy trying to deceive the public about blah blah blah. :-)”

    I kNEW it! That means all the things he’s “debunking” aren’t bunk after all! That goodness I saved my supply of tin foil. :)

  33. Tom Marking, whatever is inside the black hole is of no concern to us, because it has no observable consequences (well, it does when you cross the event horizon, but you won’t be able to report back to us). The singularity is the simplest way to describe the metric in GR.

  34. web404

    In response to plasma vs gas discussion. It’s true that matter states are both temparure and density (preasure) related. Clearly the density is increasing sufficeintly for the “gas” to cause it’s own friction, and thus tramendu heat buildups. At 1 atmosphearic preasure, sure at a few million degrees is going to be a plasma, but as the gas build up hydrostatic preasures arroudn a black hole, can we know what the density/preasure would be exerted on the subatomic components?

  35. Derek: “I mean, can’t you just measure the effect of their gravity on nearby stars, and infer what the mass must be from that?”

    That works for a very nearby supermassive black hole, namely, Sgr A*, The Milky Way’s own SMBH. Astronomers can actually see the stars very near the BH orbit over time, and can thus deduce the mass of the central object, and an upper limit on its radius. (I think this also answers NoAstronomer?)

    The problem with all the rest of the black holes in the universe is that they are so far away, that we cannot resolve the individual stars. For some nearby galaxies, you may be able to look at the integrated velocities of a group of unresolved stars and deduce a mass that way.

    QD: “Ask it.

    Unless it’s female.”

    Har har… 😛

  36. Skeptic Tim

    Jose, your comment that “One way is through the use of negative energy, which may not exist.”

    Considering that the Casimir effect is known to exist and that Morris, Thorne and Yurtsever have pointed out that the quantum mechanics of the Casimir effect can be used to produce a locally mass-negative region of space-time, suggesting that that negative effect could be used to stabilize a wormhole; we might, therefore, conclude that we do have evidence of the existence of negative energy: albeit at very small scales.

  37. occam's comic

    Am I an electric universe person? I am a chemist, and we consider the difference between an atom and an ion to be pretty darn basic and pretty darn important.

    Is it unreasonable for me to expect a scientist who is writing about science to use the correct terminology? When astronomers call what is clearly plasma, a gas you give the electric universe people a lot of ammunition. And when you get call on this basic error you say it doesn’t matter because the major difference between plasma and gas is its behavior in a magnetic field I want to bang my head because you know that the moving charged particles in plasma create magnetic fields. You have also repeatedly had posts about how there are intense magnetic fields around black holes.

  38. Torbjörn Larsson, OM

    How do you weigh a supermassive black hole?

    From here? That gravity force would be negligible.

    IIRC some call that to “mass” a body.

    And, um, we would mass it as a whole. [sic!]

    They’re black. They don’t give off any light, so that makes them pretty difficult to observe.

    Is that so? I thought Hawking radiation shows the black hole temperature, and that many of those particles would be photons. (As photons are their own anti-particles, allowing for the radiation at the event horizon.)

    @ occam’s comic:

    Is it unreasonable for me to expect a scientist who is writing about science to use the correct terminology?

    As I believe Phil says, I believe in cosmology it is the correct terminology. If it behaves as a gas for the purpose of the observation, mechanism or calculation, in physics it is called a gas for clarity. For example, AFAIU in the simplest cosmological models, when you add the simplest mass description it will be as a “gas”.

    Think of it as an approximation or simplification if you will.

  39. Torbjörn Larsson, OM

    @ TM:

    I wonder what the evidence is that the standard blackhole theory is correct instead of the gravastar theory.

    I wonder what the evidence is that the gravastar theory is correct instead of the standard blackhole theory.

    But really, black holes are derivable from GR, so it is a matter of parsimony to keep them until proven false. And, I would guess, theoretical convenience. A third argument could be that string theory can build black holes, but can it build gravastars?

    @ AP:

    Two properties of black holes are mass and angular momentum. (I don’t believe they can be charged, regardless of what Niven says.)

    Niven, as in the scifi author? That a black hole has the three properties mass, charge and angular momentum goes back to works of Werner Israel, Brandon Carter and Stephen Hawking 1967-1972 according to MTW (Missner, Thorne Wheeler).

    So presumably the singularity must be spinning infinitly fast for the BH to display even one “Plank Unit” of angular momentum.

    Spin is a QM property, so it is can’t be exactly analogous to a classical property. It is a component of angular momentum, but it is in fact not analogous to classical angular momentum. Instead it is the direction of how a particle turns along an axis; it is an intrinsic property of the particle and its field.

    Even more evidence that it isn’t analogous to classical angular momentum is that it doesn’t behave as a vector:

    Mathematically, quantum mechanical spin is not described by a vector as in classical angular momentum. It is described using a family of objects known as spinors. There are subtle differences between the behavior of spinors and vectors under coordinate rotations. For example, rotating a spin-1/2 particle by 360 degrees does not bring it back to the same quantum state, but to the state with the opposite quantum phase; this is detectable, in principle, with interference experiments. To return the particle to its exact original state, one needs a 720 degree rotation.

    Isn’t that cool? Spin has to do with geometry and knots as much as with dynamics and particles.

    AFAIU massive black holes can be approximated semi-classically because they have many degrees of freedom. So there isn’t any problem with them acquiring mass, charge or angular momentum, such as spin, from fields or particles.

    But perhaps there is an astronomer expert here that can tell us how the details work.

  40. Torbjörn Larsson, OM

    Uups. That is Misner, Thorne, Wheeler.

  41. Tom Marking

    “But really, black holes are derivable from GR, so it is a matter of parsimony to keep them until proven false. And, I would guess, theoretical convenience.”

    So in other words, no direct physical evidence for the concept.

    “That a black hole has the three properties mass, charge and angular momentum”

    What does the angular momentum of a mathematical point mean? With no size any rotational state of the singularity is identical to any other rotational state. You could not even in principle determine its rotation. Thus, angular momentum is a meaningless concept for a point object. I’m not sure why blackhole proponents would retain the concept for a supposedly point object.

    “Tom Marking, whatever is inside the black hole is of no concern to us, because it has no observable consequences”

    It is true that events within the event horizon cannot propagate out to us. However, that does not necessarily mean there is no data external to the event horizon that will tell us what type of object it is under the event horizon.

  42. Tom: Existence and uniqueness theorem. No matter what the configuration of matter is or what it’s made of, as long as it’s spherically-symmetrical it can only have a Schwarzschild gravitational field (assuming General Relativity). Only way to tell as far as I know is to go to the black hole, whack it with a hammer and see what happens.

  43. Tom Marking

    “Tom: Existence and uniqueness theorem. No matter what the configuration of matter is or what it’s made of, as long as it’s spherically-symmetrical it can only have a Schwarzschild gravitational field (assuming General Relativity). Only way to tell as far as I know is to go to the black hole, whack it with a hammer and see what happens.”

    You’re assuming it is a blackhole a priori and that Schwarzchild’s formulas apply.

    http://wwwsfb.tpi.uni-jena.de/VideoSeminar/Files/20071126-chirenti.pdf

    “…A gravastar and a black hole of the same mass cannot have the same complex eigenfrequencies: an observer can tell them apart beyond dispute”

  44. quasidog

    Re: occam’s comic’s comment and Phil’s rebuttal regarding the difference between plasma and gas. Phil, are you just being general when you call it all ‘gas’. Is there a distinct difference here, between the plasma and the gas at these temperature and in the presence of a magnetic field? … or is this just pedantic. From where I stand as a layman, but having a good general knowledge about how this stuff works, it seems he makes a good point, but your dismissal of his claim seems weak. I would really like for you to clarify if possible.

    It seems you have dismissed occam’s comic as being an electric universe proponent but it seems he is suggesting he is not. The question remains, is his point correct? Would is be called a plasma seeing as it is in this magnetic field presence, at these temperatures ?

    I ask, because I am confused as to why you are dismissing his claim so readily, by suggesting he is from some pseudo-scientific back-round, when it seems that what he says about the presence of the magnetic field playing a part makes a lot of sense. Is it a ‘gas’ generally speaking, or more specifically, would it be a ‘plasma’ ?

    Confused.

  45. Let me add my voice to quasidog’s request. I would like to know what the deal with the gas and the plasma is.

    “It is true that events within the event horizon cannot propagate out to us. However, that does not necessarily mean there is no data external to the event horizon that will tell us what type of object it is under the event horizon.”

    If this were true, changes behind the event horizon would be observable as changes outside the event horizon. In other words, it could be used to send signals accross the horizon. This is why black holes “have no hair”.

  46. Should we not be calling these things Supermassive Singularities now, so we do not fall foul of the political correctness thought police? 😮

  47. rob

    @ Michael L:

    that was a gAstley thing to do.

    Oh, what sad times are these when passing ruffians can rickroll at will to old ladies. There is a pestilence upon this land, nothing is sacred.

  48. !AstralProjectile

    TKU, Torbjörn. I stand corrected on the charge issue. As always I should havw researched the subject first. P.S. I meant “spin” in the angular momentum sense, not the QM sense. And I was referring to Larry Niven’s Hole Man

    Tom Marking: That all encompassing store of Humanity’s knowledge (Wikipedia) tells me that the singularity in BH with angular momentum atually forms a disk, not a point. (It still has no volume, since it’s 2 dimensional.)

    Io reiterate, I wish I’d done the research before posting.

  49. Tom Marking

    “If this were true, changes behind the event horizon would be observable as changes outside the event horizon. In other words, it could be used to send signals accross the horizon. This is why black holes “have no hair”.”

    In the standard blackhole model what is the only entity containing the mass? Is it not the singularity? And is not the singularity behind the event horizon? What happens to the gravitational field lines leaving the singularity? If they cannot make it through the event horizon then no mass outside the event horizon can sense the blackhole’s gravitational field. It seems to me that the gravitational field line propagates from inside the event horizon to outside it.

  50. Torbjörn Larsson, OM

    Life intervened, but finally returning to old threads FWIW:

    @ Tom Marking:

    So in other words, no direct physical evidence for the concept.

    Oh, there is direct physical evidence ot there such as accretion disks and their radiation without any central stars. But not every astronomer is convinced, I hear.

    What does the angular momentum of a mathematical point mean?

    I already explained in another comment, spin applies to pointlike particles.

    You can derive spin and angular momentum by looking at the objects response to disturbances in its environment, such as when NMR is used.

    In the standard blackhole model what is the only entity containing the mass?

    The mass et cetera is defined by its imprint far from the object. It isn’t localized.

  51. michael

    So the question still isn’t answer … Here goes u can weight a black hole. It is physically impossible u show me a person went there and weighted it with a scale Haha it hasn’t happen

  52. michael

    So the question still isn’t answer … Here goes u can weight a black hole. It is physically impossible u show me a person went there and weighted it with a scale Haha it hasn’t happen in the center of all galaxies u Thu.k is a black hole.ur theory is ur opion not fact.

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