Desktop Project Part 3: The massive massiveness of M54

By Phil Plait | March 28, 2012 7:00 am

[Over the past few weeks, I’ve collected a metric ton of cool pictures to post, but somehow have never gotten around to actually posting them. Sometimes I was too busy, sometimes too lazy, sometimes they just fell by the wayside… but I decided my computer’s desktop was getting cluttered, and I’ll never clean it up without some sort of incentive. I’ve therefore made a pact with myself to post one of the pictures with an abbreviated description every day until they’re gone, thus cleaning up my desktop, showing you neat and/or beautiful pictures, and making me feel better about my work habits. Enjoy.]

I post a lot of pictures of globular clusters, because they’re just so darn pretty. They’re roughly spherical collections of thousands of stars, all born at the same time, and bound together by their own gravity. But M54 is special, for more than one reason… but mostly because in this Hubble image it’s jaw-droppingly gorgeous:

See? [Click to embiggen, or grab the huge 15Mb version.]

It’s the second most massive globular known (the most massive is Omega Centauri), with well over a million times the mass of the Sun. That’s a lot. It’s very far away, too, at a distance of about 90,000 light years, making it one of the more distant globulars known. It’s big — 300 light years across, and also incredibly luminous, shining with over 800,000 times the Sun’s light. Only its vast distance dims that glory to us.

Also among its oddities is that it harbors what’s called an intermediate mass black hole, one that’s far larger than the kind you get when a star explodes (which have usually up to a few dozen times the Sun’s mass), but still far less than the huge ones found in the centers of galaxies like ours (which are millions or billions of times the Sun’s mass). M54’s black hole has something like 9400 times the Sun’s mass, which is still substantial! These kinds of black holes were only discovered a few years ago for the first time, and how they form is still something of a mystery.

But I haven’t even mentioned the weirdest thing: M54 may be part of another galaxy! It actually sits right at the center of a dwarf galaxy called the Sagittarius Dwarf Elliptical Galaxy, a small galaxy currently being torn apart and assimilated by the Milky Way. That’s right: we’re eating it. It’s likely that it formed in that galaxy and has been co-opted by the Milky Way. Since M 54 was discovered by Charles Messier in 1778, this makes it the first extragalactic globular cluster ever seen!

Image credit: ESA/Hubble & NASA

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CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (16)

  1. Beautiful pic! Share them when you get the time. No worries!

  2. Chris

    So what is the average distance between stars in M54? What about the probability of collision? Maybe that’s why it has an intermediate mass black hole.

  3. Nigel Depledge

    That’s a very pretty pic!

    If the galaxy it’s in is being asimilated by the Milky Way (resistance is futile), though, does that still count as extragalactic?

  4. Nigel Depledge

    @ Chris (2) –
    The main debate (IIUC) about greater-than-stellar-mass black holes is : which came first, the black hole or the cluster / galaxy?

    If the BH came first, leading to the formation of the cluster (or galaxy), then it requires a different mechanism for forming the BH.

  5. Someone destroyed another Death Star.

  6. Keith Hearn

    So where does one draw the line between a large globular cluster and a small galaxy?

  7. A recent Scientific American had an article about these intermediate black holes. Was great reading.

    And what a pretty picture. I look forward to you unloading that metric ton of them!

  8. Tim K


    So how far is it between the stars in the globular? Do they have planets? How would it be to stand on a planet and look at the sky?


  9. Chris

    @8 Tim K
    Actually I think globular clusters are the one place where there hasn’t been much luck (if any) in finding planets. They tried looking for transits and with so many stars, you should easily find some, but I think the search came up empty. Basically the stars are so close (don’t know the exact distances) they create a lot of gravitational instability that even if you get a planet to form, it’ll easily be ejected in millions of years. At the core, the density of stars is ~100 to 1000 per cubic parsec. For comparison, the nearest star (other than the sun) from the earth is 1.3 parsecs, so our local density is less than 1 per cubic parsec.

  10. ctj

    do we have good enough observations of any globular that we can use to map out the orbits of its stars and calculate the mass of the central black hole (or for that matter, confirm that it’s there)?

  11. Nigel Depledge

    Keith Hearn (6) said:

    So where does one draw the line between a large globular cluster and a small galaxy?

    Good question.

    I don’t think it’s exclusively a question of size, I think it also has to do with what else is there other than stars. IIUC, most (all?) galaxies contain gas and dust between the stars, but globular clusters not so much.

    One hypothesis I read a few years ago was that at least some globulars are the remnant cores of small galaxies that the Milky Way is in the process of assimilating. This at least explains why they don’t orbit only in the disc of the Milky Way like nearly all the other stars.

  12. @9. Chris :

    @8 Tim K – “Actually I think globular clusters are the one place where there hasn’t been much luck (if any) in finding planets. They tried looking for transits and with so many stars, you should easily find some, but I think the search came up empty.”

    As far as I am aware – & please feel free to correct me if I’m wrong y’all – there’s only been one exoplanet ever discovered inside a globular cluster and that is PSR B1620-26 b sometimes referred to as PSR B1620-26 c and also dubbed “The Genesis Planet” or “Methuselah” planet given its vast age – having presumably formed 12.7 billion years ago. This exotic pulsar planet orbits a white dwarf – pulsar pair in the Messier 4 globular cluster – near Antares as seen from Earth’s perspective – and is physically the largest although the least massive object in its system having 3 jovian masses.

    (Click on my name here for its wikipage.)

    Like Chris, I’m pretty sure I’ve read about several seraches for exoplanets in globulars which have failed to detect anything. Unfortunately I can’t recall the exact specifics of those globular exoplanet searches currently. This apparent absence of planetary bodies may be linked with the extreme age and thus low metallicity of most globulars.

    Oddly enough (before we understood this?) I think I vaguely recall one early SETI project beaming a signal out to globular cluster Messier 13.

  13. Captn Tommy

    @8 TomK

    Issac Asimov’s short story and subsequent Novel NIGHT FALL is a tale about a civilization living on a planet inside a globular cluster(GC)… It’s very good!

    My question is what is keeping a GC from collapsing altogether? There is a lot of mass there and thus gravity in a very tiny space. Perhaps like gallaxies that is why there is a black hole inside…

    Then again, since they are so old, maybe these GCs are the reminants of all those very very young red gallaxies we see in the distance, stripped of their dust and gas by the epics of time.

    “I love this place! Can I stay?” – Montgomery Scott, Engineer

    Copn Tommy

  14. Sam

    Thanks for the pics.

    I love it when science articles are well written and drafted in everyday language and I still don’t understand what I’m reading.

    “M54 has a black hole 9400 times the sun…” Can it be seen in the picture?

    “Galaxies eating each other” Not the first time I’ve heard that but at the same time I’m told all galaxies are moving away from each other at increasingly faster speeds.

  15. Gunnar

    Beautiful picture! Thanks also to those who have already commented and asked and/or answered most of the questions I was most interested in. I understand that the high density of stars per cubic parsec could result in numerous gravitational instabilities that could work against the formation and/or survival of numerous planets with stable orbits around their primaries, and that this alone might well account for the apparent scarcity of discovered planets associated with stars in globular clusters.

    However, if the reason for the apparent scarcity of planets in globular clusters is also largely due to the stars in these clusters being of low metallicity, thus being mostly first generation stars, doesn’t this also imply that most of those stars, besides being very old, are also considerably smaller than average? After all, IIUC, larger stars burn up their fuel much faster than smaller stars, and even first generation stars the size of our sun and larger should have long ago expired and turned into white dwarfs or nova-ed and turned into neutron stars or black holes by now, unless there are still pockets of primordial hydrogen and helium that have not yet been enriched by material from previously expired stars, where first generation stars can still form. Can anyone further enlighten me on this?

  16. Nigel Depledge

    Sam (14) said:

    “M54 has a black hole 9400 times the sun…” Can it be seen in the picture?

    No. Black holes cannot be seen at all, hence the name.

    If you mean, “can its accretion disc be seen in the picture?” this would require that there is an accretion disc around the BH, i.e. that it is actively consuming matter at the moment. Given the scarcity of gas and dust in a GC, I think this would be unlikely.

    If you mean “can its effects be seen in the picture?”, then sure. It is through measuring the velocities of the visible stars that astronomers can estimate the mass of the BH in the first place.

    “Galaxies eating each other” Not the first time I’ve heard that but at the same time I’m told all galaxies are moving away from each other at increasingly faster speeds.

    First off, it is not true that all galaxies are moving away from each other. Some galaxies are orbiting other galaxies, while others are moving towards one another (for example, the Andromeda Galaxy is moving towards our own Milky Way).

    There are two different effects here. First, there is gravity, that causes galaxies to move through space towards one another. Thus, galaxies tend to be found in clusters and there is often a very large elliptical galaxy at the centre of a galaxy cluster. Large elliptical galaxies are formed from the merging of smaller galaxies.

    Second, there is the expansion of the universe, whereby space itself is expanding. Over large distances, this expansion velocity is very high, but locally it is quite modest. Thus, although the space between the Andromeda Galaxy and the Milky Way is expanding, the two galaxies are moving fast enough through space that they will indeed meet in a couple of billion years’ time.

    This universal expansion appears to be accelerating. No solid explanation yet exists for how this might be occurring, but the observation won’t simply go away through re-evaluation of the data. As far as we can tell, the acceleration of universal expansion is a real phenomenon.

    Thus, over large scales galaxies are indeed moving apart at ever-increasing velocity; but locally, galaxies may be moving towards one another or be in orbit around one another.


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