Alien clusters invade our galaxy!

By Phil Plait | February 24, 2010 9:29 am

Is this the face of an alien?

sdss_palomar5

According to a new study, the answer is probably yes.

That’s Palomar 5, a globular cluster very roughly 75,000 light years away. Globulars are ball-shaped collections of hundreds of thousands of stars, and surround many large galaxies, kinda like bees swarming around a hive. There are at least 150 orbiting our own galaxy, the Milky Way.

The question is, how many of these formed here, along with our galaxy 12 billion years ago, and how many formed around other galaxies and were subsequently subsumed into us?

The Milky Way is a giant galaxy, and we know it got that way by eating — astronomers call it cannibalizing, because we’re zombie fans — smaller galaxies. We see the remnants of some of those meals as streams of stars that got torn out of the galaxies as they got digested, and sometimes we see the residual core of stars from the galaxy itself still somewhat intact — indigestible bit of chewing gum, you might say.

So the astronomers in the new study asked themselves: just how many of the Milky Way’s globular clusters formed along with the original Milky Way, and how many came from other galaxies that were eaten? The answer they found was surprisingly high: it may be as many as 1/4 of them!

They examined what’s called the Age-Metallicity Relation in the clusters, a way of figuring out the age of a cluster by looking at the relative numbers of heavy elements in it (astronomers call any element heavier than helium a metal). What they found is that most of the globular clusters in and around the Milky Way are about the same age as the galaxy itself: 12 billion years, give or take. However, quite a few are much younger, by several billion years.

Some we already know about, and are associated with known cannibal events (the Sagittarius dwarf galaxy upon which we’re currently dining, for one). But still, something like 30 – 50 clusters still remain that are too young to have been native to the Milky Way — and Palomar 5 (in the picture above) is among them.

This implies something like 6 – 8 galaxies have been eaten by our galaxy to make it what it is today. That’s pretty neat. I’ve often wondered just how many galaxies were sacrificed to make the Milky Way one of the biggest galaxies in the Universe — and it really is; while there are plenty that are bigger, and some that are a lot bigger, we’re still in the upper echelons of the cosmos if you rate galaxies by sheer size and mass. Now it looks like we had to eat a half dozen less fortunate galaxies to get where we are today.

And we’re not done yet. In a billion years, maybe two, it’s likely that the Milky Way and the massive Andromeda galaxy will collide — perhaps not directly at first, but over hundreds of millions of years they’ll merge into one even more gianter galaxy, potentially igniting a burst of star formation and tossing around stars like bugs in the wind. We may wind up consuming the score of dwarf galaxies in our own Local Group as well.

What I find interesting is that the Sun will still be around then; it won’t go red giant on us for another few billion years after the galactic merger. We may very well get a pretty good view of the coming cosmic collision. Well, maybe not us in particular, but whoever’s still around in a billion years. What a view they’ll have!

Image credit: Sloan Digital Sky Survey

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (41)

  1. ” it may be as many as 1/4 of them!”

    That’s really interesting. Furthermore, I guess the age of these globulars tells us a lot about the age of the galaxies we like to eat: young and innocent.

  2. oldamateurastronomer

    I remember reading some years ago about the discovery of a small galaxy being ‘devoured’ by the Milky Way. Apparently as a joke (maybe?) the galaxy being consumed was given the name ‘Snickers’ in deference to the name of our home galaxy. Well, apparently more learned heads prevailed and named the doomed morsel… er galaxy something else (party poopers!)

    BTW, as a retired chemist, I hereby protest that all elements higher on the periodic table than helium are called metals!!!

  3. Charlie Young

    That may be the only reason you would want to be immortal, except for the sheer boredom after all these cool events are over. However, if you subscribe to Ray Kurzweil’s view of the future, we’ll all be able to transfer our consciousness to a nanocloud and travel the universe at near the speed of light and also we’ll be immortal. That is if you don’t mind being our version of the Borg. (You have to say that with Jean Luc Picard’s voice, though.)

  4. Chris

    When Phil used to do his web chats, I asked him whether the view of the Milky Way-Andromeda collision would really be that great, since galaxies (despite their size) are faint objects that need time exposure photographs and Hubble type Photoshopping to look like the beautiful images we all enjoy. When I asked that question, Phil seemed to ponder it a while (“No, it’d be bright… Then again, the closer a galaxy gets, the bigger it looks, and the more diffuse it is… hmm…”) But I don’t remember getting a conclusive answer.

    That brings up a good question: If I could park myself outside the Whirlpool Galaxy, or whatever, would I really get the kind of beautiful view that I currently have on my desktop?

    http://en.wikipedia.org/wiki/File:Messier51_sRGB.jpg

  5. Acronym Jim

    Attention Snickers; we are the Milky Way. You will be assimilated. Resistance is futile.

  6. I was under the impression that, when structured galaxies merge they become irregular or elliptical galaxies. The Milky Way remains a spiral. What don’t I understand?

  7. Yoweigh

    “So the astronomers in the new study asked themselves: just how many of the Milky Way’s globular clusters formed along with the original Milky Way, and how many came from other galaxies that were eaten? The answer they found was surprisingly high: it may be as many as 1/4 of them!”

    I tripped over this paragraph. Does this mean 1/4 of them formed with the Milky Way, or 1/4 of them were cannibalized?

  8. I’m missing something in your paragraph 5: the way I am reading it you covered both the possible sources for globular clusters, but then say that they only account for 1/4 of the globular clusters. So I’m wondering where did the other 3/4 come from? Which is the source for the reported 1/4? I hope you don’t consider this comment to be too nit-picky. If so, sorry.

  9. CW

    We need to spend money on reverse/stop-aging technology, because I ‘really’ want to see the universe a few billions years from now.

  10. Ted

    This ideas has been around for a while, since the late 70′s at least (Searle and Zinn). But the last year has seen much more constraint, including data from an HST Survey.

  11. So are the Magellanic Clouds included in the list of local globular clusters? I seem to recall reading somewhere that they are being consumed at a pretty good clip.
    P.S. I’m writing this on my Droid and it actually suggested “Magellanic” by the time I got to the first ‘l’.

  12. Plutonium being from Pluto

    Cool photo & article. Thanks BA. :-)

    Omega Centauri is one of these potentially alien clusters – perhaps the core of a former dwarf elliptical.

    In a billion years, maybe two, it’s likely that the Milky Way and the massive Andromeda galaxy will collide — perhaps not directly at first, but over hundreds of millions of years they’ll merge into one even more gianter galaxy, potentially igniting a burst of star formation and tossing around stars like bugs in the wind.

    Yes, I seem to recall reading about that in a book somewhere. One that had the words “death” and “skies” in its title which was by a certain doctor, P …something? Pl-something was it? Ah well, I’m sure you’ll have heard of that guy and his book, BA! ;-)

    @ 9. CW Says:

    We need to spend money on reverse/stop-aging technology, because I ‘really’ want to see the universe a few billions years from now.

    Or invest in time travel and cryogenic freezing technologies eh? ;-)

    Actually, travelling at relativistic speeds would be another possibility too I gather .. ;-)

  13. Pi-needles

    they’ll [Milky Way & Andromeda] merge into one even more gianter galaxy

    Gianter? Don’t you mean an even more “embiggened” galaxy there? ;-)

  14. TMB

    Minor quibble about this sentence:

    > They examined what’s called the Age-Metallicity Relation in the clusters, a way of figuring out the age of a cluster by looking at the relative numbers of heavy elements in it.

    The ages and metallicities are measured independently. It’s true that younger clusters tend to have more heavy elements (hence the Relation), but this is an observed trend, not the method used to determine the ages.

    [TMB]

  15. DrFlimmer

    Hail our invading star-cluster-overlords ;)

  16. John Paradox

    12. Plutonium being from Pluto Says:

    Actually, travelling at relativistic speeds would be another possibility too I gather .. ;-)

    Tau Zero?

    J/P=?

  17. Wowbagger

    Question from a middle school instructor/aide. When a galaxy devours another galaxy which may have a large black hole at its center, what happens to the black hole? Does it go into orbit or move through the devouring galaxy wreaking havoc?

  18. Why can’t globular clusters form in the Milky Way recently?

  19. Sure looks like an open cluster, Palomar 5, not a glob… Pretty, though. Must be a new category, a skimpy glob. ;)

  20. Our Galaxy is quite big? I always had the impression (from Hitchhiker’s Guide) that we were small and insignificant. How cool. I feel better now.

    And with all this eating and cannibalizing, I kept reading “Gastronomers”… :D

    d

  21. Gary Ansorge

    Big galaxies eating smaller galaxies and those eating globular clusters and those eating gas and dust and,,,

    Why does this sound so biological to me? Could it just be an example of the fractal nature of reality?

    Go, Milky Way! Eat, drink and be Merry, for tomorrow comes the Dark.

    Gary 7

  22. Jon Hanford

    Phil, I would mention that ‘alien globular cluster’ infall has been well studied before. In fact, at least two papers have been published on the many-degree tail associated with Palomar 5 ( astro-ph/0603062v1 and astro-ph/0307446v1 , for example).

  23. Brian

    #6@Thomas:

    I was under the impression that, when structured galaxies merge they become irregular or elliptical galaxies. The Milky Way remains a spiral. What don’t I understand?

    Eventually they do, yes. But it doesn’t necessarily happen after a single merge event, particularly if the eater is significantly bigger than the eaten. If and when the Milky Way and Andromeda merge, the result is more likely to be an elliptical galaxy.

  24. Lawrence

    Eventually, the two black holes merge (at least that is the prevailing theory). They orbit each other for a few million years before they finally combine into one larger black hole – that may explain why the Milky Way has retains its relative shape, given the increased rotational push created by the merger (just a thought) to maintain the spiral arms.

  25. Jonathan

    I’ve always been curious about the pending Andromeda/Milky way collision. So it’s taken light from that galaxy and it’s components x amount of time to reach us. So actually, right now, the physical matter from that galaxy is actually well closer than what we can see from the light. Has anyone modeled the two galaxies and calculated/subtracted the ‘light delay’ so we can know physically how close the matter from both of our galaxies actually is?

    Furthermore, why not model everything we can see and do the same? Create sort of a ‘real time universe’ as a running model where we’re not just looking at everything we can see with the delay of time and light, but something totally real-time that predicts and projects what will have happened based on our observations and current understanding of astrophysics? I mean, the actual matter of the Andromeda galaxy (and thus it’s gravity) is not where we ‘see’ it today, right? That’s where it was x light years ago.

  26. amphiox

    So, when the Milky Way and Andromeda merge, who’s going to be cannibalizing whom?

  27. J Ferguson

    I read recently that the sun is getting brighter as it ages and the Earth’s oceans will evaporate in about 500 m.y. So, maybe we won’t be around for the merger!

  28. Torbjörn Larsson, OM

    I was under the impression that, when structured galaxies merge they become irregular or elliptical galaxies. The Milky Way remains a spiral. What don’t I understand?

    Yes, I’m a bit fuzzy on that too. I just read that spirals (re)establish themselves after collisions, apparently it’s an attractor for the system.

    Perhaps eventually the ellipticity from mergers that we also read about dominates over the basic spiral because enough imparted mass and momentum wins out. It would be unlikely to see ellipticals as a major class if they are just transient phenomena. Unless mergers are really frequent, of course.

  29. Markle

    @anothermike #11

    So are the Magellanic Clouds included in the list of local globular clusters? I seem to recall reading somewhere that they are being consumed at a pretty good clip.

    No. They are the two nearest galaxies in the Local Group. They are Dwarf Irregulars in the 7G and 10G solar mass range. Both are much more metal poor than the Milky Way. If you look in the recent history, Phil did a story on the gas tail streaming out from the SMC. I’d link it, but this would get thrown into moderation limbo.

  30. Jon Hanford

    ‘Universe Today” has a nice picture of the ‘Seven Dwarfs’, even though most of the Magellanic Stream is not included at – universetoday.com/wp-content/uploads/2007/01/2007-0112dwarfs.jpg

  31. Grimbold

    @TMB #14

    Strictly speaking, you need to measure the metallicity of stars before you can really know their ages. The more metal-rich a star is, the redder and cooler it will look and the more metal-poor it is, the bluer and younger it will look.

    @Wowbagger #17

    There’s evidence that sometimes the black holes go into orbit around each other, so that you have an elliptical with two central supermassive black holes. The reason astronomers think this happens is that some giant ellipticals are not as luminous near their centers than you’d expect. What they think is happening is that the two black holes sweep the area between them clear of stars, so that you have a relatively dark patch. Often, however, the giant black holes merge to form an even more ginormous one.

  32. Revyloution

    @TMB and Grimbold

    Ok, im confused about matallicity and dating globular clusters.

    Unlike our friendly chemist at 2 above, I get the idea that anything heavier than helium is a metal :) and that as galaxies age they convert more of their mass to heavier metals through fusion and nova explosions.

    What I’m missing is how some can be older than others. 13.7b years ago, the universe expanded, 600-800m years later we saw the cooling and coalescing of matter. That matter was separated into clusters by gravity. As the separation continued, stars formed, and they gathered into the first galaxies.

    Ok, here is my stumbling block. At this point, all galaxies are roughly the same age. As expansion continues, shouldn’t they remain the same age? If they are still close to each other today, doesn’t that mean they have been traveling at roughly the same velocity since day 1? If they have similar velocities, and the original matter coalesced into stars at the same time, shouldn’t they have similar levels of metallicity?

    I’m guessing this is yet another part of relativity that will twist my mind in odd directions, but I just can’t see how. I would love it if anyone could explain it.

    Thanks

    Revy

  33. TMB

    Grimbold: Yes, of course that’s true. :) So my use of the word “independently” was probably idealistic. ;) But the point is, both the age and metallicity are determined by measurement for each cluster… not, as Phil said, that measuring the metallicity is what tells you the age.

    Revyloution: In the early universe, although the universe was all roughly the same density, there were tiny fluctuations, so some parts of the universe were a little denser than others. Those denser regions attracted more matter to them (via gravity, because they were denser), and become even denser relative to their surroundings. So the attracted more matter to them, and become even denser, etc. This is the gravitational instability that turned the almost-smooth early universe into the lumpy galaxy-filled universe we live in today. But the time it takes for a density enhancement to grow depends on the density – in denser regions, gravity is stronger, and so processes driven by gravity happen faster. So galaxies form in denser regions faster than in less dense regions, and are older.

    [TMB]

  34. Wowbagger

    Lawrence, thanks for answering my question.

  35. Revyloution

    Ok, im crunching brain cells, TMB.

    A denser region should contract more, making it spin faster (the old figure skater metaphor), correct? This increase in orbital velocity makes it move through time faster, so it is ‘older’ in reference to our own galaxy that had less density?

    Relativity is just a brain scramble, I hope I got that somewhere in the ball park.

    Either way, thanks for the reply.

  36. Grimbold

    @Revylution #32
    “Ok, im confused about matallicity and dating globular clusters.”

    Here cometh a long explanation, and apologies if at any point I sound condescending. But I don’t know how much you know and I’d rather give too much information than not enough.

    One way to determine a star’s age is to look at its position on a colour-magnitude diagram. This is a graph that shows a star’s colour on the x-axis and its brightness on the y-axis. The majority of stars when they are first formed lie on a diagonal line running from hot, blue stars in the upper left corner to cool, red ones in the lower right. This is called the Zero Age Main Sequence (ZAMS), for obvious reasons.

    As the stars age, they move around on the colour-magnitude diagram and the nice diagonal line that was the ZAMS becomes a more complicated curve. These curves are called isochrones, because they describe the distribution of stars of all different masses at the same age. Thus the ZAMS is an isochrone for age zero. What happens as time passes is that the hottest stars exhaust their hydrogen fuel first and rapidly become vast, bloated and rather redder than they were- so they peel off to the right on the colour-magnitude diagram. It’s hard to describe without being able to draw pictures, but the basic point is that an isochrone for 1 billion years looks different to one at 2 billion, or 5 billion or 10 billion years. So if you plot a single star on a colour-magnitude diagram, you can find out which isochrone it lies on, and then you know its age.

    The trouble is, this only really works if you assume the stars all have the same composition. The metallicity (astrophysicists call anything other than Hydrogen or Helium metals) plays a small but important role in the position of a star on a colour-magnitude diagram. To understand why, it’s important to remember that all the information we have about a star comes from the photons that leave it and reach us. Before they escape the star, they bounce around in its interior many, many times, bouncing off particles (mostly electrons) until they finally can get away. So if a photon escapes from deep within the star, it will carry with it information about a part of the star that is hot and bluer looking. If it escapes from near the surface it will appear cooler and redder. Now, metals contribute a lot of electrons to the star. The more metal-rich the star, the more photon-scattering electrons, and the less chance any photon has of having a long run to the surface. Hence metal-rich stars look cooler and redder than metal-poor ones.

    This effect moves a star’s position on the colour-magnitude diagram, and puts it onto a different isochrone than it actually belongs to. So metal-rich stars fool you into thinking they’re older than they are, and metal-poor ones look younger.

    Clusters are terrific because the stars in them were all formed at once, from the same material. They should therefore form a single, well-defined isochrone- and it should be no problem figuring out how the whole thing has been shifted due to metallicity effects. For stars outside a cluster, you’ve got no such guarantees. You’ve got to measure its metallicity before you can know its age.

  37. TMB

    Revy:

    Relativity isn’t important on these scales… galaxies spin at typically ~200 km/s, while the speed of light is ~300,000 km/s… so you’re talking about 0.1% of the speed of light. You don’t get important relativistic effects until you get a lot closer, like at least 10%.

    [TMB]

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