Lonely sentinel of the galaxy

By Phil Plait | September 14, 2011 10:00 am

I’ve posted lots of pictures of globular clusters in the past, but this new one is something special. And not just because it’s stunningly beautiful… which it is:

[Click to spheroidenate, or grab the massively embiggened 3850×3850 pixel version.]

This is Hubble’s view of NGC 7006, a relatively faint cluster of a hundred or so thousand stars located in the constellation of Delphinus, the dolphin. It appears relatively faint and small as globulars go, but as it turns out it’s a very, very interesting object indeed.

Why? Distance.

NGC 7006 is one of the most remote globular clusters orbiting the Milky Way; it’s currently 135,000 light years away – 1.35 quintillion (1,350,000,000,000,000,000) kilometers distant. For comparison, the Milky Way itself is about 100,000 light years across. Not only that, but NGC 7006 is still heading out, increasing that distance, and it’s not exactly taking its time: it’s moving away at an astonishing 380 km/sec (250 miles/sec)! Measurements of its position taken over a 40 year time span also give an indication of the shape of its orbit. The most far-flung part of its path — called apogalacticon, a word you gotta love — is a stunning 330,000 light years distant!

That is a long, long way off. As far as I can tell, that will make it one day the most distant globular cluster orbiting the Milky Way. The only things farther out are dwarf satellite galaxies slowly revolving around us. NGC 7006’s orbit is so huge that it’s hard to see how it could’ve gotten there if it formed along with our own galaxy… which may be an indication it didn’t. A good guess is that it was part of some other galaxy that either still orbits us, or was torn from its home when the Milky Way collided with and ate its parent galaxy. We know that happens pretty commonly in the Universe, so it’s a fair bet.

Either way, this cluster is incredibly far off, and increasing that distance every second of every day as it heads out into space. Eventually it will reach the apex of its orbit, turn around, and head back in, like some vast comet. At that apex, once per orbit, it just skims into the region of empty intergalactic space which stretches for millions of light years, deep into the Local Group of galaxies which includes Andromeda, Triangulum, and a handful of others.

Past that? True space, deep space, lightly sprinkled with galaxies almost as an afterthought. You can even see some of these island universes in the photo above: dimmed and dwarfed by their mind-crushing distances of hundreds of millions or even billions of light years.

It’s hard to grasp how terribly deep space is, but then something like this comes along and forces us to look into the abyss.

But you know what? I don’t mind. It’s pretty, and it’s interesting, and it’s real, and it’s where we live. Learning more about it seems the least we can do.

Image credit: ESA/Hubble & NASA


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CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Hubble, NGC 7006

Comments (32)

Links to this Post

  1. New image of a distant globular cluster « Science Notes | September 14, 2011
  1. John O'Meara

    I’m very surprised that a globular could retain its shape after a major merger.

  2. uncleMonty

    Is there any reason it would be harder for life to take hold in a cluster like this that periodically heads out into deep space far from the galactic center? In other words do the cozy confines of the Milky Way protect us from anything? And how long would the orbit take, around the Milky Way – presumably for any civilisations in the cluster the changes in neighbourhood would take place on evolutionary or even geological time scales?

  3. Jim Howard

    If this thing is moving that fast now what kind of speed can it attain when is on its way back?

  4. oldebabe

    Lovely, yes; but why does it maintain its shape at all?

  5. Ben H.

    Apogalacticon is my new favorite word. Thanks, Phil!

    – Ben H.

  6. OtherRob

    How long does it take to orbit the Milky Way?

    And if I ever find myself staring a sci-fi con, I’m going to name it ApoGalactiCon. :)

  7. @OtherRob: I was wondering that myself. In the paper cited above, it says it’s 2.1 billion years, +/- 0.6 billion years. So if you’re going to look at it, better go soon. :-)

  8. pumpkinpie

    Phil,
    Like many who post and read here, I feel I am more knowledgable of astronomy than the average person. (Heck, I gotta be, I teach it!) But I learn so much from you every day! Thanks for the great posts!

  9. M Tadano

    @pumpkinpie: Well… I’m just a dumb computer programmer who just happens to have an interest in cosmology. Not an expert by ANY stretch of the imagination. But I come here every day and learn so much, see cool pictures, and really like this guy’s enthusiasm. Phil has a way to make this so interesting to you folks of greater knowledge of this stuff without going too far over the heads of the rest of us.

  10. OtherRob

    @SaintAardvark: Thanks. I wonder if it’s even made one orbit yet.

  11. Jess Tauber

    These are just the stars that weren’t dragged off during the merger, and the spherical shape is a dynamic construction- these stars are constantly interacting in complex orbital paths. After the merger there was likely a big bedraggled mess (as one sees after galaxies collide) but there has been sufficient time to exchange energy and bring about the current equilibrated configuration. It would be very interesting if we could image pre-globulars during their formational crises. Would we even recognize them?

  12. I love all the red background galaxies.

  13. “Perigalacticon”, the point in its orbit at which it’s closest to the center of the Milky Way, is also a pretty neat word.

  14. chris j.

    It’s hard to grasp how terribly deep space is, but then something like this comes along and forces us to look into the abyss.

    tell me about it! i mean, you may think it’s a long walk down the road to the chemist, but that’s just peanuts to space.

  15. Christopher Jablonski

    This is the part that makes me convexed and queasy. So it’s 135,000 light years away and orbiting our galaxy. Are we being affected by the gravity of where it appears to be, or on where it actually is? I’m pretty sure gravity waves are propagated at the speed of light, but then isn’t the system orbiting around the point where we were and not where we are? Why do I find this so hard to grasp?

    And is the Milky Way affected by its tides? How can the forces be acting on opposite sides of the galaxy equally when there’s 100,000 light years separating them?

  16. t-storm

    maybe they think we orbit them.

  17. Grimbold

    @3- “If this thing is moving that fast now what kind of speed can it attain when is on its way back?”

    The same speed as going out. It’s one of the fundamental properties of elliptical orbits; points on the ellipse with the same distance have the same speed associated with them. It doesn’t matter if the object is heading outwards or inwards. So by the time the globular cluster is 135,000 light years away again, heading inwards, the speed will again be 380km/s. The fastest speed will be when the globular is nearest the Galaxy.

  18. Jon Hanford

    “NGC 7006′s orbit is so huge that it’s hard to see how it could’ve gotten there if it formed along with our own galaxy… which may be an indication it didn’t. A good guess is that it was part of some other galaxy that either still orbits us, or was torn from its home when the Milky Way collided with and ate its parent galaxy. We know that happens pretty commonly in the Universe, so it’s a fair bet.”

    Indeed, the globular cluster M 54 along with several other clusters are thought to be torn from the Sagittarius Dwarf Elliptical Galaxy, currently being shredded by our Milky Way: http://en.wikipedia.org/wiki/Sagittarius_Dwarf_Elliptical_Galaxy

    A news story out today suggests interactions with this dwarf galaxy in the past may be responsible of some of the spiral arms in our Milky Way Galaxy: http://www.newscientist.com/article/dn20910-multiple-dwarf-strikes-gave-milky-way-its-spirals.html

    Other studies suggest extragalactic origins for several other globular clusters (notably Omega Centauri and 47 Tucanae).

  19. Pete Jackson

    Despite its remoteness, NGC7006 is not particularly metal-poor as globular clusters go (although it only has about 1/30 as much by proportion of elements heavier than helium as the Sun does). It presumably belonged at one time to an evolved dwarf galaxy that merged with our Milky Way galaxy and yes, John (No.1), the globulars resist tidal disruption quite well which is why so many still orbit around our galaxy.

  20. OnlySlightlySane

    @uncleMonty: I don’t know that there’s any particular reason why existing life would benefit or not from living within a globular cluster, but globular clusters are not conducive to the creation of life. The reason being they have an incredibly low metallicity (elements heavier than helium). With such a low metallicity, the chance of elements like carbon, oxygen, or iron which are critical to life being available in suitable amounts are almost nil.

  21. Ganzy

    As much as I love Hubble’s images of colourful Nebulae and expansive galactic vistas, I am especially drawn to the simple beauty of globular clusters. N9C7006 being an excellent example. Soaking up the Starscape in full screen mode invokes within me the same feelings of timeless serenity as gazing at a collection of Silently burning candles in a darkened room. Helped along with a nice glass of wine of course…

    The peppering of background galaxies are the icing on the cake too. This Image has been my iphone wallpaper for the last week, it makes a lovely backdrop for all my astronomy app icons:)

    Man..there’s nothing like 380km a second to put you in your place. It would be Sweet if there was a flythrough animation of this cluster to watch, other than the one in my imagination ;)

    Excellent post Phil, cheers.

  22. Chris A.

    Globulars retain their shape simply because they are small compared to the size of their orbits. Put another way, the ratio of their gravitational binding energy to the energy of their orbits is high.

    Still, they are affected over time. The gravity of their parent galaxy tidally distorts them slightly, and the energy to do this comes from their orbits. Thus, over time, they slowly spiral into their host galaxy, a process which accelerates as the tidal distortions increase (rapidly!) with decreasing distance. An example is Palomar 5, which appears to be in the process of being eaten by the Milky Way.

  23. Adam

    How would the sky look if we were in a middle of a globular cluster? Would we see the other stars as dots or a more sizeable sources of light?

  24. Andrew

    The night sky for a planet inside a globular cluster? My god- it’s full of stars! http://www.iac.es/gabinete/iacnoticias/winter98/xplaneta.htm.

    Hundreds or thousands of stars within a parsec, mostly old and red, orange or yellow, but still points (even a tenth of a parsec is still a long way away).

  25. Messier Tidy Upper

    “Few men realise the immensity of the vacancy in which the dust of the material universe swims.”
    – Page 7, ‘The War of the Worlds’, H.G. Wells, first published 1898, this edition : Aerie books, 1987.

    This stunning globular swims in an even immenser gulf than any we can fathom here.

    Superbly splendid magnificent image – and near intergalactic wandering heavenly object. :-)

  26. Dragonchild

    @ 2. uncleMonty
    “Is there any reason it would be harder for life to take hold in a cluster like this that periodically heads out into deep space far from the galactic center? In other words do the cozy confines of the Milky Way protect us from anything?”

    Cozy isn’t necessarily a good thing in this case. In addition to having relatively low metal content, the problem with globular clusters is that they’re like moving stellar mosh pits. Actual collisions are rare, but there’s conjecture that the stars can wreak havoc on each other’s planets every few hundred million years. We needed 2-3 billion just to get started. The Milky Way is more like a freeway with light traffic; the stars are farther apart and mostly moving in the same direction. Here, traffic doesn’t get logjammed until you approach the center.

    @ 4. oldebabe
    “Lovely, yes; but why does it maintain its shape at all?”
    Gravity interaction, but there isn’t much of a shape to speak of. Your brain sees the “shape” of a diffuse sphere, but if this image was more like a movie sped up to the point where you could witness several billion years in a few seconds, it’d look more like a cloud of fireflies — always shifting, always moving, but never dissipating.

    That said, if its orbit takes it through the galactic plane, it’s possible that it could get some stars stripped off every go-round. This would be especially true if its orbit took it near the galaxy’s core. These stars could even be ejected from the galaxy itself, billions of years later becoming truly lonely, homeless rogue stars in the intergalactic void.

  27. Messier Tidy Upper

    Um, BA, I think you’ve got a typo in the tags :

    Tags : globular clsuter, Hubble, NGC 7006
    by Phil Plait in Astronomy, Pretty pictures |

    Which you may want to fix.

    @10. OtherRob : “I wonder if it’s even made one orbit yet.”

    I’m sure someone who is better at maths than I am can probably calculate it. It’s moving (visibly remarkably enough!) at 380 km/sec (250 miles/sec) out to 330,000 light years distant and back.

    It’s ancient so if its been orbiting the Milky Way since our Galaxy formed it’s had 12 billion years or so to complete a few orbits – but then it may have been captured – and, if so, when was that? So maybe not.

    For reference point / comparison :

    Since our Sun was formed more than 4 billion years ago, it has travelled around the Galaxy 16 times.
    “Two of the Milky Way’s Spiral Arms Go Missing.” NASA e-newsletter news release 2008-June-4th.

    This globular won’t have done that many I’m sure! ;-)

  28. Brad

    I am stunned by the quality of this photo. How in the world is Hubble able to clearly see the individual stars of this small cluster that is further away than the size of the whole galaxy?!!?!

  29. Ronn! Blankenship

    What? No one who has commented on the possibility of life on or the appearance of the sky from a planet in the heart of a globular cluster has yet mentioned Asimov’s “Nightfall”?

  30. Patrik

    @Brad (29)

    It’s actually possible to see it with larger amateur telescoper. We spotted it but could not quite resolve individual stars using the 45 cm Newton telescope our astronomy club owns.

  31. Matt B.

    I would expect that this cluster simply got ejected by a gravity assist, same as Kuiper Belt objects from a planetary system. A galactic merger isn’t necessary, though still likely, as a contributing cause.

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