A distant sparkling eruption of diamonds

By Phil Plait | September 27, 2010 9:30 am

Globular clusters have always been one of my favorite astronomical objects. These balls of stars — sometimes hundreds of thousands strong — are easy targets through a small telescope and are fun and beautiful to see.

But when you train a big space telescope on them, well, their beauty is magnified spectacularly:

hst_ngc6934

You really want to click that to get the very beefy 4000 x 4000 pixel (11 Mb) version. It’ll knock your socks off!

This Hubble image shows NGC 6934, an ancient ball of stars located about 50,000 light years away. Globular clusters are made of stars that are bound to each other gravitationally and orbiting the center on a myriad different paths — think of it as a beehive except with a hundred thousand bees each a million kilometers across. There are about 150 of these guys orbiting the Milky Way, each a dozen or so light years across and containing upwards of a million stars. NGC 6934 is pretty typical of its class, but its great distance dims it to near-obscurity. If it were as close as M 13 or Omega Centauri — both roughly half as far as NGC 6934 — it would be heralded as a gem of the night sky.

Globulars are old. We think they form all at once, with all the stars being born at the same time. Massive stars, which are blue, don’t live long before exploding as supernovae (leaving behind black holes or dense neutrons stars), so they’re all long gone in these billions-of-years-old objects. In fact, in many globulars even stars like the Sun are gone, having used up their fuel and faded away. All that’s left are low mass stars, which means all that remains are red stars.

So why are there so many blue stars in this picture? Ah, it’s false color! It was taken through two filters, one in the red, and the other in the infrared. In the picture, the red filter image is colored blue, and the infrared one is colored red. Note that the brightest stars in the picture are red (meaning they’re bright in the infrared); this is because these are red giants, stars that are nearing the ends of their lives. They’ve swollen up and cooled off, glowing brilliantly.

Even though they’re not actually blue, the ones that look blue in the image are most likely the "normal" stars that are left in the cluster, that is, stars still fusing hydrogen into helium like the Sun is, and are not yet red giants.

Globular clusters like NGC 6934 are incredibly important to our understanding of our galaxy. Because all their stars formed at once and all from the same cloud of gas, they’re a laboratory experiment in astronomy! We don’t have to correct for age or composition of stars (or at least not very much) allowing us to examine other characteristics. They’re located all over the sky, so they’re always around for viewing, and many are isolated in space, making them easy to examine. Much of what we’ve learned about how stars age and die was gleaned from globulars like NGC 6934.

Globular clusters tell us secrets of the Universe, and all we have to do is pay attention. And when they’re as stunningly beautiful as this one, that’s really easy to do.


Related posts:

- Vampires and thrillseekers rejuvenate dead stars
- Spitzer bags… Omega Cen
- Glob smacked
- Alien clusters invade our galaxy


CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

Comments (39)

  1. Messier Tidy Upper

    So why are there so many blue stars in this picture? Ah, it’s false color! It was taken through two filters, one in the red, and the other in the infrared.

    So its a case of false colour artificial “diamonds” then? ;-)

    Awesome image nonetheless. :-)

    Oddly enough, just tonight I’ve learnt from reading an old New Scientist magazine (7th November 2009) that Kapteyn’s Star, our Suns 25th nearest neighbour and a red dwarf that boasts the second greatest proper motion in our skies beaten only by Barnard’s Star, is almost certainly an ex-member of the glorious globular and ex-dwarf galaxy Omega Centauri.

  2. MarkW

    Cliché I know, but…

    Oh my God, it’s full of stars!

  3. Messier Tidy Upper

    @ ^ MarkW :

    .. and perhaps an intermediate mass black hole at its core too maybe? ;-)

  4. chris.j.

    if there’s a black hole there, shouldn’t we be able to weigh it? we weighed the milky way’s black hole by tracking star orbits around it, and this image shows that we can resolve pretty much all of the stars in this globular (which shouldn’t have any pesky dust, like our galaxy does). if it’s only 10-15 l.y. in diameter, we should be focusing on the core region with what appear to be at least a few hundred stars within a half l.y. radius. and since we’ve been photographing it for decades, there ought to be some pictures with enough resolution to discern star movements (although, granted, at 50k l.y. away, that’s easier said than done).

  5. Just enjoy the picture Messier, just enjoy the picture.

  6. Demitri Morgan

    I spy galaxies in the background. Dozens of galaxies. Superb image, thanks once again for sharing!

  7. Oli

    @MarkW: He didn’t say “Oh” ;-)

  8. Cowboy Dan

    Sweet picture! That’s quite a beautiful spiral galaxy in the top left corner as well.

  9. Sarah

    Just joined a local ameteur astronomical society recently and was trying my hand at the telescopes. Globular clusters are so fun to catch between in the eye piece: like a little pile of spilled salt. Remembering what I’m really looking at just gives me chills :)

  10. Orlando

    Whenever I hear about globular clusters, I always think of “Nightfall”, by Asimov. Have you ever imagined how that night sky would be??? :D DD

  11. Matthew Ota

    Globular Clusters are popular with amateur astronomers because their appearance to the eye through the eyepiece closely matches what photographic film or CCDs capture.
    It is an awesome thing to see for real. If you do not have a telescope, find an astronomy club or museum that has a star party, and ask them to put their telescope on M13 in Hercules or Omega Centauri for you. You can see them even from the city, with the right equipment.

  12. BILL7718

    If globular clusters are old, what were the when they were young?

  13. timebinder

    It’s like an angel shat on my screen.

  14. Brian

    It’s weird how just swapping out red for blue makes the picture seem so much prettier.

  15. zamia

    Beautiful.

    Are there any observations of stars in globular clusters colliding?

    Are the stars packed denser than in galaxies?

    Are there studies that these clusters have condensed due to evaporation during their lives?

  16. Nigel Depledge
  17. geekGirl

    Sorry if this is a dumb question, but, something I’ve wondered for a while – what’s the difference between a cluster and a galaxy? Is it that all the stars in the cluster are the same age?

  18. Pat

    Well, at least nothing valuable was lost when we emptied the hold.

  19. John

    What is the average distance between stars in that cluster?

    Is there any reason to believe that they are too close together to allow
    life forms similar to us to evolve?

    Interstellar travel would be so much easier if proxima centaur was less than a quarter of a light year away…

  20. Number 6

    #15 (Zamia):

    I was going to ask the same question. We do know that stars there do collide
    to yield “blue stragglers”, but I was going to ask if we know how often they occur,
    and whether or not such a collision had ever been observed.

  21. Tom

    @12 (Bill7718): They were globular clusters when they were young, too. Globular clusters are formed when the galaxy is formed. The environment no longer exists within our galaxy where you could get 10,000+ stars forming all at once. Now, all you get are new open clusters.

  22. J Earley

    Besides the coolness of this primary subject, there are numerous faint galaxies visible in the background of 11MB image. A few of these are visible in the smaller image

  23. Chris A.

    @Phil:
    You state:

    “These balls of stars — sometimes hundreds of thousands strong…”

    Later:

    “…think of (a globular cluster) as a beehive except with a hundred thousand bees…”

    And, just after that:

    “There are about 150 of these (globular clusters) orbiting the Milky Way, each a dozen or so light years across and containing upwards of a million stars.”

    My understanding is that only the exceptionally large ones (e.g. Omega Cen or M22) might contain over 10^6 stars, and that the rest have stars number in the few hundred thousands or less.

  24. Mary

    Okay, if you have not already clicked this image to see the full size–do it. It is spectacular!

  25. Anon

    This was asked here before, but I’ll ask again since it has not been answered. Does anyone know what the average distance in light years is between the stars in the densest part of the cluster?

  26. Michael Swanson

    @ 25. Anon

    “This was asked here before, but I’ll ask again since it has not been answered. Does anyone know what the average distance in light years is between the stars in the densest part of the cluster?”

    http://en.wikipedia.org/wiki/Globular_cluster

    “Globular clusters can contain a high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster.”

    Referenced from here:

    http://www.astro.keele.ac.uk/workx/globulars/globulars.html

  27. Lavocat

    Lucy in the sky … with diamonds.

    Just … wow.

  28. Oli

    @10. Orlando: Probably full of deadly radiation.

  29. JB of Brisbane

    Why do I keep hearing the theme music from the closing credits of “Buck Rogers In The 25th Century” when I look at this?

  30. Spaceman Spiff

    “Blue” stars do inhabit old globular clusters, depending on how you define “blue”. These are the so-called “blue stragglers” and stars occupying the horizontal branch (core helium burning phase) — especially if their heavy element abundances are very low. Both of these types of stars often appear “blue-ish” in an image using typical optical B and V filters.

  31. Gary Ansorge

    “There are about 150 of these guys orbiting the Milky Way, each a dozen or so light years across and containing upwards of a million stars.”

    So, maybe the premise for Firefly wasn’t as far off as I surmised.

    Having access to several thousand solar systems within a few light years would allow a LOT of SciFi story possibilities. No FTL required, but the times required to get from one system to another, while long from a human perspective, would be short enough to allow stellar societies to remain in communication yet evolve in different ways.

    Gee, now I wish WE’D been part of such a cluster,,,except for the pesky problem of so many close systems initiating extinction events.

    It just takes one big blue star going super nova to mess it up for any ecosystem in the cluster.

    I guess being all alone in the local galactic area is our big advantage.

    Gary 7

  32. Number 6

    It just takes one big blue star going super nova to mess it up for any ecosystem in the cluster.

    With all old stars, there aren’t any big blue stars being created…

    What does get created are ‘blue stragglers’, which are believed to be the result of collision between smaller stars. The results of such collisions could be anything from supernova equivalent yielding a neutron star to relatively mild events. Any kind of direct collision would probably yield a shock front moving through the star and probably a huge kaboom. I don’t know if one of these has ever been observed.

  33. Jeff

    “Even though they’re not actually blue, the ones that look blue in the image are most likely the “normal” stars that are left in the cluster, that is, stars still fusing hydrogen into helium like the Sun is, and are not yet red giants”

    It is likely they are main sequence “red dwarfs” on the lower right of HR diagram, and is why they are still “alive”.

    Yes, it is fascinating how uniform they are and speaks to their birth from the same cloud at the same time in history, but it is interesting to work out the mechanism how those early nebula could collapse into this many uniform stars, is this unprecedented? I can’t think of any other situation where so many stars were born together like this. Even the “beehive” star cluster is “open” and doesn’t have so many.

  34. Gary Ansorge

    33. Number 6

    “With all old stars, there aren’t any big blue stars being created…”

    They’re all old stars NOW but there must have been a few big ones when the cluster was younger(say, 3 billion years ago), which could have put a big crimp in budding eco-systems.

    I rather like being far from the madding crowd. It’s so nice and quiet in our current neighborhood, no super novas or gamma ray bursters to mess with us.

    Gary 7

  35. Awful lot of obvious linear structures there; more than I would expect by chance appearance. Is there a possible mechanism by which stars might be arrayed into linearity?

  36. Decurion505

    Yowza! As a strictly amateur stargazer, I am always amazed with the beauty and majesty of sights like this one.

  37. Chris

    If somebody asked me what were the ten most amazing experiences of my life, one of them would have to be seeing a globular cluster through a decent telescope. Pictures don’t do them justice.

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