Spitzer bags… Omega Cen

By Phil Plait | April 10, 2008 1:41 pm

Yeah, not what you thought, eh? But still… pretty.

Spitzer Space Telescope caught this image of the cluster Omega Centauri, which I recently wrote about.

Whether Omega Cen is a galaxy or a globular cluster doesn’t change the fact that this is a striking image. It’s actually a combo of Spitzer and ground-based images taken with a 4-meter telescope in Chile. Stars shown here as blue (more on that in a sec) are low-mass stars seen in both the 4-meter ‘scope and Spitzer. Yellow and red stars were seen only by Spitzer. Since it looks in the infrared, that means these stars are probably more evolved.


Clusters (or galaxies) like Omega Cen form pretty much all at once, using up all their gas to make stars. Since high-mass stars age more rapidly, and clusters like this are old, all the more massive stars have already either blown up (if they were massive enough) leaving behind black holes or neutron stars, or have long since turned into red giants and blown off their outer layers. That leaves behind a very faint white dwarf.

Omega Cen is very old, so only lower mass stars are still around, but even they have evolved into red giants. Those stars tend to emit more light in the infrared, both due to their low temperature as well as blowing off lots of dust, which enshrouds them and reddens their light. In fact, these images were taken to investigate how much dust these stars are generating. Fewer dusty stars were found than expected, which is interesting. Clusters like this tend to have lower metals (the term astronomers use for anything on the periodic table heavier than helium) because they formed early in the Universe, before massive stars could make those heavy elements and seed them into space. Could this lack of metals affect the dust formation? Images like this one can lead us to those answers.

By the way, in Omega Cen the lowest mass stars are still going strong. Ironically, in this false-color image, those stars are colored blue (meaning visible light), but if you looked at them with your eye they’d look orange or red. False color images can be tricky, and in this case things are really topsy-turvy.

But it sure is a pretty picture.

CATEGORIZED UNDER: Astronomy, NASA, Pretty pictures, Science

Comments (8)

  1. Crux Australis

    Oooh…first post again. That’s the extent of my input. Apart from, oooooooh, aaaaaaah.

  2. The dividing line between big globular clusters and dwarf elliptical galaxies does seem to be arbitrary. They seem to form one continuous “spectrum” of things.

    They also make me a sad panda.


    Because in both cases, all star formation happened in one big burst when the cluster/elliptical first formed. No stars in them will have the heavy elements necessary for the formation of rocky planets, or organic life. And no possibility of life-as-we-know-it means no space aliens for us to talk to.

  3. Kaptain K

    Come on BA! Use the “duck test”. Ya know…if it looks like a duck, etc. Omega Cen may have started “life” as a dwarf elliptical or even the core of a dwarf spiral, but a trip (or two) through the Milky Way mixmaster, and its a globular cluster now!

    Almost forgot! Ooooooooooh…Aaaaaaaaaah!

  4. Zarquon

    I spent a cold night in 1986 mistaking Omega Cen for Halley’s Comet.

  5. George Greene

    Nice picture. A question: why do the visible stars seem to be all in the middle of the cluster? By the way, the NASA web page says they are rendered in green. That’s not how I see it; but the IR/visible ones are definitely yellow, and that’s supposed to be the combination of the red (for IR) and whatever for visibile. Green would then seem to be correct.

    Two observations. First, it must be the nerd in me, but when I see “Spitzer” I think first of the space telescope. But then I don’t live in NY. Until I read your opening comment, I hadn’t made that connection. Then there was the context.

    Second, I wouldn’t describe the coloring as topsy-turvy. To me that word means upside down or reversed. Blue-shifted would work for me. Visible (probably red-orange as you say) becomes blue/green, IR/visible becomes yellow, and IR only red.

  6. StevoR

    Hmm … Do [B] Crux Australis [/B] & [B] Kaptain K [/B] have bad stomach aches or soemthing? 😉

    And Tracer is really a panda – an intelligent (Giant or Red variety?) panda with acccess to the internet and typing abilities! How remarkable! Hold the press! 😉

    Silliness aside, awesome image & good write up.

    Incidentally planets may be possible … the second (& only other?) known pulsar planetary system PSR B 1620-something ?? aka the Genesis or Methuselah planet is located in the globular cluster Messier 4 (M4) next to Antares in our (apparent) skies.

    Plus Omega Centauri apparently has got several generations of stars that being one of the clues that make some astronomers consider it more of a dwarf elliptical galaxy than your usual globular …

    … So all may not be lost exoplanets & potential life~wise.

    Then again, I vaguely recall they searched Omega Centauri for exoplanets & found none … or was that 47 Tucanae?

    I think the best thing to conclude is that at present given what we know its unlikely they’ll be habitable planets and life in any globular cluster – but we don’t know enough to rule it out for sure .. The chances against it may be 80-95 % but not 100%!

  7. StevoR

    D’oh! How do you create Bold and underline & stuff like that here …? :-(

    I agree with George Greene when it comes to Spitzer – the ‘scope is the first thing Ithink of too. Outsidethe USA & proably even o/s his immediate state and more so as the news fades away, the telescope is / will be better known and the first association folks make.

    I hope they don’t rename it or anything. :-(

    OTOH I disagree with George Greene when it comes to the BA’s use of topsy turvey : a red star looking blue does seem the exact opposite star types ~wise to me! 😉

  8. StevoR

    I posted this before – on the “Black holes & forming planets” thread linked to here as well – Sorry if this breeches netiquette, but if anyone has any answers on the questiosn here I’d love to hera ’em! :

    # Astrostevoon 05 Apr 2008 at 10:50 pm

    Fascinating news about Omega Centauri – some implications and questions I’ve got are :

    1) If Omega Centauri is reclassified as a dwarf galaxy does that make it the nearest dwarf elliptical or is the Saggitarius dwarf or another of those captured shredded dwarfs actually closer?

    2) If it is a galaxy won’t it join the list of just a handful of galaxies visible to the unaided eye – the two Magellanic Clouds, Andromeda, Triangulum – M33, & now the Omega Centauri dwarf galaxy? Has the Local Group of galaxies gained a new member and, if so, where does Omega Cen sit in the heierachy – would it be the smallest dwarf or is it larger than some of the other very faint dwarfs?

    3) Can Omega be considered as being *both* a galaxy and a globular cluster (like eg. Ceres = largest asteroid & dwarf planet) or would we it be better to say instead that it used to be an independent galaxy but has since been transformed via capture (?) into a globular cluster? Can a galaxy be part of another galaxy and how long should it be before a smaller galaxy is considered fusedinto or a component partof a larger galaxy?

    4) To be a galaxy does Omega need to boast globular clusters of its own? Does it have – or are there any signs it might have had – its own orbiting globulars or extended stellar halo or even dark matter halo?

    Which leads on to the question of : Should the IAU be trusted to define what a galaxy is after their botch-up with Pluto! 😉

    5) Is it the only object like this or could there be more – both around our galaxy and inside others? What are the implications for other globulars and their evolution? Could there be two separate populations or types of globulars – those that used to be separate galaxies and those that were always part of their primary galaxy?


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