It's full of stars!

By Phil Plait | October 25, 2010 11:00 am

The good folks at the Space Telescope Science Institute European Space Agency just released this gorgeous Hubble picture of the globular cluster NGC 1806:

hst_ngc1806

Wow! I actually cropped it a bit and shrank it to get it to fit correctly on the blog, so click it to see it in all its 3741 x 2303 pixel glory.

Globular clusters are spherical collections of hundreds of thousands and even sometimes millions of stars, held together by their mutual gravity. The stars orbit every which-way, and I like to think of them as stellar beehives. The clusters as a whole orbit galaxies on long paths that sometimes take them well away from their parent galaxy, so we see them scattered across the sky.

NGC 1806 is actually part of another galaxy: the Large Magellanic Cloud (or LMC to those in the know), an irregular smear of a billion or so stars that orbits the Milky Way itself as a satellite galaxy. Given that this means the globular cluster is something like 170,000 light years away — 1.7 quintillion km, or a quintillion miles — it’s a pretty clear picture!


I was curious as to why the scientists took images of this cluster, so I went to the Hubble archive to look it up. The results were interesting to me: for these observations, NGC 1806 was targeted to get data on the stellar populations in globular clusters in the LMC. That is, scientists wanted to see what kinds of stars were in the cluster, to better understand how these clusters form and how they age. Globulars in the LMC are probably different than those that are part of the Milky Way, and understanding those differences in the clusters helps us understand the differences in the galaxies themselves.

But the search I did turned up other observations too: NGC 1806 was observed at a different time, by different astronomers to look for dark matter! This is the invisible stuff that we know is out there because it affects galaxies through its gravity. The problem is, it’s dark, so we don’t know what it is. One possible candidate was thought to be planets or dark stars (black holes, very old dead white dwarfs, and so on) that might orbit galaxies out in the halo. These objects were termed Massive Compact Halo Objects, or MACHOs. And while they give off almost no light and are completely invisible at their great distance, there’s a way they can be detected: if they happen to pass directly between us and a background star — say, a star in another nearby galaxy — their gravity will distort the light from the background star, making it get brighter (this effect is called a gravitational lens). If you observe enough stars, you might be able to see this change in brightness and thereby spot dark matter!

That’s why NGC 1806 was observed by this second team: they were looking for MACHOs. The more stars in an image, the more likely a MACHO might pass in front of one, so NGC 1806 is a good hunting ground. The astronomers can take short exposures spread out over time, observing the cluster over and again to look for any brightness changes in stars.

To date, no MACHOs have been found, and the more we look for them and don’t find them, the less likely it is they exist in great numbers. If they were common, we’d have spotted them by now. That’s why astronomers are looking to more and more exotic candidates for dark matter. We’ve eliminated all the most obvious stuff.

This made me smile. Sure, NGC 1806 is a stunning, sparkling beehive of stars (and my new computer wallpaper). But it’s also a target for science, and actually a target for multiple threads in science. We want to learn about globular clusters themselves, but they also tell us something about the Universe at large as well. Thrifty folks, those astronomers.

Image credit: ESA/Hubble and NASA


Related posts:

A distant sparkling eruption of diamonds
Omega Cen’s millions of stars
Spitzer bags… Omega Cen
Dating a globular may give you a case of the X-ray binaries


CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (28)

  1. Miles

    Great picture and explanation. Just curious, what are the green dots scattered throughout the high res image?

  2. Astronomers have such interesting names for things. MACHOs, WIMPs, etc… Not only are astronomers thrifty, but they are funny!

  3. Levi in NY

    Miss Globular Cluster, the doctor has some news for you. You’re having jillion-tuplets!

  4. Egad

    Maybe it’s just pareidolia on my part , but there appear to be curving strings of stars here and there in the cluster.

  5. Jack Mitcham

    Phil:

    I thought a small number of MACHOs had been detected, but I’m having trouble finding papers that confirm it. It appears that several microlensing events have been detected in the LMC, but there is some debate as to whether the cause was a halo object, an object in the galactic disk, an object in the LMC, or some other phenomenon.

    Still, I think saying “no MACHOs have been found” is a little strong, I might have said “no MACHOs have been confirmed.”

  6. Oli

    Does this cluster have a central black hole?

  7. Kaptain K

    I, too, like globular clusters. my wallpaper is the big daddy of ‘em all – omega Centauri!

  8. Donnie B.

    Globular clusters like this one had an important role in the history of astronomy, and of our understanding of the true scale of the universe. Long before Edwin Hubble and the recognition that many nebulae are other, very distant galaxies, Harlow Shapley used his measurements of the distances to globular clusters to conclude that the Milky Way was about ten times larger than previously thought. He was off by a bit, it’s not quite as big as he thought — but he was right that the universe was considerably larger than we’d imagined.

  9. love the term, “stellar beehives” & thanks for the suggestion to use the image as wallpaper!

  10. Great post, Phil. Even at 170,000 light years, in as dense a globular cluster as this, a proliferation of black holes should be evident since the stars are so close together and much more susceptible to the black hole’s gravity. Same with eclipsing MACHOs. Here’s a question, why don’t we see active, star-eating black holes in globular clusters? It would seem a perfect observation place for them. Why are globular clusters, despite their density, so quiescent?

    Here’s a question you smart folks can answer. We know that dark matter has been presumed, and nay, proven, to exist in and around galaxies because they rotate far too fast for their visible mass, and the stars on the outer visible edges rotate far too quickly if all that was on the outer edges was just the visible stuff. Some guy named Phil Plait mentioned this in a book. Do globular clusters also exhibit the same “too fast” rotation rates from which we infer dark matter from galaxies; and the same disconnect between rotation rates of stars near the center of the cluster and stars on the outskirts that we have seen in galaxies?

  11. So the question is if dark matter pervades the universe, shouldn’t it also affect the apparent rate at which stars rotate around the gravitational center of a globular cluster? And if not, why are globular clusters somehow devoid of dark matter? Or is there just not ‘enough’ of it in globular clusters for it to exert a measurable effect?

  12. andy

    Talking of the LMC, what’s the current view on whether it is actually a satellite galaxy? A while back there was some news that measurements had indicated that the Magellanic Clouds were in fact on their first approach to the Milky Way.

  13. Michael Swanson

    ” I actually cropped it a bit and shrank it to get it to fit correctly on the blog, so click it to see it in all its 3741 x 2303 pixel glory.”

    “Click it”? That’s it? Just “click it”? Not “englobulate” or “hyperinstellarize” or…I dunno…something!

    You’ve let me down, Phil. [<– insert disapproving and dour expression here]

  14. ErnestPayne

    Carl Sagan would approve.

  15. Messier Tidy Upper

    @ ^ ErnestPayne : Yes, he sure would. :-)

    Great image. Fascinating object. Last time I was up at Stockport Observatory with the local astronomical society I viewed Omega Centauri, 47 Tucane, The Great Hercules Globular (M13) and M15 near the nose of Pegasus among many others.

    @6. Jack Mitcham Says:

    … I thought a small number of MACHOs had been detected, but I’m having trouble finding papers that confirm it. It appears that several microlensing events have been detected in the LMC, but there is some debate as to whether the cause was a halo object, an object in the galactic disk, an object in the LMC, or some other phenomenon. Still, I think saying “no MACHOs have been found” is a little strong, I might have said “no MACHOs have been confirmed.”

    I’ll second that. It’s what I thought too.

    @13. andy Says:

    Talking of the LMC, what’s the current view on whether it is actually a satellite galaxy? A while back there was some news that measurements had indicated that the Magellanic Clouds were in fact on their first approach to the Milky Way.

    I also remember reading about that. If I recall correctly – a later study showed that both the Magellanic Clouds are actually gravitationally bound to the Milky Way after all. But I might be mistaken here &, alas, I can’t recall a source to cite for you on this. Sorry, I guess that doesn’t help all that much does it? :-(

    @11. Doug Watts :

    if dark matter pervades the universe, shouldn’t it also affect the apparent rate at which stars rotate around the gravitational center of a globular cluster? And if not, why are globular clusters somehow devoid of dark matter? Or is there just not ‘enough’ of it in globular clusters for it to exert a measurable effect?

    I’m not sure of that either, sorry. :-(

    I do know however that some globulars have been severly disrupted by passing through the plane of the Milky Way and had stars torn off them. For instance, Kapteyn’s Star a nearby red dwarf with the second highest proper motion (apparent speed across the sky) is thought to have been part of Omega Centauri. Kapteyn’s Star is now 12 ly away and Omega Cen is 15,800 so its travelled a *very* long way from home! ;-)

    Omega Cen is actually believed to be a old dwarf galaxy since incorporated nto our own Milky Way so its not typical but other globulars it seems have also lost stars and been tidally disrupted. Other globular clusters have undergone core collapse where the stars have collapsed in towards the centre.

    Not sure if these observations mean that the globulars don’t have *any* dark matter in them but it could possibly indicate that – perhaps?

  16. @#1 Miles: Most likely just CCD noise or hot/dead pixels. Or possibly artifacts produced during post-processing.

  17. TMB

    Re: dark matter in GCs

    Stars in globular clusters don’t show any evidence of a mass discrepancy, i.e. no dark matter associated with the GCs. That’s the basic difference between the most massive GCs and the smallest dwarf galaxies – they actually both have similar numbers of stars, but the galaxies have dark matter too.

    But it’s not that there’s dark matter everywhere and somehow a big hole in the dark matter where the GC is! There’s no dark matter *associated* with the cluster, i.e. it concentrated where the stars are concentrated. There’s still a smooth background distribution of dark matter associated with the galaxy in which the cluster resides.

    The key is that because there’s no concentration of dark matter in the cluster, the density inside and outside is the same, so there’s no net inward gravitational force on the stars due to the dark matter. It’s that net force that would make them move faster.

    [TMB]

  18. andy

    TMB: any idea on whether there are significant amounts of dark matter in Omega Centauri, would be interesting to know given that there are indications that it is a former dwarf galaxy? (Terzan 5 as well, but I would guess that it is too hard to observe for that kind of measurement?)

  19. Arundo Donax

    That is one stupendous image! Particularly because it isn’t even in our own galaxy!

    And Phil: nice Arthur C. Clarke reference in the title. ;)

  20. jick

    So the scientists are turning their back on MACHOs and instead rely on WIMPs to explain their pretty “theory.”

    No wonder hard-working Americans turn their back on the “theory” of global warming.

    :P

  21. Messier Tidy Upper

    BTW. For more on MACHO’s & their detection see :

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

    Which notes that :

    Several groups have searched for MACHOs by searching for the microlensing amplification of light. These groups have ruled out dark matter being explained by MACHOs with mass in the range 0.00000001 solar masses to 100 solar masses. One group, the MACHO collaboration, claims to have found enough microlensing to predict the existence of many MACHOs with mass of about 0.5 solar masses, enough to make up perhaps 20% of the dark matter in the galaxy. This suggests that MACHOs could be white dwarfs or red dwarfs which have similar masses. However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Telescope and with proper motion surveys. These searches have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy.

    It also looks from there as though cosmologists now don’t think that enough Massive Compact Halo Objects would’ve been created in the first place to explain more than a fraction of Dark Matter.

  22. Messier Tidy Upper

    @ 18. TMB Says:

    Re: dark matter in GCs : Stars in globular clusters don’t show any evidence of a mass discrepancy, i.e. no dark matter associated with the GCs. That’s the basic difference between the most massive GCs and the smallest dwarf galaxies – they actually both have similar numbers of stars, but the galaxies have dark matter too.

    Thanks for that answer, much appreciated. :-)

    PS. I also second (#19.) andy’s question on the level of dark matter associated with Omega Centauri.

  23. TMB

    The case of omega Cen is ambiguous right now… there are claims both that it does and and that it doesn’t require dark matter, often using the same data but different analyses.

    It’s also not clear from a theoretical perspective whether you’d expect there to be much dark matter left in the nucleus of a dwarf galaxy that has been so stripped. The dark matter distributions in galaxies are much more extended than the stars, so if you strip off 90% of the stars (which you’d need to have done to be left with just omega Cen), you have to strip off 99% of the dark matter. Whether that remaining 1% would be enough to cause a measurable difference to the orbits of the stars depends on the details of how the dark matter is distributed, which is still a matter of debate.

    [TMB]

  24. Messier Tidy Upper

    @ ^ TMB : Thanks for that. :-)

  25. MACHO confirmation

    As an earlier post states that “no MACHO confirmations” is a little strong, see the following article by Bennett et al.

    The Astrophysical Journal, 631:301–311, 2005 September 20

    PHOTOMETRIC CONFIRMATION OF MACHO LARGE MAGELLANIC CLOUD MICROLENSING EVENTS

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