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