The Milky Way's buried treasures

Contributed by
Aug 3, 2011
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When I was younger, I'd haul my 'scope out to the driveway and peruse the heavens. Some of my favorite targets were open clusters: loose aggregations of stars that had dozens or even hundreds of members. These stars are all in the family, born by the same cloud of gas and still bound together by their own gravity.

A lot of these objects are big and bright, easily visible in binoculars -- those tend to be close to us in space -- and others fainter, harder to spot. Most of those latter ones are just farther away, but some are partially obscured by galactic dust, which robs them of light, like a curtain in a window partially blocking the light form outside.

I used to wonder how many open clusters the Milky Way sported. Thousands, I figured, and I also guessed a lot had been cataloged. But how many were behind such thick curtains of dust, hidden from our view? Some estimates are that there as many as 30,000 such groups. And now we're starting to find them.

[Click to embiggen, or grab the ginormous 2400 x 2100 pixel version.]

That gorgeous shot is from the European Southern Observatory's Visible and Infrared Survey Telescope for Astronomy (VISTA), a 4.1 meter 'scope sitting in northern Chile that sports a 67 megapixel camera. It's optimized to view the skies in infrared light, which can travel through dust clouds, revealing what lies behind. This picture shows 9 of the 96 new clusters detected by a survey that started only last year! And I had to crop it to fit the blog; the hi-res shot shows 30 clusters in total.

That's pretty amazing. Those clusters are lovely, but as I was reading the news I wondered how they could differentiate stars in the foreground and background from true members of the cluster. I had a sneaky suspicion it had to do with star colors, and after reading the journal paper I was right. The process involved looking at the colors of all the stars in the image, and comparing them to the colors of the stars that are well away from the cluster (assuming the latter are stars not in the cluster). This creates a way of identifying non-cluster stars, so what remains must be cluster members. I imagine there are probably a few stars left over whose membership is questionable, but given the clusters have hundreds or thousands of stars, a handful mixed up here or there shouldn't hurt the statistics.

One of the other things that pops out of the analysis is how thick the dust is that's obscuring the clusters. One of them shocked me: it dims the cluster light by 20 magnitudes, which is a factor of 100 million!*

I wonder how bright it would appear if all that dust weren't in the way?

The ages of the clusters can be estimated as well (massive stars tend to be blue, and explode when still young, so older clusters are redder). Some are as young as 5 million years, quite young in astronomical terms, while others are more than 400 million years old -- meaning some of these clusters formed when animals on Earth first ventured out of the oceans and onto land.

What VISTA is revealing is a previously hidden treasure trove of objects, and I imagine astronomers who study such things are having the times of their lives. And with only 2500 or so clusters cataloged -- less than 10% of the estimated total! -- there are quite a few left to find.

ESO/J. Borissova

* Each magnitude step is a factor of 2.512 in brightness -- that means that a 5 mag difference between stars is 2.5125 = 100. So a 20 mag difference is 2.51220 = 100 million.

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