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SYFY WIRE Bad Astronomy

A Heavy Metal Cosmic Whodunit

By Phil Plait

Sometimes you just need a pretty picture of a star field, right?

But sometimes it’s fun to find out a little bit more, too, and I’m happy to help!

That image above is NGC 6496, a globular cluster as seen by the Hubble Space Telescope. Globulars are roughly spherical collections of hundreds of thousands of stars, bound together by their mutual gravity. I did an episode of Crash Course Astronomy about them if you want more (and you do, because they’re wonderful):

Our Milky Way galaxy has at least 150 such clusters orbiting it, and in general they can be divided into two groups: halo clusters and disk clusters. As you might expect from the names, halo globulars tend to have orbits that are way far removed from the galaxy itself, staying out in its halo. Disk clusters stick closer in to the flat plane of the galaxy.

Compared with the Sun, both kinds of globulars have stars that are deficient in heavy elements (for astronomers, that means any element heavier than hydrogen and helium, which, for historical and mainly traditional reasons nowadays, astronomers confusingly call “metals”). For example, the amount of iron in globular cluster stars never gets much more than about 1/3 as much as you see in the Sun, and can be as little as a fraction of a percent. But they are different: Halo cluster stars tend to have lower abundances of metals in them compared with disk clusters.

NGC 6496, as it happens, is seen very near the plane of our galaxy, close enough that we have to look through stars and gas and dust in our galaxy to see it. That makes it a bit harder to observe and measure its chemistry. Astronomers have argued whether it’s a halo cluster or a disk cluster for some time; even though it’s in the galactic plane it could be a halo cluster on a highly tilted orbit that we happen to see when it’s plunging through the galactic equator.

The Hubble observations here indicate the stars have more heavy elements in them than you’d expect for a halo cluster, but about the right amount for a disk cluster. While it may not be conclusive, that’s pretty good evidence for it being a disk system globular.

Why does it matter? Globulars are very old, 10 billion to 12 billion years old, about the same age as the galaxy itself, so they probably formed along with it. Being able to categorize them, assess their similarities and differences, tells us more about how the galaxy formed, and how things have changed in the ensuing eons.

It’s a detective story. The more clues you have, the more you understand the situation. The Universe is pretty good at providing us with clues, and we just have to be clever enough to put them together.

P.S. This image is made up of public Hubble observations, processed expertly and artistically by Judy Schmidt, who’s made a name for herself doing just this. Follow her on Twitter and on Flickr for more of her imagery.

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