Dating a Star ... a Few Hundred Thousand, in Fact

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Aug 21, 2014
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Globular clusters are too cool. For one thing, they’re gorgeous. I have proof!

That is IC 4499, a tight ball of tens or hundreds of thousands of stars located roughly 60,000 light years away. This image was taken by Hubble, and besides being spectacular, it also was used to nail down the age of the cluster, which until recently has been a bit controversial. This is another reason globulars are cool …

Getting the age of the cluster is possible because globulars have a very helpful characteristic: The stars are all the same distance away. That means if a star is brighter than another in the cluster, it really is more luminous. That makes comparing the stars directly to each other easier.

At first it was assumed that all globulars are very old—as old as the Milky Way itself, 12 billion years or so—and that all the stars in each were born at the same time. But it gets a bit more complicated. Some, it turns out, clearly have stars that are old, mixed in with ones that are younger. The thinking is that these clusters are more massive, could draw in more gas over time, and then could have a second bout of star formation after the initial one.

The trick to getting the age for a cluster is that stars age at different rates. More massive stars burn through their nuclear fuel faster, so they run out before their smaller, more miserly brethren. When that happens the core of the star contracts and heats up, and the outer layers respond by inflating hugely, like a hot air balloon. The heat from the interior gets spread out through the much larger surface area, so weirdly the star gets much brighter but also much cooler. We call it a red giant (or a red supergiant if the star is particular massive).

That’s the key. If you measure the stars’ colors, the ones that have run through their fuel and turned (or are currently turning) into red giants become very obvious. Theoretical models are pretty good at showing just how old the stars are that are right at that point in their lives, so that in turn must be the age of the cluster.

IC 4499 has always been a problem here. It has an intermediate mass between the lower-mass globulars that have a single population of stars and those heavier ones with two stellar populations. Knowing its age would be very helpful to nail down the difference between the two. Different studies have come up with different ages for it, with pretty large uncertainties, too.

The good news is that the Hubble observations easily cover the stars that are starting to turn in IC 4499, and the telescope’s ability to accurately resolve all the stars really nails down the age: IC 4499 is 12.0 ± 0.75 billion years. It’s old.

This helps. Astronomers like to study extremes, since that tells us what physics is doing at the edge of what it can do. But we also need to figure out intermediate cases, too, if we’re ever to have a fully filled-in picture of what’s really going on in the Universe. IC 4499 is another piece of that puzzle for which we’ve managed to find its place.