Q: How do astronomers get dates?
A: Ask a lot of heavenly bodies.
Go ahead and ogle NGC 6791:
Whoa. That's an open cluster, a collection of thousands of stars that are (in general) gravitationally bound to one another. In reality, over millions of years, the stars interact with each gravitationally, and a lot of the stars get flung out of the cluster, becoming loners. But a large fraction of the stars stick around, aging and eventually dying while still in the cluster. They're like city dwellers who never feel the need to leave town.
In this newly released Hubble image, you can see thousands of stars in just this one small patch of the cluster. You can see far more distant background galaxies, too (I love that kind of stuff).
But how old are these clusters, and the stars in them? Those are good questions, and important ones. The age tells us a lot about the environment of the cluster. For example, more massive stars tend to "sink" down to the center, and less massive ones move out away from the middle. How long does that take? The age of the cluster can tell us about how it moves around the Milky Way, and how stars behave in a cluster. All kinds of cool stuff can be figured out if we just know how long this guy has been around.
One advantage we have is that we're pretty sure all the stars in the cluster formed at about the same time. Not exactly, but probably not off by that much. So if we can find the age of any of the stars, then we should know the age of all of them. Still, it turns out that's not easy to determine. One way to is to look at the stars that have died already. We know that stars with more mass live their lives more quickly than low mass stars, eventually either exploding (if they are really massive) or blowing off their outer layers and leaving behind a white dwarf, a dense hot cinder.
So, if you want to date the cluster, look at the white dwarfs. Once formed, they don't generate any more heat, so they simply sit there and cool off like a chunk of charcoal. We know how that works, so we can work backwards to get the age of the cluster.
Astronomers used Hubble to observe NGC 6791, a cluster that sits a little over 13,000 light years away toward the summer constellation of Lyra. They made that gorgeous image above, and looked for white dwarfs. They found a bunch, got their ages... and immediately had a problem: they got two different ages. Some of the dead stars appeared to be 4 billions years old (a little younger than the Sun), and others appeared to be 6 billion years old. Ouch. Worse, another technique used to get the ages of normal stars showed them to be 8 billion years old.
In the image above, a zoom of the previous image, the younger appearing white dwarfs are circled in blue, and the older ones in red. Why would there be two separate populations of white dwarfs?
Well, there probably aren't! It turns out that 13,000 light years is a long way off. The younger-seeming white dwarfs actually are binary stars, white dwarfs orbiting normal low mass stars, but they're so far away from us they look like one single star (and it's easier to date single stars than ones in a committed relationship). The light from the normal star changes the color we see, making us think the star is younger, when in fact it's not.
So that fixes the 4 and 6 billion year issue; the white dwarfs are probably all 6 billion years old (why do stars want to look younger all the time?). But there's still the problem that the normal stars in the cluster look like they're 8 billion years old. Why would the dwarfs look younger?
Maybe they evolved differently than we expected while they were still alive. Maybe there's something about the cool-down rates of white dwarfs we don't understand. Maybe there's something about the normal stars in the cluster that make them look older. It's hard to say.
My suspicion is that the white dwarfs cool more slowly than we think. It takes them longer to get to a lower temperature, so when we look at them now they're warmer than we expect, so we think they're younger. What could do that? It might be that they have an odd chemical composition which affects their cooling rate (the presence or absence of some elements can affect how well a star radiates away its heat). I wonder if stellar encounters might play a role as well: stars are densely distributed in clusters, and there are more encounters between stars than out here in the suburbs of space. I don't know how that might play a part... but it usually pays to look at the environment. How is a cluster different than other parts of the galaxy? More stars, more encounters, more binaries... somewhere in there is the key to the mystery of the discrepant cluster star ages.
Only by studying more stars and more clusters are astronomers going to get answers to these questions. Happily there are lots of clusters to observe, and lots of stars in them. So truly, I was right before: how do astronomers get dates? Volume.