Thursday, July 10, 2008

One really strange star cluster

Part of the open star cluster NGC 6791
Image Credit: NASA, ESA, and L. Bedin (STScI)

Sometimes in astronomy we come across a really odd object. And the hard part is knowing whether the object is telling us something fundamental about physics and astronomy, or whether the object is just unique, or some combination of those. Today's example is based on a new Hubble Space Telescope press release (that I was griping a bit about yesterday).

The object in question is an open star cluster in our Milky Way galaxy called NGC 6791. First, some background. Star clusters are groups of stars born at the pretty much same time (a new star cluster is being formed right now in the Orion Nebula) and held together by their own gravity. Star clusters are not individual galaxies, but are parts of our own Milky Way galaxy.

There are two types of star clusters. Globular star clusters (like Messier 13 in the constellation Hercules) are groups of millions of stars. All globulars seemed to have been born in the very early universe, about 12 or 13 billion years ago. Globular star clusters also are very poor in iron and other metals relative to the sun. While almost 2% of the sun is made out of iron, uranium, nickel ,oxygen, carbon, silicon and other elements heavier than hydrogen and helium, only about 0.02% of a globular cluster star is made out of these elements. The reason for this is that stars are factories for the elements, and have been constantly making new elements, so the fraction of those elements goes up over time.

Open star clusters are the other kind of star cluster. They tend to have only a few thousand stars, they tend to be relatively young (most are under 1 billion years old), and they tend to have about the same amount of metal as the sun, give or take a little bit.

But NGC 6791 is, frankly, quite odd. It is old; it's stars seem to be about 8 billion years old (more on that later). This is much younger than the globular clusters, but older than any other open star cluster. Since it is so old, you might expect that its stars have a metal content between that of the globular clusters and the sun, but in fact it has over twice as much metal as the sun! And NGC 6791 has many more stars than any other open cluster, despite the fact that open clusters tend to lose stars over time. Lastly, NGC 6791 has a lot of really strange types of stars in it. So, it has always been viewed as odd, and its true nature has always been a source of controversy.

A group of scientists led by Italian astronomer Luigi Bedin looked at NGC 6791 with the Hubble Space Telescope in order to study its white dwarfs. White dwarfs are the ashes of dead stars; over time, a white dwarf cools and slowly fades away (just like what happens when you turn off the heat on an element on an electric stove, or when a blacksmith pulls red-hot metal out of a fire). We can estimate how old a white dwarf is by looking at how cool it is and using models to tell us when the star that made the white dwarf died (this is some of my own research).

When Bedin and his collaborators did this for NGC 6791, they found something odd. First, there were two groups of white dwarfs, one that seemed to be 4 billion years old, and another that seemed to be 6 billion years old. But, as I said above, the still-living stars in the star cluster look to be 8 billion years old. What's going on? We don't really know. But we do have some ideas.

First, we use models to get ages of stars and ages of white dwarfs. Those models are not perfect, and there are some uncertainties about them. In stars, a process called convection (which is like boiling or the turbulence that creates thunderstorms) mixes the stars, and that mixing can bring some new fuel to the star's nuclear furnace. We don't really know exactly how much of this mixing happens, so we guess. And that while these are highly educated guesses, they might be wrong. So, the 8 billion year age for the cluster may be wrong by as much as a billion years.

Also, the white dwarf ages have some uncertainties, especially for old white dwarfs. Old white dwarfs, as they cool, start to crystallize, forming giant, ultra-dense diamonds in space. That process of crystallization is not fully understood. Also, old white dwarfs may undergo a process called fractionation, where the heavier metals sink toward the center of the white dwarf. That sinking process can slow down the white dwarf's cooling rate, making an old white dwarf appear younger than it really is. This process (which may not even happen; it is very controversial) would be more likely to happen in white dwarfs with more metal. And, remember, this star cluster has more metal than any other star cluster!

So, I don't think that the 6-billion year age of one set of white dwarfs necessarily disagrees with the 8-billion year age from the stars. It would be kinda like looking at two separate people, saying, "Mr. X looks to be between 50 an 60 years old, and Mr. Y looks to be between 60 and 70, so I'll guess that Mr. X is 55 and Mr. Y is 65." Then you find out that they are both 60 years old. I think the same thing is happening here.

The bigger question is with the second group of white dwarfs, those that look only 4 billion years old. Here there are two ideas. Bedin and collaborators have suggested that these white dwarfs may be binary stars. Binary stars look like a single star from the Earth, but are really two stars orbiting each other closely enough that we can't tell them apart. The main effect of binary white dwarfs would be to produce a single point that looks brighter than a lone white dwarf would be. Since we get white dwarf ages from how bright they look (because they fade over time), we would mis-interpret binary white dwarfs as younger white dwarfs. In most open star clusters, about half of the stars are in binaries, and if that holds true in NGC 6791, then maybe the "young" white dwarfs are really just old twins.

But there is another option. An acquaintance of mine, Jason Kalirai at the University of California Santa Cruz, looked in detail at the brightest white dwarfs in this star cluster. He found that these white dwarfs were kinda wimpy, only about 2/3 as massive as they should be. (Most white dwarfs are about 60% the mass of the sun; these were about 40% the mass of the sun). White dwarfs with masses this low cool differently than normal white dwarfs (for reasons I don't want to go into). If you analyze the apparently younger white dwarfs as if they were merely featherweight white dwarfs, you would find that their ages are about 6 or 7 billion years, the same as the other white dwarfs.

But these featherweight white dwarfs can only exist if their parent stars ended their lives differently from the way we think most stars do. Some people (such as another acquaintance, astronomer Brad Hansen at UCLA) have suggested that the high amount of metals in the stars could cause the stars to lose a lot more matter at the end of their lives. There are other ideas that might work, though.

In short, this star cluster is a really odd one. It is very old, but it has more metal than any other star cluster (and we would think that newer star clusters should have more metals). The cluster has more stars than any other open star cluster, although it has been losing stars over its entire, long lifetime. The cluster has some really weird stars (which I haven't talked about here). The ages of the dead stars in the cluster may be different from the ages of the living stars. And even the dead stars disagree on how old they are, or they maybe they formed in multiple and unexpected ways. The jury is still out! Is this cluster just weird, or is it telling us that we don't really understand the life cycles of stars as well as we thought we did?The only thing for certain is that this cluster will continue to be studied for years to come.

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