New Facts Change Theories about Star Clusters

Thank Science for New Insights!

Flame Nebula
A new study of NGC 2024 and the Orion Nebula Cluster show stars on the outskirts of these clusters are older than those in the middle. Image Credit: X-ray: NASA/CXC/PSU/K.Getman, E.Feigelson, M.Kuhn & the MYStIX team; Infrared:NASA/JPL-Caltech

One of the things I like about science is that the story is never finished. It’s a story of continual change and learning. Ongoing research in any realm of science brings new answers to old questions and raises new questions as we refine our methods and techniques. This is why we often hear that scientists have changed their views on some topic. It’s almost always because new data are available, or new instruments allow them to probe their topics of study more deeply. In true and honest scientific research, the changes in data aren’t arbitrary or made to fit some political or business agenda. They’re always because we found a better way to learn about our topics.

Take star clusters, for example. These are groupings of stars that formed about the same time in a massive cloud of gas and dust. Astronomers could tell that the stars were of similar ages when they measured their temperatures, brightnesses, metallicities (essentially, their chemical makeup), and velocities through space. However, the sequence of starbirth in a cluster wasn’t really well understood.

For a long time, astronomers assumed that stars formed in clusters in giant clouds and that as more material got pulled into the central region of the starbirth créche, more stars would form in that region. Those stars were assumed to be the oldest and the stars elsewhere would be younger. It sounds logical, but it turns out that it’s not what happened.

Astronomers using the Chandra X-Ray telescope orbiting Earth looked at two clusters where sun-like stars are in the process of forming. One cluster is in the center of the Flame Nebula and the other is in the Orion Nebula (both in Orion). By looking at them in x-ray wavelengths, astronomers were able to calculate the ages of the stars in the clusters. It turns out the youngest stars are in the center and the older ones are in the outskirts. This has some implications for cluster development, but also means that astronomers will need to work some more on refining the models of how stars like the Sun form

The astronomers in the Chandra study, led by Konstantin Getman of Penn State University, used the Chandra data to look at the brightness of the stars in x-rays, which helped them determine the masses of those stars. They also used observations of the same stars in infrared light conducted by ground-based telescopes and also from data collected by the infrared-sensitive Spitzer Space Telescope. All that data combined helped them pinpoint the ages of the stars quite accurately.

So, stars in clusters are older in the suburbs of the cluster, and younger in the “downtown” regions. How might that happen? The astronomers have several ideas. It could be that star formation continues to occur in the inner, material-rich regions of a starforming cloud, but stops in the outer regions when material runs out. That means that no new stars form in the outer suburbs, and the older stars are the only ones left there. It’s also possible that old stars drift away from the action at the core of a cluster, and settle out in the suburbs, where star formation has stopped or slowed down drastically. Finally, there’s a possibility that stars form in massive filaments of gas that fall toward the center of a cluster, and the stars continue along the original path of the filaments.

To see if this old-stars on the outskirts, young stars in the cores trend occurs elsewhere, astronomers will need to do the same measurements in other clusters. If it does, then that gives us more data points about star formation and clusters that we can use to understand the same events elsewhere in our galaxy, and in other galaxies.

Speaking of clusters, there’s a star cluster you can find with the naked eye (if you have a good dark sky spot) or binoculars. It’s called Melotte 111, in the constellation Coma Berenices. Want to learn how to find it? Check out the May edition of “Our Night Sky” at Astrocast.TV. You can watch it below.

Happy viewing!

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