WHAT I REALLY WANT TO KNOW IS…
Why do young stars appear in old clusters?
Many stars in the Universe exist in clusters. The accepted view is that the stars in a particular cluster share a common origin – they formed from the same cloud of gas and dust at about the same time, meaning that they’re about the same age.
But around a decade ago, astronomers were looking at observations from the Hubble Space Telescope and found something rather surprising. They discovered that some galactic globular clusters contain stars with differing chemical compositions, meaning that the stars couldn’t have formed at the same time. This discovery challenged the traditional idea that star clusters form in a single episode of star formation.
It’s a conundrum that I’ve been investigating by looking at old star clusters, including globular clusters, in a companion galaxy to the Milky Way called the Large Magellanic Cloud (LMC). In the globular star cluster community, two explanations have been proposed for what Hubble observed. One is that star formation in clusters is a very long process. And the other, which is causing much debate, is that what Hubble recorded was just an observational effect, and not real.
To try to solve this puzzle, my colleagues and I started looking for a young star in its very early stages of life, just before it burns hydrogen to evolve into a main sequence star. Such stars have a very short lifetime, so if we were able to find young stellar objects in a star cluster, it would be direct evidence that star formation is currently taking place, to support the first explanation.
To try to find these stars, we’ve been using data from the European Space Agency’s Herschel Space Observatory, an infrared telescope that was able to see into the LMC’s dust and tell us the position of all the young stellar objects. Herschel allowed us to match the locations of several thousand young stars with the locations of stellar clusters and among them we found 15 candidates that were much younger than other stars within the same cluster. Now we need to understand how young stars are being born in these old clusters. In other words, we need to find out what is fuelling this second generation of star formation? One possibility is that the fuel might be gas entering the clusters from interstellar space, but observations using radio telescopes have shown no correlation between this gas and the location of the clusters we’ve been studying. Hence we’re looking at a second possibility: that an old star expels a lot of gas into the interstellar medium, which then accretes to form a second generation of stars. This is the current line of thought we’re pursuing. You may ask why we’re not examining at clusters in our own Galaxy. The reason is because some of the clusters are hidden behind the galactic plane and the dust and gas in the Milky Way. The LMC is easier to study. The next thing we have to do is produce some simulations to check whether old stars are capable of ejecting enough material to fuel star formation. Then we can compare the results we get from these simulations with the chemical compositions of the real stars that have been observed with our ground-based telescopes. NASA is set to launch the James Webb Space Telescope, the successor to Hubble, next year. What we’d like to do once the James Webb is operational is undertake a similar study of clusters, but this time in a galaxy much further away, such as M31, the Andromeda Galaxy. Looking at stellar populations in other galaxies will help tell us whether or not the processes observed in the LMC are unique to that dwarf galaxy.