AstroTalk: Behind the news headlines of November 2014 Richard de Grijs (何锐思) (Kavli Institute for Astronomy and Astrophysics, Peking University) Surprising theorists, stars within middle-aged clusters are of similar age A close look at the night sky reveals that stars don’t like to be alone; instead, they congregate in clusters, in some cases containing as many as several million stars. Until recently, the oldest of these populous star clusters—the so-called “globular” clusters—were considered well-understood, with the stars in a single group having formed at different times, over periods of more than 300 million years. Yet new research published in the journal Nature in mid-December 2014 suggests that the star formation in these massive clusters is more complex. Using data from the Hubble Space Telescope, my own team of researchers at the Kavli Institute for Astronomy and Astrophysics at Peking University and the Chinese Academy of Science’s National Astronomical Observatories in Beijing have found that, in large middle-aged clusters at least, all stars appear to be of about the same age. Stars begin their lives as billowing clouds of dust and gas. Pulled together by gravity, these clouds slowly coalesce into dense spheres that, if they grow large enough, heat up and begin to convert hydrogen into helium in their cores. This process releases energy and makes them shine. Billions of years later, when they reach the end of their core hydrogen supply, the stars begin to burn hydrogen in a shell around their cores and, as a result, their temperature changes. Previous observations of massive, old star clusters revealed a relatively large amount of variation in temperature from stars reaching the end of their core hydrogen supply, suggesting that the stars within the clusters varied in age by as much as 300 million years or more. “This has long been surprising,” said Chengyuan Li 李程远, one of my PhD students at Peking University and the lead author of our new study. “Young clusters are thought to quickly lose any remaining star-forming gas during the first 10 million years of their lifetimes,” which would make it difficult for the stars in a single cluster to vary in age by more than about 10 million years. Observing a middle-aged, 2 billion-year-old star cluster located in the Large Magellanic Cloud called NGC 1651, we looked for both the change in temperature that occurs when stars reach the end of their hydrogen supply—which is what previous studies had focused on—and a second change in temperature that occurs as the stars burn hydrogen in a shell around their core. While we found the expected wide variation in temperature of stars finishing their core hydrogen reserves, we were surprised to find very little variation when looking at the brightnesses of stars of similar temperatures burning hydrogen in the shell outside the core. The lack of variation among these stars led us to conclude that the stars in this cluster must all be within just 80 million years of the same age – a very small age range for such an old cluster. NGC 1651 is the best example found to date of a truly single-age stellar population. We have since identified a handful of other middle-aged clusters that appear to show similar features. Our research suggests that, for middle-aged clusters at least, today’s conventional wisdom may be wrong and it might be common for all stars in a single cluster to be of approximately the same age. A decade ago, astronomers actually thought that the stars within any cluster should all be about the same age, but that idea fell out of favour when clear evidence of the presence of stars of different ages within a single cluster was discovered, at least for the oldest and most populous clusters in our Milky Way. Based on our new Nature paper, a reverse shift looks necessary. Our results resolve nearly a decade of debate among scientists; as such, the results were deemed ‘solid and welcome’ by the peer- reviewers. In addition, we suggested that the wide range of brightness seen in stars reaching the end of their core hydrogen supply may actually be due to stellar rotation. That’s because two stars of exactly the same age can exhibit different levels of observed temperature if they rotate at significantly different rates. Most current models don’t take stellar rotation into account. Future studies may offer even greater insight into the age of star clusters by better modeling stellar rotation rates and using those models to interpreting the variation in temperature of stars burning the last of their core hydrogen. The oldest star clusters as “not-so-simple” stellar populations The idea that the globular clusters—dense groups of over 100,000 stars that are about as old as the Milky Way galaxy, typically around 10 billion years old—are no longer considered single-age collections of stars (usually called “simple” stellar populations) is not challenged by our results, however. “We thought we understood these old clusters very well”, said Alison Sills (McMaster University, Canada). “We taught our students that all the stars in these clusters were formed at the same time, from one giant cloud of gas. And since that time, the individual stars may have evolved and died, but no new stars were born in the cluster.” In the middle of the last century, however, a population of stars called “blue straggler” stars was discovered. These stars are apparently hotter and brighter, and more massive, than they should be for a cluster of this advanced age. The current explanations for these stars involve some kind of stellar interaction. Two normal stars get too close to each other, and the gravity of one can pull material off the surface of the other, causing the two stars to merge. “Astronomers expect that the stars get too close to each other because of the complicated dance that stars perform in these dense clusters, where thousands of stars are packed into a relatively small space, and each star is moving through this cluster under the influence of the gravity of all the other stars. Somewhat like a traffic system with no stop lights, there are a lot of close encounters and collisions,” explained Sills. Hubble Space Telescope observations of globular clusters showed evidence for two generations of star formation, not just one. But the second generation is not the same as anywhere else in the Galaxy. Instead of being made of material that came from an earlier generation of exploded stars, at least some of the second generation of stars in globular clusters seems to have come from material that was gently shed by the first generation. This link between the two generations is puzzling, and astronomers are still trying to figure out why globular clusters should behave in this way. “Studying the normal stars in clusters was instrumental in allowing astronomers to figure out how stars lived and died”, said Sills, “but now we can look even further back, to when they were born, by using the oddballs. It pays off to pay attention to the unusual individuals in any population. You never know what they’ll be able to tell you.” New insights into the oldest clusters New Hubble Space Telescope observations of globular clusters in a small, nearby galaxy known as the Fornax dwarf spheroidal galaxy show they are very similar to those found in the Milky Way, and so must have formed in a similar way. One of the leading theories on how these clusters form predicts that globular clusters should only be found nestled in among large quantities of old stars. But these old stars, though rife in the Milky Way, are not present in this small galaxy, and so, the mystery deepens. As we just saw, globular clusters remain one of the biggest cosmic mysteries. Extensive research during the past decade has shown that many of the Milky Way’s globular clusters had far more complex formation histories and are made up of at least two distinct populations of stars. Of these populations, around half the stars are a single generation of normal stars that were thought to form first, and the other half form a second generation of stars, which are polluted with different chemical elements. In particular, the polluted stars contain up to 50 to 100 times more nitrogen than the first generation of stars. The proportion of polluted stars found in the Milky Way’s globular clusters is much higher than astronomers expected, suggesting that a large chunk of the first generation star population is missing. A leading explanation for this is that the clusters once contained many more stars but a large fraction of the first generation stars were ejected from the cluster at some time in its past. This explanation makes sense for globular clusters in the Milky Way, where the ejected stars could easily hide among the many similar, old stars in the vast halo, but the new observations, which look at this type of cluster in the much smaller Fornax galaxy, call this theory into question. Astronomers used Hubble’s Wide Field Camera 3 (WFC3) to observe four globular clusters in Fornax. “We knew that the Milky Way’s clusters were more complex than was originally thought, and there are theories to explain why. But to really test our theories about how these clusters form we needed to know what happened in other environments,” says Søren Larsen of Radboud University in Nijmegen, the Netherlands. “Before now we didn’t know whether globular clusters in smaller galaxies had multiple generations or not, but our observations show clearly that they do!” The astronomers’ detailed observations of the four Fornax clusters show that they also contain a second, polluted population of stars and indicate that not only did they form in a similar way to one another, their formation process is also similar to that of clusters in the Milky Way.