Once thought to be formed soon after the Universe itself, new research from the Universities of Warwick and Auckland finds globular clusters may be only around 9 billion years old. The discovery brings into question current theories on how galaxies, including our own Milky Way Galaxy, were formed, with between 150-180 globular clusters thought to exist in the Milky Way alone.
Globular clusters could be up to 4 billion years younger than previously thought. This image shows a binary star system within a globular cluster. Image credit: Mark A. Garlick / University of Warwick.
“Determining ages for stars has always depended on comparing observations to the models which encapsulate our understanding of how stars form and evolve,” said co-author Dr. Elizabeth Stanway, of the Astronomy and Astrophysics Group at the University of Warwick, UK.
“That understanding has changed over time, and we have been increasingly aware of the effects of stellar multiplicity — the interactions between stars and their binary and tertiary companions.”
Designed to reconsider the evolution of stars, the team’s models — called Binary Population and Spectral Synthesis (BPASS) models — take the details of binary star evolution within the globular cluster into account and are used to explore the colors of light from old binary star populations — as well as the traces of chemical elements seen in their spectra.
The evolutionary process sees two stars interacting in a binary system, where one star expands into a giant whilst the gravitational force of the smaller star strips away the atmosphere, comprising hydrogen and helium amongst other elements, of the giant.
These stars are thought to be formed as the same time as the globular cluster itself.
Through using the BPASS models and calculating the age of the binary star systems, Dr. Stanway and her colleague, Dr. J.J. Eldridge of the University of Auckland, New Zealand, were able to demonstrate that the globular cluster of which they are part was not as ancient as other models had suggested.
The BPASS models had previously proven effective in exploring the properties of young stellar populations in environments ranging from our Milky Way all the way out to the edge of the Universe.
“Our findings point to new avenues of enquiry into how massive galaxies, and the stars contained within, are formed,” Dr. Stanway said.
“It’s important to note that there is still a lot of work to do — in particular looking at those very nearby systems where we can resolve individual stars rather than just considering the integrated light of a cluster — but this is an interesting and intriguing result.”
“If true, it changes our picture of the early stages of galaxy evolution and where the stars that have ended up in today’s massive galaxies, such as the Milky Way, may have formed. We aim to follow up this research in future, exploring both improvements in modeling and the observable predictions which arise from them.”
The results appear in the Monthly Notices of the Royal Astronomical Society.
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E.R. Stanway & J.J. Eldridge. Reevaluating Old Stellar Populations. MNRAS, published online May 23, 2018; doi: 10.1093/mnras/sty1353
