These stars are dying off prematurely. What’s killing them?

In a discovery that could challenge the accepted scientific view of stellar evolution, a team of researchers from Australia’s Monash University and an international team of colleagues have found evidence that a large group of stars in a nearby cluster are dying prematurely.

Monash PhD student Ben MacLean and professor John Lattanzio, along with researchers from the Max Planck Institute for Astrophysics and the Australian Astronomical Observatory (AAO), reported Thursday in the journal Monthly Notices of the Royal Astronomical Society that a large quantity of helium-burning stars are prematurely dying in the M4 globular cluster.

Globular cluster are among “the oldest objects in the universe,” Lattanzio said in a statement. “Although we have some ideas for what is going on in them, every time we look carefully we find something unexpected. They are both fascinating and frustrating at the same time.”

In this case, despite the fact that M4 is one of the nearest and brightest spherical collections of stars in our corner of the universe and has already been well studied, he and his colleagues made the startling discovery that about half of the stars found there tend to skip their red giant phases, becoming white dwarfs several million years earlier than they normally should.

Scientists searching for an explanation for this unusual phenomenon

MacLean, Lattanzio and their colleagues used a new instrument outfitted on the AAO’s Anglo Australian Telescope (AAT) called a high efficiency and resolution multi-element spectrograph or HERMES for short. Using this device, they were able to determine the chemical composition of stars in M4 using their starlight, which led them to the startling discovery.

The findings build upon previous research at Monash University which found similar premature star deaths in another globular cluster, NGC 6752. However, the stars in that cluster were sodium rich, study author Dr. Simon Campbell, adding that it was “totally unexpected” to witness similar premature stellar deaths en masse in a “normal” star cluster like M4. However, the authors of the new study have reported that all early deaths involved sodium-rich, oxygen poor-stars.

The causes of this phenomenon remain unknown, and models of the affected stars have not been able to predict their premature deaths, according to the research team. Moving forward, Lattanzio said that he and his colleagues will need to develop new computer simulations that improve upon existing models and which will better demonstrate what is happening in these stars’ cores.

AAO researcher Dr. Gayandhi De Silva also praised the HERMES instrument, which made it possible for the AAT telescope to analyze the chemical composition of as many as 400 stars at the same time. De Silva said that the spectrograph “represents a significant step forward for Australia’s observational capacity” because “it combines multi-object capability with high data quality. Otherwise we are limited to observing one star at a time.”

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Image credit: NOAO/AURA/NSF

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