Astronomers Discover Anemic Star Cluster

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Andromeda met RBC EXT8 in color. (PC: ESASky/CFHT)

Astronomers have discovered a massive star cluster in the Andromeda Galaxy, the closest large galaxy to our Milky Way, that is anemic.

Using two Maunakea Observatories in Hawaii – W. M. Keck Observatory and Canada-France-Hawaii Telescope – the team found the cluster called RBC EXT8, is so deficient in iron and magnesium, that it has broken the record for having the lowest metal content ever seen in a globular star cluster.

The cluster has on average 800 times less iron than the Sun and are three times more iron-poor than the previous globular cluster record-holder. RBC EXT8 is also extremely deficient in magnesium.


This discovery is surprising because it challenges the notion that such objects could not have formed at such low metallicities in the young universe.

Globular star clusters are a large collection of up to millions of the oldest stars that orbit together as a densely-packed group in the outskirts of a galaxy.

The study, led by Søren Larsen of Radboud University in the Netherlands, was published in a recent issue of the journal Science.


“I’m amazed that this remarkable star cluster was just sitting under our noses. It is one of the brightest clusters in the Andromeda galaxy and known for decades, yet no one had checked it out in detail,” said Aaron Romanowsky, a University of California Observatories (UCO) astronomer and professor at San José State University’s Physics and Astronomy Department who co-authored the study. “It shows how the universe still has many surprises for us to discover. It also reminds us to check our assumptions – in this case, it was assumed enough clusters had been investigated to know how anemic they can be.”

Hydrogen and helium are the two main elements created after the Big Bang. Heavier elements such as iron and magnesium formed later. Finding a massive globular cluster like RBC EXT8 that is extremely impoverished in metals defies current formation models, calling into question some of scientists’ ideas about the birth of stars and galaxies in the young universe.

“Our finding shows that massive globular clusters could form in the early universe out of gas with only a small ‘sprinkling’ of elements other than hydrogen and helium. This is surprising because such pristine gas was thought to be in building blocks too small to form such massive star clusters,” said Larsen.

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