Anemic star cluster breaks metal-poor record

October 15, 2020

Maunakea, Hawaii - In a surprising discovery, astronomers using two Maunakea Observatories - W. M. Keck Observatory and Canada-France-Hawaii Telescope (CFHT) - have found a globular star cluster in the Andromeda Galaxy that contains a record-breaking low amount of metals.

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

The study, led by Søren Larsen of Radboud University in the Netherlands, is published in today's 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."

A globular cluster is a large, dense collection of thousands to millions of ancient stars that move together as a tight-knit group through a galaxy. Until now, astronomers thought large globular clusters had to contain a considerable amount of heavy elements.

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 our 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.

"This discovery is exciting because the idea of a 'metallicity floor' for globular clusters, that must contain some minimum amount of heavy metals, underpinned so much of our thinking about how these very old star clusters formed in the early universe," said co-author Jean Brodie, Director, Centre for Astrophysics and Supercomputing at Swinburne University and Professor Emerita of Astronomy and Astrophysics at UCO. "Our finding contradicts the standard picture and that is always fun!"

The researchers observed RBC EXT8 using Keck Observatory's High-Resolution Echelle Spectrometer (HIRES) in October of 2019. The globular cluster was not originally on the program, but Larsen's team had a couple of hours of observing time left and decided to aim the Keck I telescope at the cluster, whose stellar content had not yet been studied in detail. The team made spectroscopic observations to determine RBC EXT8's metal content and used three archive images from CFHT to determine its size and estimate its mass. Their remarkable result came as quite a surprise.

"It is observationally challenging to obtain a detailed analysis of the chemical composition of globular clusters in the Andromeda Galaxy, which is in the Northern Hemisphere of the sky," said Brodie. "The HIRES capability at Keck is uniquely well-suited to meet this challenge."

In the future, the researchers hope to find more "metal-lite" globular clusters and solve the mystery about their origin.

W. M. Keck Observatory

Related Astronomers Articles from Brightsurf:

Astronomers are bulging with data
For the first time, over 250 million stars in our galaxy's bulge have been surveyed in near-ultraviolet, optical, and near-infrared light, opening the door for astronomers to reexamine key questions about the Milky Way's formation and history.

Astronomers capture a pulsar 'powering up'
A Monash-University-led collaboration has, for the first time, observed the full, 12-day process of material spiralling into a distant neutron star, triggering an X-ray outburst thousands of times brighter than our Sun.

Astronomers discover new class of cosmic explosions
Analysis of two cosmic explosions indicates to astronomers that the pair, along with a puzzling blast from 2018, constitute a new type of event, with similarities to some supernovae and gamma-ray bursts, but also with significant differences.

Astronomers discover planet that never was
What was thought to be an exoplanet in a nearby star system likely never existed in the first place, according to University of Arizona astronomers.

Canadian astronomers determine Earth's fingerprint
Two McGill University astronomers have assembled a 'fingerprint' for Earth, which could be used to identify a planet beyond our Solar System capable of supporting life.

Astronomers help wage war on cancer
Techniques developed by astronomers could help in the fight against breast and skin cancer.

Astronomers make history in a split second
In a world first, an Australian-led international team of astronomers has determined the precise location of a powerful one-off burst of cosmic radio waves.

Astronomers witness galaxy megamerger
Using the Atacama Large Millimeter/submillimeter Array (ALMA), an international team of scientists has uncovered a startlingly dense concentration of 14 galaxies that are poised to merge, forming the core of what will eventually become a colossal galaxy cluster.

Astronomers discover a star that would not die
An international team of astronomers has made a bizarre discovery; a star that refuses to stop shining.

Astronomers spun up by galaxy-shape finding
For the first time astronomers have measured how a galaxy's spin affects its shape -- something scientists have tried to do for 90 years -- using a sample of 845 galaxies.

Read More: Astronomers News and Astronomers Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to