Expert Q&A: Ancient star cluster discovery reimagines the universe

Many astronomers will use the newly launched James Webb Space Telescope to look at distant galaxies to study the ancient universe, but recently discovered galactic fossils also open a unique window into galaxy formation from when the big bang occurred.

Kim Venn, UVic Professor in Physics & Astronomy and Director of the Astronomy Research Centre (ARC), is the co-lead of an international team of research astronomers who have discovered the ruins of an ancient star cluster, as published in Nature today.

Found in the outer reaches of the Milky Way galaxy, this star cluster shows the lowest proportion of heavy elements, or lack of metals, ever found in such systems. Challenging long-standing theories on early galaxy formation, astronomers did not know that such star clusters could exist—some theories hypothesized they could not form at all—making this a key discovery in our understanding of the early universe. 

The University of Victoria research team includes scientists and engineers at the National Research Council of Canada’s (NRC) Herzberg Astronomy and Astrophysics Research Centre who are adjunct faculty at UVic, and international astrophysicist members of the Pristine collaboration who collaborate from locations around the world: Toronto, Europe, and Russia.

Interview clips of the researchers answering questions around this galactic archaeology discovery can be accessed on Dropbox.

Kim Venn answers questions about galactic fossils:

Q. What is so exciting about this discovery of ancient stars at the edge of the galaxy?

A. It is exciting to find something new, especially something that challenges our understanding of the formation of stars and galaxies in the early Universe. This unexpected discovery is exactly the kind of thing that draws people into scientific research in the first place—curiosity and wonder. We have discovered the most metal-poor star cluster ever found. This nearly pristine group of stars would have formed very early on, and we can use it to study the first stages of galaxy formation after the Big Bang. We’re all thrilled to be a part of uncovering this missing link as part of an international collaboration, but especially where Canada played so many leading roles.

Q. What are globular clusters and why do they matter?

A. A globular cluster is a particular kind of star cluster: a group of millions of stars that formed together and are now gravitationally bound together in a symmetrical, somewhat spherical form. The stars that make up globular clusters formed at the same time and from the same gas cloud.

We have seen globular clusters before: there are over 150 in the Milky Way galaxy. Some are amongst the oldest objects seen in galaxies, at 13 billion years, providing a unique glimpse of relics from the early universe.

Q. What was special about the star (globular) cluster you found?

A. We discovered the remnants of the cluster, named C19, as a stellar stream in the outer parts of the Milky Way galaxy. Its metallicity (a measurement of elements other than hydrogen and helium) is less than 0.05% the solar metallicity, which is two times lower than any previously known star cluster. This is a break-through discovery, since we didn’t know that globular clusters with so few heavy elements could exist. In fact, many astronomers theorized the opposite: that a cluster of stars could not form with so few heavy elements. As team member Alan McConnachie (NRC astronomer and UVic adjunct faculty) noticed about the findings, “This is a ridiculously low level of metals”.

Q. What steps did you and the group take to make this discovery?

A. We have been leading a scientific survey* to search for metal-poor stars in the Galactic halo which uses the Canada-France-Hawaii Telescope (CFHT) on Maunakea, Hawaii. This survey, called the Pristine survey, designed a unique filter sensitive to a specific stellar metallicity characteristic or feature–the weaker the feature then the more metal-poor the object.

We combined our results with another survey—the European Space Agency’s satellite mission Gaia, which is determining the distances and motions of the stars in the Galaxy—to find a group of extremely metal-poor stars that seemed to have a common orbit in the Galactic halo. This star cluster is not spherical like a regular globular cluster. It is arranged along a stream that extends about 20,000 light years from the Galactic Centre at its closest approach and roughly 90,000 light years at its farthest. This group of stars stretches an impressive expanse of the night sky—roughly 30 times the width of the moon—although it is not visible to the naked eye.

* An astronomical survey is a map or image of the sky that doesn’t have a specific observational target

Q. What is a spectrograph and how is it used?

A. Spectrographs are scientific instruments that split the light from the star into a rainbow. By examining the rainbow scientists can identify features of the chemical elements in the stars.

Q. What new technology allowed the two telescopes work together?

A. Victoria team member John Pazder (NRC engineer and UVic graduate student) designed a unique optical fibre system called GRACES. The Gemini Observatory has one of the largest telescopes on Maunakea, but the CFHT (also on Maunakea) has one of the best optical spectrographs. GRACES uses an optical fibre to focus starlight collected at Gemini and direct it to the spectrograph at CFHT. This is the first time an instrument in one observatory has been successfully used by another observatory, effectively allowing instruments to be shared between the community of telescopes on Maunakea.

Q. What proof did your research provide?

A. We proved that metal-poor star clusters exist. I led the observations and analysis of the spectroscopic data gathered by GRACES on Maunakea in Hawaii. International partners lead the observations and analysis of other data taken at the Gran Telescopio Canarias in La Palma, Spain. Together, we provided the proof that the stars in C19 are all extremely metal poor. However, the GRACES data was higher quality which also allowed us to determine the chemical composition of the C19 stars observed at the Gemini Observatory. The chemical composition provided a fingerprint that was critical to proving that C19 is the ruins of an ancient globular cluster system, not the more commonly found ruins of an accreted dwarf satellite galaxy.

Q. What was University of Victoria’s role? What partnerships made this discovery possible?

A. We were very fortunate to have had the opportunity to conduct the observations from Maunakea. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Indigenous Hawaiian community.

Working with our research partners here and around the world has been particularly stimulating. Here in Victoria our team included professors, postdocs and students at UVic as well as astronomers, data scientists and engineers at the NRC’s Herzberg Astronomy and Astrophysics Research Centre. We also worked with our international collaborators, including people with access to other observatories and several previously in Victoria researchers who returned to Europe. We used the instruments and observatories in Hawaii, part of Canada's ongoing international partnerships, which enable Canadians to lead major scientific discoveries (like this), but importantly provide Canada’s high-tech industry opportunities to develop new instruments and data methodologies. Technological down flows from these opportunities benefit the Canadian economy (such as optical fiber communications, security systems, laser eye surgery, WIFI, etc.). UVic supports this group of expertise and our productive partnerships through its Astronomy Research Centre (ARC).

Key UVic members:

1. Kim Venn (UVic Physics & Astronomy, and ARC Director):  lead scientist on the C19 GRACES spectral observations and data analysis, and co-founder of the Pristine survey in Canada.
2. Alan McConnachie (NRC astronomer, adjunct faculty UVic) : co-founder of the Pristine survey in Canada, and scientific contributor to the C19 paper
3. Federico Sestito (UVic Hess postdoctoral researcher): He was a PhD student with the Pristine survey co-lead Nicolas Martin (Observatoire Astronomique de Strasbourg, France) when C19 was discovered. He has provided calculations of the stellar parameters of the stars in C19 as a postdoc at UVic with Kim Venn.
4. Julio Navarro (UVic, Physics & Astronomy): member of the Pristine survey, and scientific contributor to the C19 paper.
5. Sebastien Fabbro (NRC data scientist, adjunct faculty UVic): data analyst for the Pristine survey, and scientific contributor to the C19 paper.
6. John Pazder (NRC engineer, and UVic graduate student): senior optical engineer who designed, built, and tested the GRACES spectrograph – a unique 270 metre optical fibre cable connecting the Cassegrain focus at the Gemini-North Telescope to the ESPaDoNS spectrograph in the Canada-France-Hawaii Telescope building.

International partners:

1. France: Lead author Nicolas Martin (Observatoire astronomique de Strasbourg).  Co-lead of the Pristine survey, and co-lead for the Streamfinder algorithm used on Gaia data for this discovery.
2. United Kingdom: David Aguado (University Cambridge) analyzed OSIRIS spectra, and scientific contributor to the C19 paper.
3. Spain: Jonay Gonzalez-Hernandez (Grantecam Telescope, La Palma) took the OSIRIS spectra, and scientific contributor to the C19 paper.
4. Netherlands: Else Starkenburg (Rijksuniversiteit Groningen). Co-lead Pristine survey.  Helped determine the stellar parameters for the stars in C19, and scientific contributor to the C19 paper.
5. France: Rodrigo Ibata (University Strasbourg), Co-lead for the Streamfinder algorithm used on Gaia data for this discovery, he calculated the orbits for the C19 stream, and provided scientific contributions to the C19 paper.

Q. Why is this research so important?

A. This discovery tells us that cosmologists need to rethink our views of the formation of stars and galaxies in the early Universe. When we talk about what happened after the Big Bang it has been necessary to estimate some timelines between events, such as when the first stars formed. It has been thought that the first stars would increase the overall number of heavy elements in the Universe through the usual known steps in stellar nucleosynthesis and supernova explosions—up to a ‘metallicity floor’—before galaxies and star clusters could form. That ‘metallicity floor’ is higher than the metal abundances in C19. The ruins of this ancient stellar cluster provide us with a new, direct and unique window into the earliest epochs of star formation in the Universe. As team member Julio Navarro explained, “We now know it is possible to study the oldest structures within our own galaxy as fossils from those ancient times.”

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