Image of the Week

A Milky-Way relic of the formation of the Universe

Figure 1: The Milky Way globular clusters on top of our Galaxy as seen by the Gaia space telescope. Like the insert image, each cluster is a grouping of thousands to millions of stars. The colour of the symbols represents their “metallicity,” i.e., their fraction in heavy atomic elements compared to the Sun. The stars of the C-19 stream (star symbols) have a 2,500 times lower density of heavy elements when compared to the Sun. C-19 is the Milky-Way structure with the lowest metallicity known to date. Image credit: N. Martin; Observatoire astronomique de Strasbourg; Canada-France-Hawaii Telescope / Coelum; ESA/Gaia/DPAC

An international team of researchers discovered the remnants of a star cluster whose stars are uniquely pristine. The stars in this cluster have an extremely low metallicity and must have formed in the metal-poor early Universe. Stars gradually enrich the interstellar gas with heavy elements which allows future stars born from this enriched interstellar gas to contain more heavy elements. The found cluster constitutes of only very early generations of stars hence provides a remarkable relic from when the first stellar structures were assembling. Some theories hypothesized clusters of such pristine stars could not form at all, others that they would have all vanished by now. This finding makes this a fundamental discovery for our understanding of how stars form in the early Universe.

To study the formation of the first stellar structures in the universe, astronomers can look at the most distant galaxies. As light takes time to travel, the observations we make now originate from a long time ago, from a time these galaxies and their constituent stars were in their infancy. Alternatively, astronomers may study the oldest structures in our galaxy, the Milky Way, in great detail. This approach is called “Galactic Archeology.” Like the Sun, most stars surrounding us formed in our Galaxy. However, favourably found in its outskirts, a small fraction of the Milky Way’s stars come from smaller galaxies that passed nearby, and donated stars and stellar clusters like comets leave debris behind them. One of these encounters left behind the discovered cluster that now sheds a stream of stars travelling through the strong Galaxy’s tides.

The team led by Nicolas Martin (CNRS/Strasbourg University) explored the unprecedentedly detailed map of the location and movement of stars seen by Gaia with a novel algorithm to isolate the rare groupings of stars moving in concert. One of the structures discovered is a new candidate stellar stream called “C-19”. In parallel, at the Canada-France-Hawaii Telescope (CFHT) in Hawaii, the Pristine survey mapped the sky to systematically measure the fraction of heavy elements in millions of stars (see Figure 1). When combined, these two studies revealed the surprising fact that C-19 contains stars with an extremely low heavy element fraction.

"It is only through the combination of the exquisite Gaia data and observations from multiple ground based telescopes that the Pristine team unveiled this remarkable stellar stream," said Dr. Nicolas Martin, Observatoire astronomique de Strasbourg and Pristine team lead. "This exciting discovery illustrates the power of bringing together observers and theorists from across the world."

Follow-up spectroscopic observations with the Gemini North Telescope in Hawaii and Gran Telescopio Canarias in La Palma allowed the astronomers to measure the chemical content of these stars. They found that they share a chemical signature specific to star clusters. The spectra also confirmed that their heavy element fraction, otherwise called “metallicity,” is 2,500 times lower than the Sun’s, making C-19’s level well below any other known structure in the universe.

This remnant from ancient times opens a direct and unique window into the early epochs of star formation in the Universe and the build-up of stellar structures in these very early times. The continuously improving maps of the Milky Way from the Gaia mission, the Pristine survey, and future telescopes will help the discoveries of such exceptional groupings of stars as precious fossils from ancient times.

Gaia Data Release 3, expected in Q2 of 2022, will provide us with astrophysical parameters determined from the BP/RP and RVS spectra. These will open many more dimensions to explore to find rare chemo-dynamical structures allowing us to better understand the formation and evolution of our Galaxy.

The “metallicity” of stars

The Sun is composed of 98% of the two lightest elements in the universe: hydrogen and helium. The sum of all the other heavier atomic elements (carbon, oxygen, iron, etc.) is what astronomers call “metallicity” and represents only 1.5% of the total mass of our star. Massive stars (5 times the Sun and above) produce most of these heavy elements. As these stars reach their final stages of evolution, they release these elements into the interstellar gas through winds or when they explode as supernovae. Like the Sun 4.5 billion years ago, newborn stars from this interstellar gas contain the elements formed in the previous generations of stars; they are metal-enriched. Consequently, stars impoverished in metals must be of the first generations in the entire Universe. The stars that make up the C-19 stream show abundances in heavy elements 2,500 times lower than those found in our Sun.

Animation of the C-19 stellar stream

An animation was created to show the evolution of the C-19 stream. This animation was created by Gabriel Pérez Díaz (SMM, IAC).

References and further reading:

• A stellar stream remnant of a globular cluster below the metallicity floor, 2021, Nicolas F. Martin et al., Nature
• Press release from Observatoire Astronomique de Strasbourg
• Press release from the Instituto de Astrofísica de Canarias - IAC
• Science release of NOIRLAB
• Press release from the Canada-France-Hawaii telescope

Credits: Nicolas F. Martin, M. Fouesneau, et al., 2021

Published: 05/01/2022