Ancient stars somehow survived near the center of the Milky Way

The core of our Milky Way attracts astronomers’ attention like moths to a flame. That’s because there’s a lot to do there. Not only is there a supermassive black hole, but also populations of very ancient stars teeming at the center. Most of them date back to at least the formation of the Galaxy.

Astronomers used the Canada-France-Hawai’i Telescope in Hawaii to study a selection of these stars. They wanted to understand its characteristics and longevity. The observations are part of the Pristine Inner Galaxy Survey (PIGS) project. He has been studying the nucleus of the Milky Way for several years now to understand these ancient stars.

PIGS Ancient stars in the core

Other studies have shown that stars began their formation in what is now the core of the Milky Way. PIG stars are the remaining members of the Milky Way’s “inner court”. They slowly circle the nucleus and spend most of their lives there. Chemically, they are very different from the younger stars scattered across the Galaxy. The old ones are mostly hydrogen and helium, which indicates that they were formed in a very ancient epoch of cosmic history. By comparison, younger stars in the Milky Way have greater abundances of “metals” such as carbon, oxygen, nitrogen, and so on.

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There are many stars in the Universe itself that formed shortly after the Big Bang from the largely hydrogen and helium-rich environment. Astronomers can use them as “templates” to study the stars in our Milky Way to determine what it was like in its infancy. Typically, these old stars exist in the central region of the Galaxy and in the swarm of stars that form a low-density halo around the Milky Way.

According to PIGS team member Anke Arentsen, this study appears to confirm that the survey is seeing the oldest stars in the Galaxy. “It’s exciting to think that we’re seeing stars that formed in the early stages of the Milky Way, previously largely out of reach,” Arentsen said. “These stars probably formed less than a billion years after the Big Bang, as well as relics of the early Universe.

What ancient stars say about the history of the Milky Way

When you look at galaxy formation patterns to try to understand how the Milky Way came to be, there’s a common theme: material comes together. It does it again and again, from the protogalaxy to galaxy collisions to star formation to planetary formation. This is what astronomers call the “hierarchical model”.

There are two main models of what comes next in the evolution of a galaxy. One says that the early Universe was a jumble of gas clumps and mostly dark matter. The lumps interacted gravitationally. Eventually, the non-dark matter component (called baryonic matter) in each cluster cooled and began to contract on itself. It spins and creates a proto-disc and eventually breaks into smaller lumps within the disc. Those lumps essentially became the first stars. Take it one step further and the infant galaxies collide with each other to form bigger galaxies. This (in a nutshell) is what astronomers call the “top-down” theory of formation.

Others propose that smaller gas clouds began the star formation process and that those became protogalaxies. Eventually, the protogalaxies merged to form larger ones that eventually began to rotate and produce disk-like structures. This is often referred to as the “bottom-up” theory of education. In both cases, the final product galaxies remained encased in dark matter halos as we see today.

Both models imply that the oldest stars in a galaxy should be present in the dense, inner regions. This certainly appears to be true for the Milky Way. And, using spectroscopy, astronomers have found that the chemical properties of those ancient stars match those of the earliest stars in the Universe.

Putting twelve billion year old stars into perspective

So where is the Milky Way in the hierarchy? It began forming about 13.6 billion years ago, and its original “core” was a smaller protogalaxy that became its first nucleus. For comparison, the Universe itself began in the Big Bang, now thought to have occurred at least 13.8 billion years ago. Thus, the first stars of the Milky Way are not exactly the same age as the Universe. But, since they formed so early, they’re excellent evidence of what conditions were like in the earliest epochs of star formation history.

To prove their ideas about these ancient stars, astronomers need to “see” the core of the Milky Way. It hasn’t always been easy. It is hidden by vast clouds of gas and dust, meaning visual observations are nearly impossible. If astronomers use special filters on telescopes or look using other regimes of light (like infrared or radio, for example), they can spot those stars easily enough. The next step is to determine their age by looking at their light to determine their metal content. There aren’t many of the stars that are older than their younger counterparts elsewhere in the Galaxy. So, astronomers look for metal-poor stars, which are probably older.

The PIGS observations, combined with previous data from the GAIA survey, tracked the movements of these stars across the galaxy. It turns out that these older stars have more chaotic motions, but tend to maintain an average rotational path around the core. Typically, most ancient stars reside within a spherical region extending about halfway between the core of the Galaxy and the Sun. However, some are found within a low-density halo that surrounds the Galaxy , where they are easier to spot.

Next steps

Arentsen presented the results of the PIGS survey at a meeting of the Royal Astronomical Society. Prior to this presentation, she and the team released other PIGS survey results, dating back several years. They also focus on the characteristics of aging stars at the heart of the Milky Way.

How can such ancient stars still exist? Presumably, most of the much older ones in the Universe have long since exploded as supernovae or met some other end. At the core of a galaxy, it is likely that very many could be swallowed up by the growing supermassive black hole. So, it will be interesting to find out why the core of our galaxy still harbors a population of ancient star citizens.

Further studies of the heart of the Milky Way should discover more about the formation, life and movements of these first stars. “The data available for these ancient objects is growing rapidly,” Arentsen said. “I’m excited to see what we learn about these first stars to populate our Galaxy in the coming years!

For more information

PIGS They find ancient stars in the heart of the Milky Way
The Pristine Inner Galaxy Survey (PIGS) V. A chemodynamic survey of the early assemblage of the Milky Way with the most metal-poor stars in the bulge