The core of our Milky Way Galaxy captures astronomers’ attention like moths plunging into flames. That’s because there’s a lot going on there. Not only is there a supermassive black hole, but also a very ancient population of stars pervading 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 the selection of these stars. They want to understand their characteristics and lifespan. The observations are part of the «Primary Inner Galaxy Survey» (PIGS) project. It has been studying the core of the Milky Way for several years now to understand these ancient stars.
Pig Ancient stars in the core
Other studies have shown that stars began to form in what is now the core of the Milky Way. The Piglets stars are the remaining members of the «inner court» of the Milky Way. They rotate slowly around the core and spend most of their lives there. Chemically, they are very different from the younger stars scattered throughout the Galaxy. The old ones are mostly hydrogen and helium, which suggests that they were formed at a very early time in the history of the universe. For comparison, younger stars in the Milky Way have higher amounts of «metals» like carbon, oxygen, nitrogen, etc.
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There are many stars in the Universe that formed shortly after the Big Bang from environments that were mainly rich in hydrogen and helium. Astronomers can use them as «samples» to study stars in our Milky Way to determine what it looked like in its infancy. Typically, such old stars exist in the core region of the Galaxy and in the cluster of stars that make up the low-density halo around the Milky Way.
According to PIGS team member Anke Arentsen, this study seems to confirm that the survey is seeing the oldest stars in the Galaxy. «It’s exciting to think we’re seeing stars form in the early stages of the Milky Way, which were previously largely out of reach,» Arentsen said. “These stars likely formed less than a billion years after the Big Bang, as are relics from the early Universe.
What ancient stars say about the history of the Milky Way
When you look at models of galaxy formation to try to understand how the Milky Way formed, there is one common theme: matter coming together. It repeats itself over and over, from pregalactic to galactic collisions, to star formation, to planet formation. This is what astronomers call «the «hierarchical model.»
There are two main models of what happens next in the evolution of a galaxy. One says that the early Universe was a mixture of masses of gas and mostly dark matter. Attractive interactive blocks. Eventually, the non-dark matter component (called baryonic matter) in each block cools and begins to shrink on its own. It spins and creates a prototype disc that eventually breaks up into smaller chunks within the disc. Those clusters essentially became the first stars. Take it one step further and newborn galaxies collide to form larger galaxies. This is (in a nutshell) what astronomers call the «top-down» theory of formation.
Others suggest that the smaller gas clouds started star formation and that they became protogalaxies. Eventually, the protogalaxies merge to form larger galaxies that eventually begin to rotate and create disk-like structures. This is often referred to as the «bottom-up» formation theory. In both cases, the final product galaxies were still enveloped in the dark matter halos we see today.
Both models imply that the oldest stars in a galaxy must be present in dense interior regions. This certainly seems true for the Milky Way. And, using spectroscopy, astronomers have found that the chemistry of those ancient stars matches 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 to form about 13.6 billion years ago, and its original «core» was a smaller protogalactic that became its first nucleus. For comparison, the Universe itself began with the Big Bang, which is now thought to have occurred at least 13.8 billion years ago. So the Milky Way’s earliest stars are not exactly the same age as the Universe. However, because they form so early, they are excellent probes of the conditions under which history’s earliest star-forming epochs occurred.
To prove their idea of these ancient stars, astronomers need to «see» the core of the Milky Way. It’s not always easy. It is obscured by huge clouds of gas and dust, which means it is nearly impossible to see with the naked eye. If astronomers use special filters on telescopes or observe with other modes of light (for example infrared or radio waves), they can detect those stars. easy. The next step is to determine their age by looking at their light to determine their metal content. There are not many older stars than younger stars in other parts of the Galaxy. So astronomers look for metal-poor stars, which are likely older.
The PIGS observations, combined with previous GAIA survey data, tracked the motions of these stars in the Galaxy. It turns out that these older stars have more chaotic motion, but they tend to maintain an average rotation around the core. Normally, most ancient stars lie inside a spherical region that extends roughly between the core of the Galaxy and the Sun. However, some are found inside the low-density halo surrounding the Galaxy, where they are easier to detect.
Next step
Arentsen presented the results from the PIGS survey at a meeting of the Royal Astronomical Society. Prior to this presentation, she and her team published other results from the PIGS survey, dating back several years. They also focused on the characteristics of old stars at the center of the Milky Way.
How are such ancient stars still around? Presumably, most of the oldest ones in the Universe either exploded long ago as a supernova or met some other end point. In the core of a galaxy, it is very likely that many galaxies will be swallowed up by the growing supermassive black hole. So it will be interesting to find out why our galactic core still contains a group of ancient stellar inhabitants.
More studies of the center of the Milky Way will uncover more about the formation, life, and motions of these early stars. “The data available on these ancient objects is rapidly increasing,” says Arentsen. «I can’t wait to see what we’ll learn about the first stars to populate our Galaxy in the next few years!»
For more information
Pigs Find Ancient Stars In The Heart Of The Milky Way
Pristine Inner Galaxy Survey (PIGS) – V. A chemodynamic investigation of the Milky Way’s early assemblage with mostly metal-poor stars in the bulge
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