Probing the Origins of Milky Way Stars: Unravelling Their Birthplaces and Orbital Histories

This work dives into the fascinating question of where stars in our Galaxy were born and how their orbits have changed over billions of years.

Stars in the Milky Way don’t stay put—they move around, influenced by structures like the Galactic bar and spiral arms. These interactions can alter their orbits in two main ways:

  • Churning (radial migration): Stars change their guiding radii, moving inward or outward (Figure 1).
  • Blurring: Stars preserve their angular momentum but their orbits are perturbed by interactions with Galactic structures.

Figure 1: Illustration of radial migration in a spiral galaxy, such as the Milky Way. Two stars are highlighted: one (red) migrating outward from the inner regions of the galaxy, and another (blue) migrating inward from the outer regions. These movements, driven by interactions with Galactic structures like spiral arms and the bar, highlight how stars can change their orbits over time, shaping the evolution of the Milky Way. Credit: Maria Luiza. L. Dantas. This figure is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

In this study, we set out to understand whether stars in the thin disc of the Milky Way (observed by the Gaia-ESO survey) have been primarily churned or if they’ve kept their original birth radii (i.e. blurred or remained undisturbed). To do this, we used a Generalised Additive Model (GAM), a statistical technique not often used in Astronomy, to estimate the birth radii of stars based on their ages and chemical compositions. We then compared these birth radii to their current guiding radii and other dynamic parameters.

What Did We Find?

Our results reveal some intriguing patterns:

  1. Metal-rich stars, born in the inner regions of the Milky Way, tend to be churned outward, while metal-poor stars, born in the outer thin disc, show the opposite trend.
  2. About 75% of the stars in our sample have been affected by churning (either inward or outward), while the remaining 25% have either been blurred or remained undisturbed. These proportions vary significantly across different metallicity groups.
  3. There’s a clear age gap between churned and blurred/undisturbed stars. Outward-churned stars are the oldest, inward-churned stars are the youngest, and blurred/undisturbed stars fall somewhere in between.
  4. We also found evidence of the Sagittarius dwarf galaxy’s influence on the most metal-poor stars in our sample, hinting at how external interactions can shape stellar orbits.
  5. Finally, we estimated that the Sun’s most probable birth radius is 7.08 ± 0.24 kpc, which aligns well with previous studies.

One of the most interesting visualizations in the paper is a barplot (Figure 2) that shows the proportion of stars in each metallicity-stratified group that have been churned outward, inward, or have kept their original guiding radii. This figure highlights how different groups of stars have distinct orbital histories.

Figure 2: Stacked barplot showing the distribution of stars across metallicity groups, ordered from most metal-rich (left) to most metal-poor (right). The bars are divided into three segments: Darker shades (right-slanted hatching): Stars that have moved outward from their birth radii; Medium shades (left-slanted hatching): Stars that have moved inward from their birth radii; Lightest shades (vertical hatching): Stars that have remained near their birth radii (within the margin of error). Percentages at the top of each bar indicate the proportion of stars that have moved outward. Credit: Maria Luiza. L. Dantas. This figure is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

Why Does This Matter?

Understanding how stars move and where they were born helps us piece together the history of the Milky Way. It’s like being a cosmic detective, tracing the clues left behind in the chemical compositions and orbits of stars to uncover the story of our Galaxy’s evolution.

But this isn’t just about stars—it’s about us, too. Our own Sun has undergone radial migration, meaning the Solar System itself was born in a different part of the Milky Way and has since moved to its current location. This migration has had profound impacts on Earth’s geological history (Tsujimoto & Baba, 2020) and, consequently, on the evolutionary processes that shaped life on our planet. In a very real sense, the journey of the Sun through the Galaxy has directly impacted the conditions that allowed life to emerge and thrive on Earth.

This study is just the beginning—there’s so much more to explore about the origins and journeys of stars in our cosmic neighbourhood. I’m excited to see where this research leads next!

Text by Maria Luiza. L. Dantas (Reproduced with permission).