Vertical Actions in our Galaxy: How Young Stars Grow Up

We often imagine the stars in our galaxy as steady, fixed lights on an eternal canvas. The reality, however, is far more dynamic, especially for young stars. In our recent study, together with researchers from the Max Planck Institute for Astronomy in Germany, we uncovered that young stars in the Milky Way’s disk don’t simply remain stationary or follow predictable orbits. Instead, they can “heat up”—that is, increase their vertical motions as they age.

Young stars in the Milky Way disk are born in thin, nearly circular orbits, staying close to the galactic midplane. They do not remain on these paths forever. Much like a marble rolling on a bumpy surface, stars in the disk are nudged around by exotic features of the galaxy’s gravitational landscape—gas clouds, dark matter, spiral arms, and more. We specifically examined the vertical action of the stars, a quantity related to the star’s up-and-down motion relative to the galactic plane. We found a remarkable regularity that vertical actions, across a wide range of stars, grow almost linearly with age.

Observing Stars and Modeling the “Heating” Process

To get there, we used data from the space telescope Gaia E/DR3, which catalogues the position, motion, and other properties of over a billion stars in our galaxy. Cross-referencing the LAMOST spectroscopic survey, we studied the orbits of bright young stars for which we had estimated ages and 3D positions. Hot, massive stars (known in the field as OBA stars) are ideal for this study because they represent the younger side of the stellar spectrum, still fresh from their formation. This freshness is crucial because the initial orbits of stars can offer insights into the conditions of the stars’ birth environment.

The first challenge was to quantify the increase in vertical action. We designed a model in which stars are assumed to begin their lives in low-vertical-action orbits close to the midplane, then gain vertical motion over time. Using a well-established statistical method for exploring complex probability distributions, the Markov chain Monte Carlo (MCMC) method, we fit this model to our data. This allowed us to track the average vertical action as a function of age across different distances from the galactic center to find a robust, almost-linear relationship between the age of the stars and their vertical action.

Explaining this new relationship was more challenging. One straightforward explanation for the pattern is vertical heating. In this context, “heating” means that stars gain more motion perpendicular to the galactic disk as they interact with their surroundings. For example, giant molecular clouds (GMCs), dense regions of gas and dust, might “bump” stars slightly off their original orbits. Over millions of years, these small perturbations can accumulate, leading to an observable increase in vertical action.

Our model, however, shows that the simple vertical heating explanation  isn’t enough. Our data reveal that the vertical action of stars doesn’t simply increase with age; it also varies depending on a star’s location in the galactic disk. Stars farther from the center tend to exhibit slightly higher vertical actions across all ages. This difference could be due to observational effects, such as galactic dust, or it might reflect a structural feature of the galaxy itself. The outer parts of the galactic disk may not be perfectly flat. That warp could explain some of the increased vertical motion, as stars born farther out may naturally start with a bit more vertical movement. Another possible factor is azimuthal mixing, where stars drift in their orbits around the galactic center, sometimes venturing closer to or farther from the midplane. This gradual redistribution in stars’ positions may give the impression that stars are gaining vertical motion over time.

Insights and Future Questions

Our findings provide a new perspective on the galaxy’s structure, adding to the growing evidence that the Milky Way is a lively, evolving system. The data show a steady growth in vertical action for young stars, but the exact mechanism, whether it’s GMCs, a warped disk, or some other dynamic process, remains an open question. Using future surveys and more precise data, we hope to build on these findings and unravel the combined influences on vertical motion in the galactic disk.

Understanding how young stars “grow up” in this way provides valuable insights into the broader forces shaping the Milky Way. By examining these early stages, we can peel back layers of stellar motion history, revealing the hidden evolution and dynamics of our galaxy.