🌀 Dozens of stellar streams discovered around the Milky Way

At the fringes of our galaxy, a multitude of fine ribbons of stars, previously unnoticed, has just been discovered. This find changes our view of the outer regions of the Milky Way.

A team of astronomers used data from the European Space Agency's Gaia mission. They applied a new algorithm called StarStream, which allowed them to increase the number of identified structures from fewer than 20 to 87 candidates. This method, based on a physical model, proved much more effective than a purely visual search.


Called stellar streams, these ribbons form when compact groups of stars, such as globular clusters, traverse our galaxy's gravitational field. They then leave behind a trail of stars, reminiscent of grains of sand escaping from a hole in a bag. The shape and motion of these trails preserve a record of the gravitational forces experienced over time.

A notable aspect of these new streams concerns their appearance. Far from the image of fine, perfectly aligned ribbons, many appear as shorter, wider, or even misaligned structures compared to the orbit of their original cluster. This diversity of shapes could explain why previous searches, targeting the sharpest structures, missed them.

Led by Yingtian Chen from the University of Michigan, the study reports that some low-density globular clusters are losing their stars at a high rate. This phenomenon could indicate that they are on the path to complete disruption due to galactic tidal forces. The team notes, however, that some detections among the 87 candidates are less certain due to contaminating stars in the background.

The next steps will rely on next-generation observatories. The future Vera C. Rubin Observatory, NASA's Nancy Grace Roman Space Telescope, and the DESI instrument will be able to test these results. The researchers believe their StarStream algorithm will adapt easily to data from these missions.

By mapping the mass distribution associated with these streams, including their invisible component, astronomers hope to better understand the evolution of the Milky Way. The details of this research are available in an article in The Astrophysical Journal.

Globular Clusters: Fossils of the Young Universe

Globular clusters are very dense spherical groupings that can contain hundreds of thousands, even millions, of stars. They orbit around the centers of galaxies, like satellites. More than a hundred are known in the Milky Way. Their major characteristic is their extremely advanced age, most having formed over ten billion years ago, shortly after the birth of the Universe.

These clusters are considered cosmic fossils. Their very old stars, poor in heavy elements, inform us about the conditions in the early ages of galactic formation. Studying their composition and dynamics allows us to travel back in time and understand how the first stellar structures assembled.

During their long lives, globular clusters interact with the gravitational field of their host galaxy. These interactions, particularly tidal forces, can tear stars away from the cluster, giving rise to the observed stellar streams. The rate at which a cluster loses its stars depends on its density, its orbit, and the mass distribution of the galaxy, including its invisible component.

Thus, observing a stellar stream linked to a still-intact globular cluster is particularly instructive. It allows for a direct comparison between the stream of stars and the parent cluster, providing a measure of the forces at play.