7% of the Stars in the Milky Way’s Center Came From a Single Globular Cluster That Got Too Close and Was Broken Up
The heart of the Milky Way can be a mysterious place. A gigantic black hole resides there, and it’s surrounded by a retinue of stars that astronomers call a Nuclear Star Cluster (NSC). The NSC is one of the densest populations of stars in the Universe. There are about 20 million stars in the innermost 26 light years of the galaxy.
New research shows that about 7% of the stars in the NSC came from a single source: a globular cluster of stars that fell into the Milky Way between 3 and 5 billion years ago.
Humans have been looking up at the gigantic starry arch of the Milky Way since the dawn of our species. But it’s only in the last century or so that astronomers discovered the galactic center. And only in recent decades, aided by the development of adaptive optics and other technologies, has the region begun to surrender its secrets, including the presence of a supermassive black hole.
The center of the Milky Way is still a region of intense astronomical study. Astronomers know that galaxies like the Milky Way grow through mergers with other galaxies, and by enveloping smaller galaxies. The Milky Way is feeding on two of its satellites, the Large and Small Magellanic Clouds. Currently, it’s consuming the gaseous halo that surrounds both dwarf galaxies, but over time, it will eventually take in the dwarf galaxies’ stars, too.
So it shouldn’t be surprising to find a distinct population of stars in the galactic center that was at one time its own, separate cluster, before being taken in by the Milky Way.
Researchers at multiple institutions around the world are working hard to understand the Milky Way in more detail, including how it grows and evolves over long time periods. Recently, a group of astronomer associated with the European Southern Observatory (ESO) and the Max Planck Institute for Astronomy (MPIA) made some progress. They observed the galactic center with the Very Large Telescope (VLT) and that data has led to some new discoveries. Three teams of researchers have published new papers on different aspects of the galactic center and the newly-discovered population of stars that resides there.
The three studies are:
- Asymmetric spatial distribution of subsolar metallicity stars in the Milky Way nuclear star cluster. Lead author is Anja Feldmeier-Krase from the ESO. Published in the Monthly Notices of the Royal Astronomical Society.
- On the Origin of a Rotating Metal-poor Stellar Population in the Milky Way Nuclear Cluster. Lead author is Manuel Arca Sedda of the Astronomisches Rechen Institut, Zentrum fur Astronomie der Universitat Heidelberg. Published in The Astrophysical Journal Letters.
- Revealing the Formation of the Milky Way Nuclear Star Cluster via Chemo-dynamical Modeling. Lead author is Tuan Do from the UCLA Division of Physical Sciences, LA. Published in The Astrophysical Journal Letters.
The galactic center is hard to observe because of the Zone of Avoidance (aka the Zone of Obscuration.) It’s a region so full of dust that it blocks visible light. It takes special instruments and telescopes that see in other wavelengths, especially infrared, to observe it. The VLT in Chile is able to peer into this region.
In “Asymmetric spatial distribution of subsolar metallicity stars in the Milky Way nuclear star cluster,” the team of researchers analyzed the speed, motion, and chemical composition of 700 stars in the galactic center.
Populations of stars aren’t only delineated by the way they move through space together. Their compositions also tell astronomers a lot about their shared origins. Specifically, their metallicity.