24 Million Stars in New Stellar Catalog Could Unlock Milky Way's Secrets

An international team of astronomers has published a new catalog containing over 24 million stars which could be used to unlock the secrets of the chemical evolution of our galaxy.

The team, including University of Notre Dame professor of physics Timothy Beers, used a new approach to measure light from each of the stars and determine the amounts of heavy metals like iron, as well as determining their distances, ages, and motion through space.

The catalog of 19 million dwarf stars and 5 million giant stars increases our knowledge of stars formed very early in the history of the universe that contain virtually no elements heavier than hydrogen or helium. Beers, one of the authors of a paper published in The Astrophysical Journal that details the construction of this new star catalog, calls these "fossils of the night sky."

These cosmic fossils could give us a clue of how elements that make up the Milky Way's stars, and ultimately everything around us, were formed and distributed. This research brings the known number of these early stars to more than half a million. When Beers began his work in the early '80s, we knew of just 20 of these stellar fossils.

Beers has spent much of his professional life planning and executing ever-larger surveys of stars to decipher the galaxy's formation and chemical evolution. This field is known as galactic archaeology and it traces the history and formation of the Milky Way from detailed observations of the stars, gas, and other cosmic structures.

Beers said in a press release from Notre Dame: "The elemental abundances of individual stars trace the chemical enrichment of the Milky Way galaxy, from when it first began to form stars shortly after the Big Bang to the present."

Beers said that by combining this information with the stellar distances and motions, galactic archaeologists can constrain the origin of different components of galaxies.

This includes stars in areas of the galaxy, including the halo region that surrounds globular clusters and those in the thick and thin disks that form important components of the structure of spiral galaxies like our own.

Beers continued: "Adding age estimates puts a 'clock' on the process so that a much more complete picture of the entire process can be drawn."

Milky Way Disk
A stock illustration of the disk of the Milky Way. Astronomers have created a new massive catalog of ancient stars that could reveal the secrets of the Milky Way. alex-mit/GETTY

Beers and his colleagues' previous work had obtained information on tens of thousands of stars and helped them perfect this new approach which is based on visible light samples, called photometric samples obtained with the Australian SkyMapper Southern Survey and the European Gaia satellite mission.

What Beers and his team wanted to calibrate specifically was estimates of "metallicity" in stars, which is the abundance of elements heavier than helium (astronomers classify any element heavy than helium as a "metal").

Until now, however, the only way to estimate the proportions of heavy metals like iron in stars was by analyzing their low- and medium-resolution light output or spectra. This was a long and painstaking process.

Of particular interest to the Notre Dame researcher are stars with a metallicity lower than one percent or "metal-poor" stars. These were formed early in the history of our 13.8 billion-year-old universe and fused together hydrogen and helium to make heavier still elements like nitrogen and nitrogen.

When this first generation of stars ran out of fuel they collapsed, triggering supernova explosions that spread these heavier elements throughout the universe. These would go on to form the building blocks of the next generation of stars.

In addition to its benefit to scientists who aim to understand the structures of galaxies and the distribution of elements, the catalog of stars could also assist researchers who aim to identify the trails of stars left behind from disrupted dwarf galaxies and globular clusters.

As impressive as the catalog built by Beers, his colleagues at Notre Dame, and researchers from China and Australia, is in listing 24 million stars, this represents just one-hundredth of a percent of the 240 billion stars in the Milky Way alone meaning there is a lot more cataloging to be done.

Despite this, Beers and his colleagues begin their paper by pointing out that thanks to large star surveys, we are now in the "golden era" of galactic archeology.