In 2006, scientists discovered fossil galaxies, ancient galaxies that left remnants within another galaxy. Sloan Digital Sky Survey discovered the first fossil galaxy Segue 1, one of the smallest galaxies ever found. It only consisted of about 1,000 stars, a microscopic amount compared to the Milky Way’s 100 billion stars.

Astronomer Anna Frebel and her team were convinced of the old age of Segue 1 because of the composition of its stars, which were deficient in heavy elements. Their composition of mainly hydrogen and helium conclude that the stars were born out of gas clouds with only these elements, hinting that they were formed around the time that the first stars in the cosmos emerged from the Big Bang.

Recently, researchers found another fossil galaxy, named Hercules, that also collided with the Milky Way during the infancy of the Milky Way galaxy. Its name, Hercules, derives from the ancient Greek hero who received immortality after completing a series of dangerous tasks. After this fossil galaxy formed 10 billion years ago, its remnants lingered in the Milky Way’s halo (the cloud of stars further away from the galaxy’s brighter center). Hercules’ stars account for one-third of the Milky Way galaxy’s galactic halo.

Although Hercules is larger than Segue 1 (the first discovered fossil galaxy), scientists still consider it to be minuscule compared to the size of the Milky Way. “It is really small in the cosmological context – only 100 million stars,” mentions Dr. Ricardo Schiavon, a scientist at the Liverpool John Moores College’s Astrophysics Analysis Institute.

How were scientists able to differentiate between the stars of Hercules and the Milky Way’s stars if they’re all part of one galaxy now? Researchers identified Hercules’ stars because of their chemical composition (hydrogen and helium) and velocity, whereas the Milky Way’s stars contain heavier elements along with the basic elements.

With the help of the Apache Point Observatory Galactic Evolution Experiment (APOGEE), Schiavon’s team was able to identify the different stars by viewing all of the Milky Way in infrared light, which allowed them to see deeper into the galaxy and observe stars towards the center of the Milky Way. Under visible light, these stars are hidden by clouds of interstellar dust, but infrared light filters through the dust to reveal the galaxy’s core. APOGEE measured the chemical makeup and velocity of the stars so that researchers could separate the stars of the Hercules fossil galaxy from the Milky Way’s stars.

The discoveries of multiple fossil galaxies that collided with the Milky Way early in its 13.5 billion-year-life indicates the galaxy’s significant history. Its eventful past distinguishes it from most spiral galaxies, which usually have calmer early lives. “As our cosmic residence, the Milky Approach is already special to us, however, this historic galaxy buried inside makes it much more particular,” Dr. Schiavon includes.