Approximately 19,000 light-years from Earth, near the center of the Milky Way galaxy, lies Terzan 5. Astronomers have previously known this celestial system as a globular cluster, but recent research has revealed anomalies that call this classification into question. In September of 2016, a team from Italy officially confirmed that Terzan 5 is not a cluster at all, but instead a fossil of the early Milky Way.
Astronomers used a complex color-analysis system, which places stars at a particular point in their lifespans, to determine that Terzan 5 developed through two phases of star formation. Its initial phase took place approximately 12 billion years ago and generated not only the oldest stars in today's structure, but also more massive stars that then exploded as supernovas and produced gas rich in heavy elements.
The Terzan 5 of 4.5 billion years ago then used these elements to produce a second group of stars. This phase indicates that the ancient Terzan 5 included not only large concentrations of gas but also a mass at least 100 million times that of the Sun.
Researchers have noted that analysis of this process can give the scientific world a more detailed understanding of the Milky Way's development. As a fossilized remnant of the early galaxy, Terzan 5 provides a connection between those ancient processes and the way that galaxies continue to form in the universe today.