Terzan 5. Astronomers classified it as a globular cluster since its detection about 40 years ago. But a new team of astronomers took another look at it. And it’s unlike any globular cluster they have ever seen.

What makes Terzan 5 different? Astronomers found convincing evidence of two distinct types of stars in the globular cluster. What surprised astronomers the most was the age discrepancy between the two types of stars. About 7 billion years separates the two stellar populations. The older stars are estimated to be 12 billion years old. While the younger set is just 4.5 billion years old.

The big takeaway? The age gap points to star formation not being continuous. Instead, Terzan 5 saw two separate bursts of star formation. “This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive. At least 100 million times the mass of the Sun,” says Davide Massari, co-author of the new study.

Massari and his fellow astronomers believe what makes Terzan 5 unusual could make it the perfect candidate for a living fossil from the Milky Way’s earliest days. Before this study, theories on galaxy formation believed the huge clumps of gas and stars merged to form the primordial bulge of the Milky Way. Any excess gas dissolved in the process.

“We think that some remnants of these gaseous clumps could remain relatively undisrupted and keep existing embedded within the galaxy,” explains Francesco Ferraro from the University of Bologna, Italy, and lead author of the study. “Such galactic fossils allow astronomers to reconstruct an important piece of the history of our Milky Way.”

Terzan 5 might not be the typical globular cluster, but similar properties are found in the galactic bulge.

“Terzan 5 could represent an intriguing link between the local and the distant Universe, a surviving witness of the Galactic bulge assembly process,” says Ferraro.

How do astronomers figure this out?

They don’t just grab the best telescope they can find and train it on their target. Though, that does help. Ferraro and his colleagues tapped several telescopes to help learn more about the unique globular cluster. Let’s take a quick peek at which telescopes they used.

Very Large Telescope array – It’s aptly named. Sitting atop Cerro Paranal in the Atacama desert of northern Chile, the VLT has unparalleled views of the night’s sky. Dubbed the “world’s most advanced optical instrument,” the VLT actually consists of four “Unit Telescopes.” Each with a main mirror measuring 8.2 meters in diameter. All four can work together as a giant interferometer. This gives astronomers the ability to see details up to 25 times finer than just using an individual telescope.

For Terzan 5, astronomers looked at data from the Multi-conjugate Adaptive Optics Demonstrator.

W.M. Keck Observatory – One of the best telescopes to hunt for ancient galaxies, Keck puts in plenty of work looking at other targets. Data from the NIRC2 (Near-Infrared Camera) helped astronomers tease new details from Terzan 5.

Hubble Space Telescope – It needs no introduction. It’s the most famous telescope out there. Ask anyone to name a telescope, and Hubble springs to mind. The team of astronomers used data from the Wide Field Camera 3 and the Advanced Camera for Surveys.