'Galactic fossil' of early Milky Way discovered

(CNN) At 12 billion years old, Terzan 5 looks pretty good for its age.

And for the curious, this living galactic fossil looks nothing like Knowhere, the gigantic celestial head on the edge of the universe seen in Marvel's "Guardians of the Galaxy" film. Rather, this fossil is made of gas and stars and exists 19,000 light-years from Earth.

Since its detection 40 years ago, scientists acknowledged Terzan 5 was a globular cluster in the Milky Way galaxy -- until now. An international team of astronomers using cameras on the Hubble Space Telescope, as well as a group of ground-based telescopes, noticed something else. Their findings were released in a study this week.

They believe Terzan 5 is really a stellar system that represents a fossilized relic of galaxy formation, perhaps one of the earliest building blocks of our galaxy. But it is also unlike any other galactic fossil known.

Galactic fossils like globular clusters are timestamps pointing back to when the galaxies initially formed. Galactic globular clusters can be found in the halo of our Milky Way, as well as the bulge at its center, and can be traced back 12.5 billion years.

"In these fossils is written the history of the formation of the first cosmic structure at the time when the universe was a baby, just 1 billion years old," said Francesco Ferraro, lead study author and professor in the physics and astronomy department at the University of Bologna in Italy.

But Terzan 5 is different. Within its gaseous clump, there are stars from two time periods, 12 billion years ago and then 4.5 billion years ago. The stars are also comprised of different elements. All this points to the fact that rather than a continuous star formation process, there were two bursts of star formation with a 7 billion year gap.

So how did this happen?

Fluctuations in size

After the first burst of star formation, there was a supernovae explosion. But at the time, Terzan 5 was so massive that rather than ejecting the iron-enriched gas created by the explosion, it retained that gas to form a new generation of stars 7 billion years later, Ferraro said.

Terzan 5 didn't start out massive and it isn't now, which means it has fluctuated in size over time. It was most likely 100 million times the mass of the sun to produce both bursts of star formation. Now, it is just a few million times the mass of the sun, orbiting the bulge at the center of the Milky Way.

What also surprised researchers is that even though Terzan 5 is depleted in mass, enough so that its former size is referred to as an "ancestor," it is still intact as a galactic fossil.

"Current models of galaxy bulge formation assume that vast clumps of gas and stars interacted to form the primordial bulges of galaxies, merging and dissolving in the process," Ferraro said. "The progenitor of Terzan 5 might have been one of these clumps which survived complete destruction."

To remain largely undisturbed for billions of years while hanging out in the dangerous zone at the center of the Milky Way, full of disruptive structures, isn't easy, which could point to how it decreased in mass.

Ferraro believes this living fossil still exists because if it was originally less massive and more compact than the other clumps, it didn't sink and merge into the central region of the bulge. Rather, it got bounced around and out as it interacted with other structures. He and his team want to investigate this and other potential answers more as they continue their research.

Studying this building block of the galaxy can provide astronomers better insight when trying to answer the questions surrounding the mysteries and complexities of galaxy formation, both within the Milky Way and across the universe.

Ferraro and his team will continue looking for other stellar systems that might also be hiding in the Milky Way to see what other secrets these ancient building blocks at the center of our galaxy might contain.