Astronomers see a 'galaxy in a bottle' near the Milky Way's core

A strange clump of stars near the Milky Way's core may be the battered remnant of something far more complex than previously thought.

They call this space region Terzan 5, named after astronomer Agop Terzan, who discovered it in 1968. At first glance, it looks like a dense ball of old stars, the kind astronomers usually label a globular cluster. But new data from NASA's James Webb Space Telescope, combined with two decades of Hubble observations, show it's much more than that.  

Though globular clusters usually have just one ancient star population, Terzan 5 appears to have at least four generations. The first stars were born 12.5 billion years ago, followed by waves dating back to roughly 4.7, 3.8, and 2.5 billion years ago. 

The chemistry inside the cluster also points to at least two kinds of supernovas happening within it. Early, massive stars exploded and enriched the system with heavier elements. Later, so‑called Type Ia supernovas — white dwarfs that blew apart — added more iron. Over time, the levels of lighter elements, such as oxygen and magnesium, dropped. That pattern fits a system that keeps recycling gas and forming new stars.

To astronomers, these clues suggest Terzan 5 started life far more massive, then shed most of its stars into the Milky Way's bulge, the dense swarm of stars around the center of the galaxy. By reconstructing its lost mass, testing whether it hides dark matter or a central black hole, and mapping how the galaxy disassembled it, researchers hope to use Terzan 5 as a sort of Rosetta Stone for studying how galaxies build their central regions.

"This is like a kind of a galaxy in a bottle," said R. Michael Rich, a research astronomer at UCLA, at the 248th meeting of the American Astronomical Society in Pasadena, California. 

Terzan 5 sits about 3,000 light-years from the Milky Way's center, in the crowded region astronomers call the bulge. 

Because Terzan 5 resembles a chunk of the material that helped build the Milky Way’s central bulge, the research team is now calling it a "bulge fossil fragment," said Francesco R. Ferraro, principal investigator for the observations at Italy's University of Bologna, in a statement. 

By comparing about two decades' worth of images, the researchers tracked how individual stars drifted across the sky. True members of Terzan 5 move together, while foreground and background stars wander in different ways. 

"We look at the cluster as if it were a flock of birds, and essentially, there are these birds flying together in the flock, but then there are a lot of other birds flying around kind of randomly," Rich said. "The other birds are stars in the galactic bulge."

Now the team wants to rewind its life story. Today, Terzan 5 weighs about 1 to 2 million times the mass of the sun, similar to a hefty cluster. But the team believes the group started life with far more mass — perhaps on par with 1 billion suns — and slowly lost most of its stars to the Milky Way's bulge. The team plans to use Webb to count low‑mass stars in Terzan 5 and estimate how strongly the galaxy stripped stars away over time.

Astronomers used new data from NASA's James Webb Space Telescope, left, combined with two decades of Hubble Space Telescope observations for the study of Terzan 5. Credit: NASA illustration

They also see hints that something unseen may lurk inside. Earlier measurements suggested that stars along the outer edges of Terzan 5 move faster than the visible matter comfortably explains. That tension helped motivate the Webb study in the first place. 

"We are presently doing detailed analysis to try to find out if we can account for that motion by a population of low-mass stars, or whether we need something else," Rich said. "That something else would possibly be the first globular cluster to host dark matter." 

Another idea points to a massive black hole in the center, which would reveal itself through very fast‑moving stars near the core. So far the team has not seen clear evidence for that.