Astronomers Find a Star System More Complex Than Expected

Astronomers using NASA's two most powerful space telescopes have discovered something unexpected hiding in Terzan 5, a star system near the center of the Milky Way. What scientists had classified as an ordinary cluster of stars turns out to be something far more interesting: it contains four separate generations of stars, born at different times across billions of years. This discovery, announced in June 2026, reveals a previously unknown type of stellar system. NASA/STScI

Most star clusters are relatively simple. When billions of stars form together from a collapsing cloud of gas, they are all born at roughly the same time and contain roughly the same mix of chemical elements — think of it as a single age cohort. ESA/Webb Terzan 5 is different. Four chemically and chronologically distinct generations of stars live within it, which means the system had enough gravitational strength and enough gas in reserve to birth new stars repeatedly over cosmic time.

Scientists believe Terzan 5 is what remains of a much larger star system. Long ago, gravitational forces from the rest of the Galaxy gradually stripped away its outer layers, leaving behind the dense core we observe today. That surviving core kept evolving on its own, which explains why each new generation of stars carries its own chemical fingerprint.

Terzan 5 is located in the Milky Way's bulge — the bright, densely packed region at the Galaxy's center. This region is notoriously hard to study because so many stars crowd the space that their light blurs together, and cosmic dust blocks much of what we try to observe with ordinary telescopes. The James Webb Space Telescope sees in infrared light, which passes through dust more easily. The Hubble Space Telescope provided additional data at other wavelengths. Together, the two telescopes could separate individual stars and measure their ages and chemical composition with enough precision to reveal the four generations. NASA

What makes this discovery valuable is that each stellar generation carries clues about how the Milky Way itself was chemically built. The oldest stars in Terzan 5 formed from pristine gas, then seeded that gas with heavier elements made in their cores. The next generation was born from that enriched gas. And so on. By studying Terzan 5, astronomers gain a historical record of chemical change compressed into one place.

This classification of Terzan 5 as a new type of star system suggests that other examples may exist — or may have existed before gravity tore them apart. Understanding how many of the Milky Way's central structure came from intact, self-enriching fragments versus diffuse accumulation of debris is an open question. The current work supplies a clear template for identifying others.

A deeper point: this result shows what modern telescopes can achieve when they work together. Webb's infrared vision cut through dust. Hubble's optical and ultraviolet measurements filled in gaps neither telescope covers well alone. The two instruments were designed to complement each other, and this discovery is proof of that principle at work.

Public images from both telescopes are available through NASA and ESA. NASA