Hubble recently captured a dazzling image of the heart of globular cluster Terzan 9, a bright ball of stars 23,000 light years away.

Globular clusters are tightly packed groups of tens of thousands — or even hundreds of thousands — of stars, bound by gravity into a dense glittering sphere about 100 light years wide. They’re among the oldest objects in the galaxy; every globular cluster astronomers have studied so far has been long past its star-forming years. But exactly how globular clusters form remains an open question.

One of the most accepted ideas about globular clusters’ origins is that they’re the remnants of dwarf galaxies, captured long ago by the Milky Way’s irresistible gravity. That would explain why so many of them have retrograde, or backwards, orbits.

The dwarf galaxy hypothesis would also explain another weird property of globular clusters. Most star clusters formed in a single cosmic event when their stars coalesced from a dense cloud of gas and dust. But globular clusters are different; they contain stars of several different ages. Mapping the ages of those stars could help astronomers better understand the origins of globular clusters.

Our galaxy contains about 150 globular clusters. Many of them, including Terzan 9, are in the central part of the galaxy, a cosmic neighborhood that’s heavily populated with stars and clouds of interstellar dust.

The dust absorbs most wavelengths of light, so for astronomers, it’s like a filter that changes the brightness and color of the stars behind it. That’s a problem, since a star’s brightness and color tell astronomers its age and what it’s made of. But Hubble’s instruments are sensitive enough to pick out the effects of the dusty filter and reveal the stars’ true colors.

Hubble captured this image as part of a series of observations of globular clusters near the center of our galaxy, using its Wide Field Camera 3 and Advanced Camera for Surveys.