Glittering star cluster image reveals missing patch of stars: 'We were not looking for the gap, but we found it'

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A gorgeous new portrait of an ancient star cluster reveals an unexpected imperfection: a visible gap in the brightness of its stars.

The stunning image, taken by the European Space Agency's (ESA) Euclid space telescope, captures NGC 6397 — one of the closest globular clusters to Earth. NGC 6397 is a glittering swarm of hundreds of thousands of stars packed tightly together, but when astronomers took a closer look, they discovered something unusual hidden within the sparkles.

A graph plotting the stars by their brightness and color revealed a conspicuous gap — a narrow region where certain stars should have appeared but didn't. The feature is so distinct that it stands out visibly in the data, appearing almost like a blemish in an otherwise smooth distribution of stars, according to a statement from the Space Telescope Science Institute (STScI).

The discovery emerged from observations collected by Euclid, which is primarily tasked with investigating dark matter and dark energy. Initially, the team was studying the motions of stars within the globular cluster using data from both Euclid and the Hubble Space Telescope. When analyzing the data from NGC 6397, the researchers weren't searching for missing stars. Instead, they stumbled across the feature while studying the cluster's stellar population.

"The discovery was serendipitous," Andrea Bellini, one of the research paper's primary authors from STScI, said in the statement. "We were not looking for the gap, but we found it."

The gap occurs among red dwarf stars, the most common type of star in the Milky Way. Researchers believe the visible void is linked to changes deep within the stars as they transition from having partially convective interiors to becoming fully convective. That shift slightly alters the stars' structure and luminosity, leaving relatively few stars at certain brightness levels.

The idea that stellar populations can contain small "missing" ranges of stars first emerged in 2018, when ESA's Gaia mission revealed a subtle gap in the brightness distribution of hundreds of thousands of nearby stars. Plotted on a Hertzsprung–Russell (HR) diagram, the data suggested that even in large stellar populations, stars do not always fall into perfectly smooth patterns.

The new Euclid observations build on that idea by identifying a similar feature inside the globular cluster NGC 6397, which is a tightly packed, roughly spherical collection of stars bound together by gravity, often found in the outskirts of galaxies and containing some of the oldest known stars. Using a HR diagram, the team again mapped stars by their luminosity and color and discovered a narrow shortage of red dwarfs at specific brightness levels.

Because relatively few stars pass through this brief transitional stage in their evolution, there is a corresponding dip in their numbers at those luminosities. On the HR diagram, that shortage appears as a thin gap cutting through the otherwise continuous band of stars.

"Globular clusters are the ideal laboratories to study stellar evolution and stellar populations," Massimo Griggio, lead author of the study from STSc, said in the statement.

The precise brightness where the gap appears, along with the properties of the stars involved, helps astronomers estimate how far away the cluster is. NGC 6397, an ancient globular cluster about 13.4 billion years old, lies roughly 8,000 light-years away in the constellation Ara, the researchers said.

This marks the first time astronomers have identified the phenomenon in a globular cluster, providing a new opportunity to test models of stellar evolution using one of the galaxy's oldest and most densely populated stellar systems.

Their findings were published May 12 in the journal Astronomy & Astrophysics.