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

The stunning image, taken ’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 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. An image of the globular cluster NGC 6397, captured , shows a dense, glittering swarm of hundreds of thousands of ancient stars packed into one of the Milky Way's closest stellar clusters. (, NASA, Euclid Consortium)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 a narrow shortage of red dwarfs at specific brightness levels.