Euclid telescope finds a missing patch of stars in globular cluster NGC 6397
Summary: A 4 June 2026 release from the Space Telescope Science Institute (STScI) reports that ESA's Euclid space telescope has identified a conspicuous gap in the brightness–color (Hertzsprung–Russell) diagram of stars in NGC 6397, a globular cluster about 7,800 light-years from Earth. The gap appears in the red-dwarf sequence and is consistent with the structural transition from partially convective to fully convective stellar interiors — a brief phase in which stars change their luminosity and surface temperature and are therefore under-represented in any narrow brightness bin. The team did not set out to look for the gap; it surfaced while they were analyzing stellar motions.
A complete-looking diagram, with a single blank stripe
NGC 6397 lies in the constellation Ara and hosts hundreds of thousands of stars bound together by gravity, with an estimated age of about 13.4 billion years. When the team combined Euclid and Hubble Space Telescope photometry and plotted each star by brightness and color (a proxy for surface temperature), the red-dwarf sequence that should have been a smooth, continuous band of points instead showed a narrow vertical stripe where stars are nearly absent.
In a press release, STScI described the visual impression: a single brightness slot in the otherwise smooth distribution, like a missing square in a tessellated floor. Adjacent brightness bins are well populated; the gap itself is conspicuous because the red-dwarf sequence is statistically the most populated part of the H-R diagram for an old, metal-poor cluster like NGC 6397.
A possible "gear shift" inside low-mass stars
The explanation the team favors involves the internal structure of red dwarfs, the most common type of star in the Milky Way, with masses between roughly 7.5% and 50% of the Sun's. The lowest-mass red dwarfs begin their lives with partially convective interiors — meaning the outer layers and the core are not fully mixed. As mass decreases or as the star evolves, it eventually transitions to a fully convective state, in which material circulates throughout the interior.
The transition between these two regimes produces a small but discontinuous change in the star's radius, effective temperature, and luminosity. The team argues that the missing brightness slot in NGC 6397's H-R diagram corresponds to stars caught in this brief transitional phase. Because their "dwell time" at that exact brightness is very short, very few stars are seen there at any given moment, even though the cluster contains hundreds of thousands of red dwarfs.
A serendipitous byproduct of a kinematic survey
Euclid's primary science mission is to map the three-dimensional distribution of dark matter and dark energy across extragalactic space. NGC 6397 was imaged as a calibrator and ancillary target within Euclid's wide survey. The team's actual research goal was to measure proper motions — the tiny angular shifts of each star against the background sky — by combining Euclid's sharp imaging with archival Hubble data.
The brightness gap was not a planned target of that analysis. It emerged when the team was characterizing the cluster's stellar population, and the gap's location on the H-R diagram was sufficiently distinct to demand an explanation.
"The discovery was serendipitous. We were not looking for the gap, but we found it."
— Andrea Bellini, lead author, STScI
Data scale and follow-up
Hundreds of thousands of stars within NGC 6397 were captured by Euclid's high-resolution imager, providing the statistical basis for the gap detection. The team's next step is to check whether the same gap signature appears in other globular clusters in Euclid's wide survey, and whether the boundary brightness matches the theoretical prediction for the partially-to-fully convective transition in metal-poor old populations. If the signatures align, the gap becomes a new observational probe that can be used to map the location of this structural transition in distant clusters where individual stars cannot be resolved at high signal-to-noise.
Significance
Globular clusters are a benchmark for stellar evolution models: stars in a single cluster share age, metallicity, and distance, which makes small statistical deviations in their H-R diagrams easier to attribute to specific physical mechanisms. The NGC 6397 gap shows that even well-studied, nearby clusters still harbor detail visible only with Euclid's photometric precision and high dynamic range. The same diagnostic will also help Euclid's main dark-matter-mapping analyses correct for red-dwarf physics in luminosity-function estimates.