Brightness ‘Gap’ Found in Ancient Star Cluster | Space Telescope Science Institute
Space Telescope Science Institute (STScI) scientists in Baltimore, Maryland, using the Euclid Space Telescope, have for the first time found a red-dwarf brightness “gap” feature in the population of a globular cluster—an ancient, crowded collection of stars. A similar gap was first identified in data from the Gaia observatory of nearby stellar populations. However, it has never before been detected in a globular cluster. The gap provides clues to processes happening deep within the stars’ interiors.
This finding would not have been possible without the software and techniques originally developed at STScI for NASA’s Hubble Space Telescope over more than two decades. These tools allowed the team to push the limits of Euclid, and in the future, the Roman Space Telescope.
Image Description: This astronomical image is speckled with thousands of stars against the black background of space. Most stars are visible only as white pinpoints. The great majority of stars are crowding the image’s center that appears as an irregular round shape. Most stars in the center of this cluster are whiter, and those along periphery edges yellower. The stars across the entire image range in color from blue to white to yellowish red. A few of the stars appear a bit larger and brighter than the rest with six diffraction spikes.
NGC 6397 (also known as Caldwell 86), pictured here, is a globular cluster in the constellation Ara that was discovered by French astronomer Nicolas-Louis de Lacaille in 1752. It is located about 7,800 light-years from Earth, making it one of the two nearest globular clusters to Earth (the other one being Messier 4). The cluster contains around 400,000 stars, and can be seen with the naked eye under good observing conditions. NGC 6397 is one of at least 20 globular clusters of the Milky Way Galaxy that have undergone a core collapse, meaning that the core has contracted to a very dense stellar agglomeration.
Using data from the European Space Agency’s (ESA’s) Euclid space telescope and NASA’s Hubble Space Telescope, the team planned to analyze the motions of stars within an ancient collection of stars called a globular cluster. However, what they found when they grouped the cluster’s stars by brightness and color as observed by Euclid was a thin “gap” of expected but missing low-mass stars called red dwarfs. This gap is thought to be linked to changes occurring within star interiors, giving astronomers a glimpse at processes happening inside stars even from thousands of light-years away.
This is the first time the gap feature was discovered in a globular cluster. “The discovery was serendipitous,” said STScI’s Andrea Bellini, one of the research paper’s primary authors. “We were not looking for the gap, but we found it.”
The presence of this gap in relatively nearby stars was discovered in 2018 by scientists analyzing data from the European Space Agency’s Gaia observatory. This team plotted nearly 250,000 stars from the Gaia archive on a Hertzsprung-Russell (HR) diagram, one of the most important tools in stellar studies. This is the graph that astronomers use to classify stars and trace their life cycles.
The Gaia data revealed a previously unknown feature—a narrow, diagonal slice of mostly missing stars through the main sequence in the middle of the red dwarf region.
So what causes this gap? It appears that in red dwarf stars, fuel built up in their centers can trigger an energy burst that results in structural instability in a star’s interior. Between 0.34 and 0.36 times the mass of the Sun, red dwarfs undergo small variations that change their size, brightness, and temperature. Since only a small number of stars are undergoing these changes, there is a dearth of red dwarfs with these specific brightnesses. This is reflected in the HR diagram as a gap.
In the Gaia case, stars were at a multitude of distances and had varying ages, histories, and chemical compositions. In contrast, stars within a globular cluster share a common history, having formed in the same environment at roughly the same point in cosmic time.
“Globular clusters are the ideal laboratories to study stellar evolution and stellar populations,” said STScI’s Massimo Griggio, the principal author on the research paper. “In this globular cluster, the stars are basically at the same distance and have approximately the same age.”
The STScI team used Euclid to study NGC 6397, one of the closest globular clusters to Earth. Located approximately 8,000 light-years away in the southern constellation Ara, it contains hundreds of thousands of stars and is estimated to be 13.4 billion years old.
“Because we can determine the brightness where the gap is with very high precision and know for what stellar masses it occurs, we can use this information to estimate the cluster’s distance,” said STScI’s Russell Ryan, another of the primary researchers.
Gaia found the gap while viewing stars in the local neighborhood, which are typically younger than stars in globular clusters. Now, the Euclid team found the exact same process happening in more distant stellar interiors.
This finding would not have been possible without the software and techniques originally developed at STScI for NASA’s Hubble Space Telescope over more than two decades. The team used these tools, pioneered primarily by STScI’s Jay Anderson, to make the high-precision measurements needed to detect this feature in the extremely crowded environment of a globular cluster. Although Hubble’s field of view is much, much smaller, when these tools were coupled with Euclid’s panoramic view, the gap clearly appeared.
“With these tools, we show that we can push the limits of Euclid, and in the future, the Roman Space Telescope, across a wide field of view,” said team member Mattia Libralato, formerly of STScI and currently with the Italian National Institute for Astrophysics (INAF) in Padova, Italy. “Further investigations with Euclid and, in the future, Roman, will hopefully allow us to better characterize this feature also in other globular clusters.”
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Image Processing: Jean-Charles Cuillandre (CEA-Saclay), Giovanni Anselmi (ESA)
Release Date: June 3, 2026
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