STScI Scientists Surprised to Find Brightness ‘Gap’ in Ancient Star Cluster

Scientists from the Space Telescope Science Institute (STScI) in Baltimore, Maryland, sought to study one stellar subject and ended up finding something even more exciting.

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. But 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 some stars' 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."

Understanding the Gap

The presence of this gap in relatively nearby stars was discovered in 2018 by scientists analyzing data from ESA's Gaia observatory. That 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.

On the HR diagram, stellar luminosities are plotted against their colors, which serve as a proxy for their temperatures. The positions of stars on the diagram reveal specific stellar evolutionary stages. Perhaps the most distinctive feature is the swath of main-sequence stars that cuts diagonally across the diagram.

As the precision and sensitivity of modern astronomy improves, astronomers can place stars more accurately on the plot. 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 some 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. Because 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.

Enabling More Accurate Distance Estimates

In the Gaia case, stars were at a multitude of different 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.

Hubble Tools Pave the Way for New Discoveries

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, which were 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. Though 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."

The team's results published today in Astronomy & Astrophysics.

The Space Telescope Science Institute is expanding the frontiers of space astronomy by hosting the science operations center of the Hubble Space Telescope, the science and mission operations centers for the James Webb Space Telescope, and the science operations center for the Nancy Grace Roman Space Telescope. STScI also houses the Barbara A. Mikulski Archive for Space Telescopes (MAST) which is a NASA-funded project to support and provide to the astronomical community a variety of astronomical data archives, and is the data repository for the Hubble, Webb, Roman, Kepler, K2, TESS missions and more. STScI is operated by the Association of Universities for Research in Astronomy in Washington, D.C.