Astronomers studying an ancient star cluster have discovered a strange gap where certain stars should exist but do not.
The finding provides new clues about what happens deep inside small stars and may help scientists improve measurements of distances across the universe.
The discovery was made by researchers from the Space Telescope Science Institute (STScI) using observations from the European Space Agency’s Euclid space telescope and NASA’s Hubble Space Telescope.
The scientists were originally investigating how stars move within a globular cluster, a dense group of very old stars. Instead, they stumbled upon an unexpected pattern.
When the team sorted the stars by their brightness and color, they noticed a narrow gap where some low-mass stars, known as red dwarfs, appeared to be missing.
Red dwarfs are the most common type of star in the universe and are much smaller and cooler than the sun.
“The discovery was serendipitous,” said astronomer Andrea Bellini. “We were not looking for the gap, but we found it.”
This mysterious feature was first noticed in 2018 when scientists analyzed data from the Gaia space observatory. Gaia mapped hundreds of thousands of nearby stars and revealed a thin gap among red dwarfs on a chart called the Hertzsprung-Russell diagram, or HR diagram.
This diagram is one of the most important tools in astronomy because it helps scientists understand how stars evolve throughout their lives.
The gap appears to be caused by changes occurring inside certain red dwarf stars. Stars with masses between about 34% and 36% of the sun’s mass can experience brief periods of instability in their interiors. During these stages, fuel builds up in their cores and triggers bursts of energy that slightly alter the stars’ size, temperature, and brightness.
Because these changes happen only during a short period of a star’s life, relatively few stars are observed in this state at any given time. As a result, a noticeable gap appears on the HR diagram.
The new study is especially important because it is the first time this feature has been found in a globular cluster. The team examined NGC 6397, one of the closest globular clusters to Earth. Located about 8,000 light-years away in the constellation Ara, the cluster contains hundreds of thousands of stars and is estimated to be around 13.4 billion years old.
Unlike nearby stars observed by Gaia, stars in a globular cluster formed at roughly the same time and are located at nearly the same distance from Earth. This makes them ideal laboratories for studying stellar evolution.
Researchers say the discovery could also help improve distance measurements. Since the gap occurs at a very specific brightness and stellar mass, astronomers can use it as a reference point to estimate how far away a cluster is.
The finding was made possible by combining Euclid’s wide-field observations with precision measurement techniques originally developed for Hubble.
Scientists hope future observations with Euclid and NASA’s upcoming Roman Space Telescope will reveal similar gaps in other star clusters, helping them better understand the hidden processes occurring inside stars.
Source: KSR.
