‘Blue Stragglers’ in Space May Mask True Age of Star Clusters in Hubble Views

By spying on star clusters in a nearby galaxy, the Hubble Space Telescope has shown that star clusters aren't always as old as they appear.

The long-running observatory turned its attention to collections of stars in the Large Magellanic Cloud (LMC), a satellite dwarf galaxy of the Milky Way located about 160,000 light-years from Earth. (A light-year is the distance light travels in a year, or roughly 6 trillion miles or 10 trillion kilometers).

In the LMC, astronomers observed the behavior of bright, blue stars known as "blue stragglers." These stars receive a bunch of extra fuel from other stars in their environment, which makes them bigger and brightens them up. This usually happens when a star steals gas from a neighbor or two stars crash into each other.

An artist's impression of "blue stragglers," bright, blue stars with masses that are higher than average for stars that belong to a cluster. Blue stragglers sink toward the center of a star cluster over time. (Image credit: ESA/Hubble/NASA/L. Calçada)

These stragglers revealed something interesting about how star clusters evolve, the researchers said. Because they are so bright, they are easy to track even if they are buried deep within the group of stars. They thus make a handy reference for star movement in a star cluster, which can consist of up to 1 million stars.

This infographic shows two ways that "blue stragglers" can form in star clusters. In one way, two low-mass stars collide head-on, combining into a single star (shown here on the left). In the other way, known as the "vampire" model, a lower-mass star drains its more massive companion of hydrogen. (Image credit: ESA/Hubble/M. Kornmesser)

The mutual gravitational pull between stars in a cluster tends to change the cluster's structure over time, a process that astronomers call "dynamical evolution." Specifically, heavy stars sink toward the middle of the cluster and low-mass stars flee to the outskirts, Hubble officials said in a statement.

An image from the Hubble Space Telescope reveals the globular star cluster NGC 1466, located in the Large Magellanic Cloud. (Image credit: ESA/Hubble/NASA)

The "stragglers" showed, for the first time, that star clusters can be different physical shapes even if they formed during the same period. That's because the varying pull between stars "causes a progressive contraction of the cluster core over different time scales," Hubble officials said, "and means that star clusters with the same chronological age can vary greatly in appearance and shape because of their different 'dynamical ages.'"

The research was led by Francesco Ferraro, an astrophysicist at the University of Bologna in Italy. The work was published Sept. 9 in the journal Nature Astronomy.

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Elizabeth Howell (she/her), Ph.D., was a staff writer in the spaceflight channel between 2022 and 2024 specializing in Canadian space news. She was contributing writer for Space.com for 10 years from 2012 to 2024. Elizabeth's reporting includes multiple exclusives with the White House, leading world coverage about a lost-and-found space tomato on the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?" (ECW Press, 2022) is co-written with astronaut Dave Williams.