The term “blue lurker” might evoke images of a villain from a comic book, but it actually refers to a rare type of star that was studied by NASA’s Hubble Space Telescope in the open star cluster known as M67, located about 2,800 light-years from Earth.
Investigations using Hubble data reveal that this star has experienced a chaotic history, having interacted gravitationally with two other stars in a fascinating triple-star configuration. It shares similarities with “blue stragglers,” which are stars that appear hotter, brighter, and bluer than average, likely due to stellar mergers.
The blue lurker has an unexpectedly high spin rate, which played a key role in its identification. Visually, it resembles a typical Sun-like star. The “blue” label can be misleading, as its color blends in with other stars of similar mass in the cluster, hence it is described as “lurking” among the more common stars.
The high spin rate indicates that the blue lurker likely absorbed material from a nearby companion star, resulting in an increased rotation speed. This rapid spin was first detected using NASA’s now-retired Kepler space telescope. While average Sun-like stars take about 30 days to complete a rotation, the blue lurker achieves this in just four days.
Emily Leiner from the Illinois Institute of Technology in Chicago describes the origins of the blue lurker as a “super complicated evolutionary story.” She notes that this star is particularly fascinating because it has interacted within a triple-star environment. Initially, the blue lurker spun more slowly and was part of a binary system with two Sun-like stars.
Approximately 500 million years ago, one of the stars in that binary system merged with the other, forming a more massive star. This new giant subsequently expanded, transferring some of its material to the blue lurker, accelerating its rotation. Currently, the blue lurker orbits a white dwarf, which is the remnant of the massive star created by the merger.
“We know that multiple star systems are relatively common and can lead to fascinating phenomena,” Leiner stated. “However, we still lack a reliable model to describe all the evolutionary stages. Triple-star systems make up about 10 percent of Sun-like stars, so piecing together this evolutionary narrative is quite complex.”
Hubble also observed the white dwarf that the blue lurker orbits. Through ultraviolet spectroscopy, it was determined that this white dwarf is extremely hot, reaching temperatures up to 23,000 degrees Fahrenheit (about three times the temperature of the Sun’s surface) and has a mass of 0.72 solar masses. Theories suggest that hot white dwarfs in M67 should have a mass closer to 0.5 solar masses, indicating that this white dwarf is indeed the result of a stellar merger within the original triple-star system.
“This is one of the few triple systems where we can narrate such a detailed evolutionary history,” remarked Leiner. “Triple systems may play a significant role in creating unique and explosive outcomes. It’s rare to be able to impose constraints on such systems as we are doing now.”
Leiner is set to present her findings at the 245th meeting of the American Astronomical Society in Washington, D.C.
