Blue Lurker Star secrets revealed by Hubble

Unlike our solitary Sun, which stands alone, at least half the stars in our galaxy are in binary systems. A notable example is seen in the Star Wars trilogy, where Luke Skywalker watches two suns set on his home planet, Tatooine. However, there is a system that once contained three co-orbiting stars, and it has a fascinating story. Using Hubble data, astronomers discovered that the stars have had a tumultuous existence.

Image credit: NASA, ESA, Leah Hustak (STScI)

Fast-spinning Blue Lurker Star secrets revealed by Hubble

About 500 million years ago, two of the stars merged into a more massive star, which eventually burned out and collapsed into an unusually massive white dwarf. The third star, a bystander to this chaotic event, siphoned material from the merged star, gaining mass and brightness. Now, this star is alone, orbiting the dead white dwarf. Hubble revealed that this surviving star has an extraordinarily fast spin rate, likely due to it feeding off the gas expelled during the stellar merger.

Evolution of the Blue Lurker Star

Image credit: NASA, ESA, Leah Hustak (STScI)

A Triple Star System Yields an Unusual Surviving Star

The term "blue lurker" might evoke images of a villain from a superhero movie, but it actually refers to a rare type of star that NASA's Hubble Space Telescope observed in the open star cluster M67, about 2,800 light-years away.

Through analysis of Hubble data, scientists uncovered the star's turbulent past, having been part of a triple-star system with two other gravitationally bound stars. The star is similar to "blue stragglers," which are hotter, brighter, and bluer than expected because they result from star mergers.

The blue lurker’s spin rate is much faster than anticipated, a behavior that made it stand out. Visually, it appears similar to a typical Sun-like star, with its color blending in with other solar-mass stars in the cluster. Thus, it "lurks" among the common stars.

The star’s high spin rate provides evidence that it must have siphoned material from a companion star, causing it to spin faster. This was first observed by NASA’s retired Kepler Space Telescope. While normal Sun-like stars take about 30 days to complete one rotation, the blue lurker takes only four.

The process that led to the blue lurker’s current state is described as a "super complicated evolutionary story," according to Emily Leiner of the Illinois Institute of Technology. Originally, the blue lurker rotated slowly and orbited a binary system with two Sun-like stars. Around 500 million years ago, these two stars merged into a much more massive star. This massive star soon became a giant, expelling material onto the blue lurker and accelerating its spin. Today, the blue lurker orbits a white dwarf, the remnant of the stellar merger.

Leiner emphasized that multiple-star systems are relatively common and can lead to fascinating and unique outcomes, though no model yet exists that can reliably connect all the stages of evolution. Triple-star systems make up about 10 percent of Sun-like stars, but the challenges of piecing together their evolutionary histories remain.

Hubble also observed the white dwarf that the blue lurker orbits. Using ultraviolet spectroscopy, Hubble determined that the white dwarf is extremely hot—about 23,000 degrees Fahrenheit, nearly three times the Sun's surface temperature—and has a mass of 0.72 solar masses. This is surprising because white dwarfs in M67 are typically expected to be around 0.5 solar masses, providing evidence that the white dwarf is the product of the merger of two stars in the triple-star system.

"This is one of the few triple systems where we can describe its evolution in such detail," said Leiner. "Triple systems are becoming recognized as important for producing interesting and explosive outcomes, and it is rare to gain such insight into such a system."

Leiner presented her findings at the 245th meeting of the American Astronomical Society in Washington, D.C.

The Hubble Space Telescope, operational for over three decades, continues to make groundbreaking discoveries that enhance our understanding of the universe. A joint project between NASA and the European Space Agency (ESA), Hubble is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland, with mission operations supported by Lockheed Martin Space. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy, handles Hubble's science operations for NASA.

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