NASA has identified a rare and unusual “blue lurker” star produced during the collapse of a triple-star system in the M67 star cluster.
Due to its remarkable speed, the star hides 2,800 light years away and is visible through NASA’s Hubble Space Telescope. Astronomer Dr. Emily Leiner of the Illinois Institute of Technology in Chicago led the work presented at the 245th Meeting of the American Astronomical Society in Washington, D.C.
Blue Lurkers And Stragglers
The blue lurkers are a subset of “blue stragglers,” stars that are hotter, brighter, and emit more blue light than would typically be expected. What separates the lurkers is their ability to blend in with other stars, if not for their telltale high spin velocity.
Scientists attribute the blue stragglers’ greater intensity to mergers between stars, and in this case, the lurker’s trademark speed caught scientists’ eye and led to its identification.
From its high spin rate, astronomers deduced that the blue lurker absorbed material from its companion star, leading it to increase its spin velocity. NASA’s Kepler space telescope, now retired, first discovered the high spin velocity. The lurker completes a rotation in only four days; compared to the roughly 30 days, it takes more sun-like stars.
Formation of a Blue Lurker
The blue lurker results from a “super complicated evolutionary story,” Leiner said. “This star is really exciting because it’s an example of a star that has interacted in a triple-star system.”
Initially, the blue lurker rotated and orbited a binary system of two sun-like stars at a far more leisurely pace. Roughly 500 million years ago, those binary stars merged into a single massive star. The product of that merger swelled into a giant star, which cast off some of its mass into the star orbiting the binary system, turning the orbiter into a blue lurker. Accepting this material led to the lurker spinning at an increased rate as it orbits the white dwarf star representing the remaining husk of that cosmic merger.
“We know these multiple star systems are fairly common and are going to lead to really interesting outcomes,” Leiner explained. “We just don’t yet have a model that can reliably connect through all of those stages of evolution. Triple-star systems are about 10 percent of the Sun-like star population. But being able to put together this evolutionary history is challenging.”
An Unusual White Dwarf
NASA trained the Hubble Space Telescope on the lurker’s white dwarf companion. The telescope’s ultraviolet spectroscopy capabilities discovered that the white dwarf runs hot, about three times the surface temperature of Earth’s Sun.
Intriguingly, the white dwarf has 0.72 solar masses, although generally, hot white dwarfs are only expected to have around 0.5 solar masses within M67. Astronomers believe this suggests that the white dwarf once belonged to a three-star system.
“This is one of the only triple systems where we can tell a story this detailed about how it evolved,” said Leiner. “Triples are emerging as potentially very important to creating interesting, explosive end products.”
“It’s really unusual to be able to put constraints on such a system as we are exploring.”
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.