Free Astronomy Magazine March-April 2025

MARCH-APRIL 2025 E volution of a “blue lurker” star in a triple system. Panel 1: A triple star system containing three Sun-like stars. Two are very tightly orbiting. The third star has a much wider orbit. Panel 2: The close stellar pair spiral together and merge to form one more massive star. Panel 3: The merged star evolves into a giant star. As the huge photosphere expands, some of the material falls onto the outer companion, causing the companion to grow larger and its rotation rate to increase. Panels 4-5: The central merged star eventually burns out and forms a massive white dwarf, and the outer companion spirals in to- wards the white dwarf, leaving a binary star system with a tighter orbit. Panel 6: The surviving outer companion is much like our Sun but nicknamed a “blue lurker.” Although it is slightly brighter bluer than expected because of the earlier mass- transfer from the central star and is now rotating very rapidly, these features are subtle. The star could easily be mistaken for a normal Sun-like star despite its exotic evolutionary history. [NASA, ESA, Leah Hustak (STScI)] companion star, causing its rotation to speed up. The star’s high spin rate was discovered with NASA’s retired Kepler space telescope. While nor- mal Sun-like stars typically take about 30 days to complete one ro- tation, the lurker takes only four days. How the blue lurker got that way is a “super complicated evolu- tionary story,” said Emily Leiner of Illinois Institute of Technology in Chicago. “This star is really exciting because it’s an example of a star that has interacted in a triple-star system.” The blue lurker originally rotated more slowly and or- bited a binary system con- sisting of two Sun-like stars. Around 500 million years ago, the two stars in that binary merged, creating a single, much more massive star. This behemoth soon swelled into a giant star, dumping some of its own material onto the blue lurker and spinning it up in the process. Today, we ob- serve that the blue lurker is orbiting a white dwarf star — the burned out remains of the massive merger. stages of evolution. Triple-star sys- tems are about 10 percent of the Sun-like star population. But being able to put together this evolution- ary history is challenging.” Hubble observed the white dwarf companion star that the lurker orbits. Using ultravio- let spectroscopy, Hubble found the white dwarf is very hot (as high as 23,000 degrees Fahrenheit, or roughly two-three times the Sun’s surface temperature) and a heavyweight at 0.72 solar masses. According to theory, hot white dwarfs in M67 should be only about 0.5 solar masses. This is evidence that the white dwarf is the byprod- uct of the merger of two stars that once were part of a triple-star system. “This is one of the only triple sys- tems where we can tell a story this detailed about how it evolved,” said Leiner. “Triples are emerging as po- tentially very important to creating interesting, explo- sive end products. It’s really unusual to be able to put constraints on such a system as we are exploring.” T his wide-field image of the sky around the old open star cluster Messier 67 was created from images form- ing part of the Digitized Sky Survey 2. The cluster appears as a rich grouping of stars at the centre of the picture. Messier 67 contains stars that are all about the same age, and have the same chemical composition, as the Sun. [ESO/Digitized Sky Survey 2. Ack.: Davide De Martin] “We know these multiple star sys- tems 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 !