Hubble Discovers a Mysterious ‘Blue Lurker’ Hiding Amidst the Stars

Our sun exists as a solitary star. Approximately half of the stars within our galaxy are paired with binary companions. This was beautifully depicted in the Star Wars film trilogy, where Luke Skywalker witnessed two suns setting on the horizon from his home planet Tatooine. Now envision three suns in the sky. This narrates the tale of a system that once held three co-orbiting stars.

Analysis of Hubble data indicates that the stars have experienced a chaotic existence. Approximately 500 million years ago, two of the stars merged to create a heavier star. Eventually, it expired and collapsed into a remarkably massive white dwarf. The bystander to this turmoil is the previous third member of the system. It siphoned material from the merged star, securing a renewed lease on life by becoming more massive and luminous.

However, it now exists in isolation, orbiting a deceased star. Hubble uncovered that the remaining star maintains an unusually rapid rotation rate, which can solely be clarified if it had consumed gas ejected by the stellar merger.

The term “blue lurker” may evoke the notion of a villainous figure from a superhero narrative. Yet it refers to a rare category of star that NASA’s Hubble Space Telescope examined by delving deep into the open star cluster M67, situated approximately 2,800 light-years away.

Analysis of Hubble data reveals that the star has undergone a tumultuous journey, mingling with two other stars bound by gravity within an extraordinary triple-star system. The star has a connection to the so-called “blue stragglers,” which appear hotter, brighter, and bluer than expected as they are likely the product of star mergers.

The blue lurker spins significantly faster than anticipated, a peculiar trait that facilitated its identification. Otherwise, it resembles a standard sun-like star. The label “blue” is somewhat misleading as the star’s color intermingles with other solar-mass stars in the cluster, rendering it somewhat “lurking” amid the typical stellar ensemble.

The rapid spin rate serves as evidence that the lurker must have drawn in material from a companion star, accelerating its rotation. The star’s swift spin rate was detected using NASA’s retired Kepler space telescope. While typical sun-like stars generally take about 30 days to complete one rotation, the lurker accomplishes this in merely four days.

The process by which the blue lurker obtained its characteristics is described as a “super complicated evolutionary story,” stated Emily Leiner from the Illinois Institute of Technology in Chicago. “This star is truly fascinating because it exemplifies a star that has interacted within a triple-star system.” Initially, the blue lurker rotated at a slower pace and orbited a binary configuration comprising two sun-like stars.

Around 500 million years ago, the two stars within that binary amalgamated, crafting a singular, much larger star. This colossal entity soon expanded into a giant star, discarding some of its own material onto the blue lurker, causing an increase in its spin in the process. Presently, we observe that the blue lurker orbits a white dwarf star—the remnants of the substantial merger.

“We recognize that these multiple star systems are relatively common and will lead to genuinely intriguing results,” Leiner clarified. “We just lack a model that can reliably connect through all of these phases of evolution. Triple-star systems constitute about 10% of the sun-like star population. However, piecing together this evolutionary narrative presents challenges.”

“`html

Hubble examined the white dwarf companion star that the lurker revolves around. By employing ultraviolet spectroscopy, Hubble determined that the white dwarf is extremely hot (reaching up to 23,000°F, or nearly threefold the sun’s surface temperature) and quite massive at 0.72 solar masses. According to predictions, hot white dwarfs in M67 should only have approximately 0.5 solar masses. This indicates that the white dwarf is the result of the fusion of two stars that were once part of a triple-star system.

“This is one of the very few triple systems where we can narrate such a detailed account of its evolution,” stated Leiner. “Triples are coming forth as potentially very significant in producing fascinating, explosive outcomes. It’s remarkably rare to impose constraints on such a system as we are studying.”

Leiner’s findings are being showcased at the 245th gathering of the American Astronomical Society in National Harbor, Maryland.