Evolution of a “Blue Lurker” star in a triple system (artist’s impression)

The name “blue lurker” might sound like a villainous character from the golden age of radio. In reality it is a rare class of star that the NASA/ESA Hubble Space Telescope explored by looking deeply into the open star cluster M67, roughly 2,800 light-years away.

Forensics with Hubble data show that the star has had a tumultuous life, mixing it up with two other stars gravitationally bound together in a remarkable triple-star system. The star has a kinship to so-called “blue stragglers” that are hotter, brighter, and bluer than expected because they’ve absorbed the mass of their companion stars.

The blue lurker was identified because it is spinning much faster than expected. Otherwise it looks like a normal Sun-like star. The term “blue” is a bit of a misnomer here because the star’s color blends in with all the other solar-mass stars in the cluster. Hence it is sort of “lurking” among the common stellar population.

The spin rate is evidence that the lurker must have siphoned in material from a companion star, causing its rotation to speed up. The star’s high spin rate was discovered with NASA’s decommissioned Kepler space telescope. While normal Sun-like stars typically take about 30 days to complete one rotation, the lurker takes only four days.

These results are being presented at the 245th meeting of the American Astronomical Society in Washington, D.C.

Panel 1: A triple star system containing three sunlike stars. Two are very tightly orbiting. The third star has a much wider orbit.

Panels 2-3: The close stellar pair spiral together and merge to form one more massive star.

Panel 4: 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.

Panel 5: The central merged star eventually burns out and forms a massive white dwarf, and the outer companion spirals in towards 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 blue 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.

[Image description: This visual features six boxes, three stacked on others. The top left shows a large circular path of a star surrounding a small circular path of two rotating stars. The top middle box shows two stars rotating around each other, shown with blue streaks. A third star is in the distance. The top right box shows two stars right next to each other, and a third star in the distance. The bottom left box shows a fiery orange star with a feeding line to another distant star. The bottom middle box shows a small yellow star on a black background. The bottom right box shows a fiery yellow star glowing into the black background.]