Between Galaxies: Lonely Supernovae Identified

If a star dies in a vacuum, and no galaxy is around to see it, did it ever truly live? Astronomers can start asking some pretty existential questions, after discovering that three massive stars went supernova while drifting in the vast darkness between galaxies - lonely deaths hundreds of light-years away from any known system.

The supernovae in question are highlighted in a series of remarkably sharp images captured by the now 25-year-old Hubble Space Telescope. The star remnants were found between galaxies in three large clusters of several thousand galaxies each - their nearest neighbors being some 300 light-years away.

Astronomers had long concluded that most supernovae will be found within galaxies - systems that contain hundreds-of-billions of stars. Among those, only a handful will die each century, and depending on their mass, they may not even go supernova.

That's what makes it so remarkable that experts have found not one, but three "intracluster" supernovae - floating within a dense galaxy cluster, but with no host galaxy to call their own. Researchers now suspect that they can use these bright explosions as guides, helping to point out what rogue stars could be like and just how many there are. After all, unlike the supernovae they give rise to, the light of intact stars floating between galaxies is too faint to be seen even by Hubble.

Melissa Graham, a University of California, Berkeley researcher who led the study, said in a recent release that the supernovae all proved to be "Type Ia" explosions, meaning that they all arose from small but extremely dense stars and an unlucky companion.

"This is no love story," she explained. "The companion was either a lower-mass white dwarf that eventually got too close and was tragically fragmented into a ring that was cannibalized by the primary star, or a regular star from which the primary white dwarf star stole sips of gas from its outer layers. Either way, this transfer of material caused the primary to become unstably massive and explode as a Type Ia supernova." (Scroll to read on...)

The Darkest of Skies

Graham added that although these stars exist outside of a host galaxy, that does not mean they didn't have their own tiny systems. An avid science fiction reader, the researcher compared unseen intracluster stars to the fictional star Thrial, which, in the Iain Banks novel Against a Dark Background, lies a million light-years from any other star.

The story takes place on Golter, a world with a starless night sky. It would be not unheard of, Graham explained, for similar worlds to exist in reality, orbiting lonely stars until they are obliterated in supernovae like the ones she and her colleagues identified.

"It would have been a fairly dark background indeed," she said, "populated only by the occasional faint and fuzzy blobs of the nearest and brightest cluster galaxies."

A Whole Other Kind of Rarity

Still, evidence that intrastellar stars exist and explode far from any galactic neighbor is not the only thing Graham and her colleagues found.

The research team also turned their attention to a fourth exploding star first identified by the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii. Closer inspection revealed that unlike the other three Type Ia explosions, this fourth supernovae was occurring within what Graham suspects is a small reddish galaxy often called a globular cluster. (Scroll to read on...)

"Since there are far fewer stars in globular clusters, only a small fraction of the supernovae are expected to occur in globular clusters," Graham she explained. "This might be the first confirmed case, and may indicate that the fraction of stars that explode as supernovae is higher in either low-mass galaxies or globular clusters."

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