Sharp images obtained by the Hubble Space Telescope confirm that three supernovae discovered several years ago exploded in the dark emptiness of intergalactic space, having been flung from their home galaxies millions or billions of years earlier.

Most supernovae are found inside galaxies containing hundreds of billions of stars, one of which might explode per century per galaxy.

These lonely supernovae, however, were found between galaxies in three large clusters of several thousand galaxies each. The stars' nearest neighbors were probably 300 light-years away, nearly 100 times farther than our sun's nearest stellar neighbor, Proxima Centauri, 4.24 light-years distant.

"We have provided the best evidence yet that intracluster stars truly do explode as Type Ia supernovae, and confirmed that hostless supernovae can be used to trace the population of intracluster stars, which is important for extending this technique to more distant clusters," said study leader Melissa Graham, a University of California, Berkeley postdoctoral fellow, according to a press release

If the supernova is in fact part of a globular cluster, it marks the first time a supernova has been confirmed to explode inside these small, dense clusters of fewer than a million stars.

"Since there are far fewer stars in globular clusters, only a small fraction of the supernovae are expected to occur in globular clusters," Graham said. "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."

Graham said that most theoretical models for Type Ia supernovae involve a binary star system, so the exploding stars would have had a companion throughout their lifetimes.

"This is no love story, though," she added. "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."

Graham and her colleagues -- David Sand of Texas Tech University, Dennis Zaritsky of the University of Arizona and Chris Pritchet of the University of Victoria in British Columbia -- will report their analysis of the three stars in a paper to be presented Friday, June 5, at a conference on supernovae at North Carolina State University in Raleigh. Their paper has also been accepted by the Astrophysical Journal.