Three supernovae imaged by the Hubble telescope involved "exiled" stars flung out of their home galaxies millions or even billions of years ago to end their lives in fiery explosions in the emptiness of intergalactic space, astronomers say.
Most supernovae occur within galaxies possessing hundreds of billions of stars, where perhaps one might explode as a supernova every century.
However, the three supernovae discovered several years ago and caught in detailed images by the Hubble Space Telescope were in the dark emptiness between three giant clusters with several thousand galaxies each, a study led by the University of California, Berkeley, found.
The stars' nearest neighbors at the time of their explosive deaths were around 300 light years away; that's almost 100 times the distance to our own sun's nearest cosmic neighbor, the star Proxima Centauri.
If any of the exploding stars had planets — which would have been incinerated in the supernovae — they would have experienced night skies without any bright stars, since the density of stars in interstellar space is about a millionth of what we observe from the Earth, says study leader Melissa Graham.
"It would have been a fairly dark background indeed," she says, "populated only by the occasional faint and fuzzy blobs of the nearest and brightest cluster galaxies."
The three apparently exiled supernovae were observed from 2008 to 2010, but the telescopes involved weren't able to rule out the possibility they may have been inhabitants of very faint galaxies.
However, new observations by sensitive, high-resolution cameras on Hubble confirmed they did indeed explode in empty space, a long way from any possible host galaxy, the researchers report in the journal Astrophysical Letters.
While stars and the supernovae sometimes end up as typically residing within galaxies, in large galactic clusters, gravitation forces can fling some 15 percent of a galaxy's stars out into the space between it and its neighbor galaxies, previous studies have shown.
However, once in those interstellar empty regions, the stars tend to remain there, gravitationally bound by the huge overall mass of the cluster.
In the darkness of their exiled wandering, they are too faint to be observed individually until they explode as supernovae, the researchers explain.
Bright visible supernovae in such intracluster spaces can serve as tracers yielding population estimates for such unseen stars, Graham says.
"We have provided the best evidence yet that intracluster stars truly do explode as Type Ia supernovae," she says, "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."
Astronomers say that could help them understand the processes that form galaxy clusters and how such clusters evolved during the history of the universe.
