When runaway stars are flung from their host galaxies and lost to intergalactic space, what happens next? Well, they get old, run out of fuel and, if they are massive enough, explode as a supernova - as would be expected for any regular massive star.
However, the explosive final moments of these elusive stellar outcasts are hard to find.
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But in new research to be published in the Astrophysical Journal, the neighborhoods of a handful of odd Type Ia supernovae between the galaxies inside bustling galactic clusters have been studied by the Hubble Space Telescope, revealing that these specific stellar detonations were triggered in the depths of "intracluster" space and not in bustling stellar metropolises.
Discovering supernovae far from their host galaxy is extremely rare. In fact, because supernova events can be millions or billions of light-years from Earth, the host galaxy or star cluster hosting a supernova can be too dim for us to see; the supernova becomes the only visible indicator of that galaxy's existence.
Type Ia supernovae are thought to be spawned in stellar binaries, where material is siphoned from one, less massive partner to the other, usually a dense white dwarf. When the stellar plasma reaches a threshold, the system detonates and produces a supernova.
"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," said lead researcher Melissa Graham, of the University of California, Berkeley. "Either way, this transfer of material caused the primary to become unstably massive and explode as a Type Ia supernova."
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Using Hubble, Graham's team zoomed in on the sites of 3 apparently galaxy-less supernovae that occurred between 2008 and 2010. They found that these examples were indeed exiled stars that died alone at least 300 light-years from the nearest stellar neighbor. That's almost 100 times the distance between our sun and its nearest neighbor, Proxima Centauri, which is over 4 light-years away.
Before these stars died, it's hard not to ponder whether they hosted planets, as most of the stars in our galaxy are thought to contain. The night sky seen from any of these hypothetical planets orbiting "intracluster" stars would appear very different than the sky as seen from Earth, however, reminding Graham, who is an avid sci-fi fan, of particular science fiction imagining of one such world:
The solitary worlds reminded study leader Melissa Graham ... of the fictional star Thrial, which, in the Iain Banks novel Against a Dark Background, lies a million light years from any other star. One of its inhabited planets, Golter, has a nearly starless night sky. - UC Berkeley press release.
"It would have been a fairly dark background indeed, populated only by the occasional faint and fuzzy blobs of the nearest and brightest cluster galaxies," said Graham.
Of course, any planets orbiting these vagabond stars would have likely been destroyed, or at least severely sterilized, by the resulting Type Ia supernovae.
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These particular stellar explosions were observed inside massive galactic clusters where powerful gravitational interactions are known to strip neighboring galaxies of around 15 percent of their stars. It is in these regions where there is a higher likelihood of finding the supernovae of stripped stars, like the sparks that fly from a welders torch.
"We have provided the best evidence yet that intracluster stars truly do explode as Type Ia supernovae," added Graham, "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."
A fourth supernova was also studied by Graham's team, but found that it resided in a small, faint and red region that could be a small galaxy or a globular cluster of old stars. If this feature is identified as a globular cluster, this would be the first confirmed supernova detection in such a cluster.
"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."
Source: UC Berkeley