Three exiled stars, stellar objects that were flung from their host galaxies as early as billions of years ago, exploded as supernovae in the dark emptiness of intergalactic space. The phenomenon was revealed by images obtained by NASA's Hubble Space Telescope.

Supernovae, stars that explode at the end of their lives, are often found inside galaxies that host billions of stars but the stellar trio went supernova alone in the emptiness between galaxies in three large clusters hundreds of light years away from their nearest neighbors.

The solitary supernovae are rare compared with other supernovae and because the stars are in the void of intergalactic space, they faint and are nearly impossible to observe unless they explode. Scientists could estimate the amount of stars that float freely through space outside the bounds of any galaxy by studying these stars.

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

The reason why stars explode and how they explode vary. Thus, scientists have classified supernovae into different types. The three stars observed by the Hubble appeared to have exploded as Type Ia supernova, which scientists believe occurs when a small star that orbits a larger star disintegrates as its bigger companion feeds on its material. This means that the stars that exploded may have companion stars in the void.

Another theory for the formation of Type 1a involves the merger of two white dwarfs. The combination of their masses causes the explosion.

Other types of supernovae include the Type II, which involves a star having at least nine times the solar mass. As its fuel runs out, the star experiences core collapse leading to its explosion. Type 1b and 1c also involves a star that goes through core collapse albeit majority of its outer hydrogen is already lost to stellar winds.

The hypernova, on the other hand, is an extreme type of supernova characterized by powerful energy 50 times more energetic compared with other supernovae. This type of explosion can result in a black hold albeit the exact cause of the explosion remains unknown. 

Photo: NASA Goddard Space Flight Center | Flickr