‘Hostless’ stars explode in solitary space

The confirmation that three supernovae exploded alone in deep space is helping astronomers learn more about the empty spaces between galaxies.

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One of the four supernovae (left, 2009) could be part of a dwarf galaxy or globular cluster, as visible on the 2013 HST image (right). Image Credit: Melissa Graham, CFHT and HST)

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Three supernovae have exploded in the emptiness of space, far from any hosting galaxy, according to new data collected by the Hubble Space Telescope.

The stars, which eventually exploded as Type Ia supernovae, were likely about 300 lightyears from their nearest neighbours.

The study is helping astronomers decipher how galaxy clusters formed and evolved through the history of the Universe, as well as how to detect other intracluster stars that cannot be observed by normal techniques.

The news confirms previous observations of three suspected hostless supernovae between 2008 and 2010 by the Multi-Epoch Nearby Cluster Survey, using the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea in Hawaii.

Astronomers were initially unable to ascertain whether the supernovae were hostless and solitary, but the higher resolution provided by the Hubble Space Telescope in this latest study has confirmed that they exploded in empty space.

Stars and supernovae are usually hosted within galaxies, which themselves are found in massive clusters.

These clusters, however, experience gravitational forces that fling some of their stars into empty space.

The stars, once separated from their host cluster, are too faint to be viewed individually, but can be detected once they explode as supernovae.

The team behind the study, led by Melissa Graham, postdoctoral fellow at University of California, Berkley, are searching for supernovae in intracluster space in order to learn more about the population of unseen stars.

“We have provided the best evidence yet that intracluster stars truly do explode as Type Ia supernovae,” Graham 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.”

The team also found that a fourth exploding star that had been discovered by CFHT is apparently in a red, round region that could be a small galaxy or a globular cluster.

If the latter, it would be the first time a supernova has been confirmed to have exploded inside these small, dense clusters of fewer than a millon 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.”

This study is also helping scientists piece together more information on Type Ia supernovae, which are thought to exist mostly in a binary star system.

If true, it would mean that the solitary stars had a companion during their lifetime.

“This is no love story, though,” Graham says.

“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.

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Either way, this transfer of material caused the primary to become unstably massive and explode as a Type Ia supernova.”