Stars that hurtle through space at speeds of up to 7.2 million kilometres an hour, only to explode all alone in deep intergalactic space, have perplexed astronomers for more than a decade.
What sets these doomed stars, known as hyper-velocity supernovae, speeding along this path? Ryan Foley from the University of Illinois claims to have solved the puzzle. Like passengers thrown out of a vehicle in a head-on collision, he thinks these stars are the victims of a collision between two galaxies. It’s “a very nice piece of detective work”, says Curtin University astronomer James Miller-Jones. The study was published in the Monthly Notices of the Royal Astronomical Society in August.
Most stars live out their lives within the galaxy in which they were born. And in the centre of every galaxy, astronomers believe, lurks a supermassive black hole that is millions or billions of times more massive than our Sun, and that provides part of the gravitational glue holding the galaxy’s stars in place.
No one knew how these rogue stars had escaped their galaxies or
how they achieved such tremendous speeds.
In 2000, astronomers spotted a star that exploded while racing through space. It was at least 12,000 light years away from the galaxy where it was born.
Similar sightings followed. “It took until 2003, when four had been discovered, for astronomers to understand that they were something weird and different,” Foley says. But no one knew how these rogue stars had escaped their galaxies or how they achieved such tremendous speeds.
Foley used NASA’s Hubble space telescope to trace 13 hyper-velocity supernovae back to their host galaxies. He discovered these host galaxies were ancient and packed with old, dense stars that have used up all their fuel, known as white dwarfs.
When he examined these galaxies more closely, he uncovered the smoking gun. Most of them had either collided with another galaxy or had narrowly escaped. From there, his theory fell into place.
Foley thinks each hyper-velocity supernova began not as a single star, but as a white dwarf star in a binary “dance” with another stellar object – maybe another white dwarf. This binary pair happily circled each other, until their host galaxy collided or had a near-miss with another galaxy. As the supermassive black hole at the centre of each galaxy close in on each other, they form a vast binary pair themselves. And as these supermassive partners start to dance, the white dwarf pair (minuscule in comparison) get momentarily caught up in their gravity, before being launched off of the dance floor.
(Space agency scientists deliberately use a similar manoeuvre, called “gravity assist”, to turbocharge the journey of their space probes. In 2007, NASA sent its New Horizons probe skimming past Jupiter, boosting its speed by 14,000 kilometres per hour and cutting four years from its journey time to Pluto.)
Foley says that when a binary white dwarf is evicted from a galaxy in this way, it “shortens the fuse between star formation and death”. That’s because the ejection unbalances the dance of the white dwarf and its partner, sending the pair spiralling toward each other until they collide and explode in the middle of nowhere.
“They explode after only 10 to 100 million years, rather than billions of years,” Miller-Jones adds. It is a short lifespan, in cosmological terms, and that makes it easier to trace the supernovae back to their host galaxy.
According to Foley, astronomers could start to use hyper-velocity supernovae to hunt down and study binary supermassive black holes. Ordinarily, most supermassive black holes are by definition dark and hard to spot. As a result, Foley says, astronomers have only ever found a dozen or so binary supermassive black holes within around 100 million light-years of Earth.
By retracing the path of hyper-velocity supernovae, which are super-bright and easy to see, Foley has potentially doubled this number. “As with all things in science, it’s good to have other avenues” to detect binary supermassive black holes, he says, and perhaps solve more mysteries of our Universe.
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