Space is big. Really big. Even the most inept science-fiction highlights the immense nature of space as a silent empty void which consists mostly of not very much at all. But astronomers are now challenging this view of space as the cosmological equivalent of North Wales, in a new report which suggests the galaxy may be loaded with 'ticking time bombs' - white dwarf stars which could be poised to go supernova at any moment.
A report in Astrobiology Magazine explains this startling claim with reference to the Keanu Reeves film Speed, which just goes to show that scientists are so worried about the prospect of white dwarfs going supernova that they've not been to the cinema in 20 years. Still, the analogy holds water, new research from the Harvard-Smithsonian Center for Astrophysics suggests that there are white dwarf stars which are only prevented from turning into supernovae by their rapid spins, and that these stellar bodies will explode when their spin begins to slow.
The CfA team, lead by astophysicist Rosanne Di Stefano, commented: “We haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this research suggests that we’ve been looking for the wrong signs. Our work points to a new way of searching for supernova precursors.”
The research focused on type la supernovae, which occur when a white dwarf star destabilises. White dwarfs are dense remnants of stars, which weigh up to about 1.4 times the amount of our own Sun and have ceased nuclear fusion, having burned through their fuel of hydrogen atoms and attempted to fuse heavier and heavier elements. This weight limit is called the Chandrasekhar mass after the astronomer who first calculated it, and refers to the mass beyond which a white dwarf will go supernova.
The problem which Di Stefano and her team were studying was the fact that the theory on how white dwarfs might gain the mass required to exceed the Chandrasekhar limit is not quite matched by the observable facts. It had been theorised that white dwarfs would accrete mass from nearby stars, but this would result in traces of helium and hydrogen gases being detectable near the site of the exploding star, and for the moment this has not been the case.
The suggestion made by the researchers is that as a white dwarf accretes mass from a donor star, it also gains angular momentum, which speeds up its spin to the point where it can exceed the Chandrasekar mass limit - until it begins to slow down again. Once the star's spin is no longer enough to counteract its own gravitational forces, and it goes supernova.
The research suggests that the delay between a white dwarf accreting mass and then slowing down enough to explode could be as much as a billion years, which would make the analogy rather more like Speed 2: Cruise Control!
It has been estimated that there are around three type la supernovae in our galaxy every thousand years, which suggests that there could be dozens of pre-explosion white dwarf stars spinning down within a few thousand light years of Earth.
Radboud University Nijmegen's Rasmus Voss was a co-author on the research and has said: “We don’t know of any super-Chandrasekhar-mass white dwarfs in the Milky Way yet, but we’re looking forward to hunting them out.”
Usually when astrophysicists talk about cosmic devastation, they are working on a scale of hundreds of millions, if not billions of years. The idea that our part of the galaxy could be littered with stars which could explode at a rate of one every three centuries or so presents a much more dramatic and dynamic view of the cosmos.
Even more exciting is the fact that according to Astrobiology Magazine the study of Type la supernovae and similar phenomena might well lend clues as to areas where suns supporting inhabitable planets such as the murky expolanet Gilese 581d might be. The galaxy is a more dangerous, and yet more exciting place, than we had hitherto suspected.