White dwarfs scattered throughout our galaxy could be ticking time bombs ready to explode as Type Ia supernovae once their rapid spins slow down.

These burned out old stars usually weigh up to 1.4 times the sun’s mass, a measurement known as the Chandrasekhar mass. If a white dwarf was heavier, gravity would compact it, causing nuclear fusion and a supernova.

Extra mass could be acquired via two white dwarfs merging or gas accretion from a companion star, the more likely scenario, but evidence in support of this theory is scarcely seen in Type Ia supernovae. No donor stars have been found, and traces of hydrogen and helium near supernovae are absent, even though small amounts of unaccreted matter would be expected.

Instead, white dwarf spin could provide an explanation because a spin-up/spin-down process would create a delay between the time of accretion and the supernova.

“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,” astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA) said in a press release.

While gaining mass, a white dwarf’s angular momentum and spin would increase, thus crossing the 1.4-solar-mass barrier to form a super-Chandrasekhar-mass star.

As accretion diminishes, so does the spin rate, generating a supernova once gravity cannot be counteracted.

“Astronomers therefore must take angular momentum of accreting white dwarfs seriously, even though it’s very difficult science,” De Stefano said.

The time delay from the spin-down process could be up to a billion years between the end of accretion and the explosion, allowing the companion star to evolve into a second white dwarf, and any surrounding matter to dissipate.

On this basis, astronomers estimate that there are three Type Ia supernovae every one thousand years in the Milky Way, meaning there could be plenty of supernova precursors within a few thousand light-years of Earth.

“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,” said co-author Rasmus Voss at Holland’s Radboud University Nijmegen in the release.

The study was published online in The Astrophysical Journal Letters on Sept. 1.

Milky Way Could Be Loaded With Time Bombs