Milky Way s graveyard of dead stars found

The visible Milky Way compared with the galactic underworld [Credit: University of Sydney].

The first map of the ‘galactic underworld’ – a chart of the corpses of once massive suns that have since collapsed into black holes and neutron stars – has revealed a graveyard that stretches three times the height of the Milky Way, and that almost a third of the objects have been flung out from the galaxy altogether.

“These compact remnants of dead stars show a fundamentally different distribution and structure to the visible galaxy,” said David Sweeney, a PhD student at the Sydney Institute for Astronomy of the University of Sydney, and lead author of the paper in the latest issue of Monthly Notices of the Royal Astronomical Society.

“The ‘height’ of the galactic underworld is over three times larger in the Milky Way itself,” he added. “And an amazing 30 percent of objects have been completely ejected from the galaxy.”

Neutron stars and black holes are formed when massive stars – more than eight times larger than our Sun – exhaust their fuel and suddenly collapse. This triggers a runaway reaction that blows the outer portions of the star apart in a titanic supernova explosion, while the core keeps compressing in on itself until – depending on its starting mass – it becomes either a neutron star or a black hole.

In neutron stars, the core is so dense that electrons and protons are forced to combine at the subatomic level into neutrons, squeezing its total mass into a sphere smaller than a city. If the mass of the original star is greater than 25 times our Sun’s, that gravity-driven collapse continues until the core is so dense that not even light can escape. Both types of stellar corpses warp space, time, and matter around them.

Although billions must have been formed since the galaxy was young, these exotic carcasses were flung out into the darkness of interstellar space by the supernova that created them, hence slipped beyond sight and knowledge of astronomers – until now. 

By carefully recreating the full lifecycle of the ancient dead stars, the researchers have constructed the first detailed map showing where their corpses lie.