Researchers create map of Milky Way’s graveyard of dead stars

Researchers have created a chart of the corpses of massive stars that have since collapsed into black holes and neutron stars. The study reveals that this “galactic graveyard” stretches three times the height of the Milky Way and that close to one-third of the objects have been flung out from the galaxy.
“These compact remnants of dead stars show a fundamentally different dribution 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 Monthly Notices of the Royal Astronomical Society, in a press statement. “The ‘height’ of the galactic underworld is over three times larger in the Milky Way itself. And an amazing 30 per cent of objects have been completely ejected from the galaxy,” added Sweeney.
Black holes and neutron stars are formed when stars which are more than eight times larger than our Sun exhaust their fuel supply and suddenly collapse. When this happens, it triggers a reaction that blows apart the outer portion of the star in a supernova explosion. Meanwhile, the core keeps compressing itself until it becomes either a neutron star or a black hole, depending on its starting mass.

Neutron stars have cores so dense that electrons and protons combine at the subatomic level to form neutrons. This squeezes its total mass into a sphere that is “smaller than a city.” If the starting mass of the star is more than 25 times that of our Sun, this collapse will continue until the core gets so dense that even light cannot escape, creating a black hole. Both kinds of stellar remnants warp space, time and matter around them due to their density.
The researchers carefully recreated the full lifecycle of these ancient star corpses to construct the first detailed map of the “galactic underworld” where they lie.
A rendition of the Milky Way Galaxy (top), compared with its galactic graveyard. (bottom) (Image credit: Sydney University)
“One of the problems for finding these ancient objects is that, until now, we had no idea where to look. The oldest neutron stars and black holes were created when the galaxy was younger and shaped differently and then subjected to complex changes spanning billions of years. It has been a major task to model all of this to find them,” said Sydney Institute for Astronomy’s Professor Peter Tuthill in a press statement. Tuthill is a co-author of the paper.
According to the researchers, newly-formed neutron stars and black holes stay conforming to today’s galaxy, letting astronomers know where to look. But the oldest neutron stars and black holes are like “ghosts still haunting a house demolished long ago.”

“The hardest problem I had to solve in hunting down their true dribution was to account for the ‘kicks’ they receive in the violent moments of their creation. Supernova explosions are asymmetric, and the remnants are ejected at high speed – up to millions of kilometres per hour – and, even worse, this happens in an unknown and random direction for every object,” explained Sweeney.
Point-cloud chart of the Milky Way (top) and its ‘galactic underworld’. (Image credit: Sydney University
In order to find out the location of these dead stars, the researchers had to look deep into the depths of cosmic time and use complex models to reconstruct how they behaved over billions of years. The researchers compared it to a snooker table, where you might be able to find out where a ball went based on how hard it was hit and how it was hit.
The University of Sydney researchers collaborated with Monash University researchers to build intricate models that encoded where the stars were born, where they met their end and how they were eventually dispersed as our galaxy evolved.