Milky Way's Graveyard of Dead Stars Found

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. Phys.Org reports: 'These compact remnants of dead stars show a fundamentally different distribution and structure to the visible galaxy,' said David Sweeney, a Ph.D. student at the Sydney Institute for Astronomy at 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.' Sweeney added that 'the hardest problem I had to solve in hunting down their true distribution 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 kilometers per hour -- and, even worse, this happens in an unknown and random direction for every object.' But nothing in the universe sits still for long, so even knowing the likely magnitudes of the explosive kicks was not enough: the researchers had to delve into the depths of cosmic time and reconstruct how they behaved over billions of years.

The intricate models they built -- together with University of Sydney Research Fellow Dr. Sanjib Sharma and Dr. Ryosuke Hirai of Monash University -- encoded where the stars were born, where they met their fiery end and their eventual dispersal as the galaxy evolved. The final outcome is a distribution map of the Milky Way's stellar necropolis.

In the maps generated, the characteristic spiral arms of the Milky Way vanish in the 'galactic underworld' version. These are entirely washed out because of the age of most of the remnants, and the blurring effects of the energetic kicks from the supernovae which created them. Even more intriguing, the side-on view shows that the galactic underworld is much more 'puffed up' than the Milky Way -- a result of kinetic energy injected by supernovae elevating them into a halo around the visible Milky Way. 'One of the problems for finding these ancient objects is that, until now, we had no idea where to look,' said Sydney Institute for Astronomy's Professor Peter Tuthill, co-author on the paper. '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.'

'It's a little like in snooker,' said Sweeney. 'If you know which direction the ball is hit, and how hard, then you can work out where it will end up. But in space, the objects and speeds are just vastly bigger. Plus, the table's not flat, so the stellar remnants go on complex orbits threading through the galaxy.' He added: 'Finally, unlike a snooker table, there is no friction -- so they never slow down. Almost all the remnants ever formed are still out there, sliding like ghosts through interstellar space.'

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