The Milky Way is a massive cemetery. Stars are born, burn out, and die, but they do not simply vanish — and their corpses haunt the galaxy.
Massive stars that died billions of years ago in the Milky Way went supernova and morphed into two types of objects. The cores that were left after their outer layers were blasted away by the force of the explosion either entered the afterlife as extremely compact neutron stars or collapsed in on themselves and formed black holes. Scientists refer to what remains of these ancient stars as the “galactic underworld,” which has kept most of its secrets hidden until now.
Researchers have created the first digital map of the galactic underworld after virtually rewinding time to see how and when these early stars were born, lived, and died. They were able to do so by analysing observations of dead stars scattered throughout the galaxy, such as neutron stars and black holes, and determining when and how they were born. They discovered a sprawling necropolis three times the size of the Milky Way.
What took so long to imagine the catacombs of the galaxy? The nascent Milky Way in which these early stars lived looked very different from the galaxy we see today, so much so that its spiral arms had not yet fully unfurled. It was difficult to even guess where to look for hidden black holes and neutron stars, as opposed to younger ones, which are scattered across the Milky Way’s current shape.
The new map, created by astronomer David Sweeney and his colleagues at the University of Sydney, reveals not only where the bones of these old stars could be hidden, but also that roughly one-third of the remnants lying around have already been ejected from the galaxy or are on their way to being ejected. Supernovae erupt with enormous but random amounts of energy, accelerating dust and gas to millions of miles per hour. It is difficult to predict where greater or lesser amounts of energy will be generated. The researchers had a particularly difficult time calculating the energy involved in each supernova burst. If the star ejects gas and dust in an area where it is exerting more force, it will be propelled farther than neighbouring gobs of star stuff.
The researchers discovered that entire neutron stars could have been ejected from the galaxy. Because black holes can travel through space as rogue black holes, they can be flung into the void.