Chapter 1: Introduction to the Galactic Underworld

The Milky Way is encircled by an enormous graveyard of ancient stars, far more extensive than the galaxy itself.

Recent research has uncovered that billions of black holes and neutron stars have formed since the inception of our galaxy, but their locations have remained elusive. This groundbreaking study has revealed that this concealed galactic burial ground reaches a height three times greater than that of the Milky Way.

Moreover, the structure of this stellar necropolis—coined the ‘galactic underworld’ by the astronomers who mapped it—differs significantly from the galaxy's shape. Surprisingly, nearly one-third of these remnants of former massive stars have been expelled from the galaxy entirely.

David Sweeney, a PhD candidate at the Sydney Institute for Astronomy at the University of Sydney and lead author of the research published in the Monthly Notices of the Royal Astronomical Society, stated, “The compact remnants of dead stars exhibit a fundamentally different arrangement compared to the visible galaxy.” He further elaborated, “The height of this galactic underworld exceeds three times that of the Milky Way, and approximately 30 percent of these objects have been completely cast out of the galaxy.”

Section 1.1: The Formation of Neutron Stars and Black Holes

Neutron stars and black holes form when massive stars, those exceeding eight times the mass of our Sun, exhaust their nuclear fuel and collapse. This collapse ignites a supernova explosion that ejects the outer layers of the star while the core continues to compress, ultimately becoming either a neutron star or a black hole.

Neutron stars possess such dense cores that electrons and protons combine to form neutrons, condensing the star's mass into a sphere smaller than a city. If the original star's mass exceeds 25 solar masses, the collapse continues until the core becomes so dense that not even light can escape. Both neutron stars and black holes warp the fabric of space, time, and matter around them.

The Milky Way, which formed around 13 billion years ago, has hosted billions of stars. Over time, many of these massive objects have transformed into dense remnants. Although billions of these exotic remnants likely formed in the galaxy's early years, they were ejected into the vastness of interstellar space, eluding astronomers until now.

Section 1.2: Mapping the Stellar Graveyard

In their research, scientists painstakingly recreated the lifecycle of ancient stars to create a detailed map of where these remnants are located. “One of the challenges in locating these ancient objects was our previous ignorance about where to search,” noted Professor Peter Tuthill from the Sydney Institute for Astronomy, co-author of the study. The oldest neutron stars and black holes, formed in a younger galaxy, have undergone complex changes over billions of years, making them difficult to pinpoint.

The newly formed remnants align with our current understanding of the galaxy's structure, but the oldest remnants are like specters in a long-demolished house, making them elusive. “It was akin to searching for the mythical elephant’s graveyard,” Professor Tuthill remarked, referring to the legendary place where old elephants supposedly go to die.

The researchers faced the additional challenge of accounting for the ‘kicks’ these remnants receive during their explosive formation. Supernovae are asymmetric, ejecting remnants at high velocities—sometimes millions of kilometers per hour—in random directions.

Chapter 2: Discoveries and Implications

The intricate models developed by the researchers, along with collaborators from Monash University, encoded the birthplaces and final resting places of these stars, resulting in a comprehensive distribution map of the Milky Way's stellar graveyard.

“It was quite surprising,” Dr. Sanjib Sharma commented. “I work with contemporary images of the visible galaxy daily and expected the galactic underworld to mirror it closely. I did not anticipate such a dramatic divergence in structure.”

The generated maps revealed that the Milky Way’s characteristic spiral arms are absent in the galactic underworld, obscured by the age of the remnants and the disruptive effects of the supernovae that created them.

Surprisingly, the side view indicates that the galactic underworld is considerably more voluminous than the Milky Way, a result of kinetic energy from supernovae elevating remnants into a halo around the visible galaxy. “Our study’s most astonishing finding is that the kicks are so potent that the Milky Way will lose some of these remnants entirely,” Dr. Hirai stated. “Approximately 30 percent of the neutron stars are expelled into intergalactic space, never to return.”

Professor Tuthill added, “One of the most fascinating aspects of this research is that even our local stellar neighborhood may be frequented by these ghostly remnants. Statistically, the nearest remnant should be a mere 65 light-years away—essentially in our cosmic backyard.”

“The most thrilling phase of this research is still to come,” Sweeney concluded. “Now that we know where to look, we’re advancing technologies to search for them. I believe the ‘galactic underworld’ will soon be unveiled from its shroud of mystery.”

The first video, "Thousands of Black Holes Discovered in 'Cosmic Graveyard'," explores the recent findings regarding the immense number of black holes and neutron stars located around our galaxy.

The second video, "First Ever Map of Milky Way's Galactic Graveyard Revealed," details the groundbreaking study that visualizes the locations of these stellar remnants in the Milky Way.