Chapter 1: The Ancient Gazes at the Night Sky

When the Khoisan hunter-gatherers of sub-Saharan Africa looked up at the shimmering trail of stars and cosmic dust streaking across the night sky, they interpreted it as the glowing remnants of a campfire. Polynesian navigators envisioned a shark that feasted on clouds, while the ancient Greeks saw a milky river, which eventually inspired the term "galaxy." This intriguing view of the cosmos has evolved dramatically since the 20th century when astronomers began to uncover the Milky Way's composition as part of a much larger galactic structure.

In a simplified narrative, it is believed that the Milky Way began to form around 14 billion years ago, as colossal clouds of gas and dust came together due to gravitational forces. This process eventually led to the creation of a vast spherical halo, followed by the formation of a dense, luminous disk. Our solar system formed within this disk, which is why, when we gaze at the night sky, we see what appears to be a spill of milk — an edge-on view of the galactic disk.

However, the last couple of years have seen researchers revising nearly every aspect of this galactic story, thanks in large part to improved data.

On April 25, 2018, the Gaia satellite released an astonishing volume of information about the cosmos. This data included the detailed movements of around 1 billion stars, a leap from previous surveys that had only tracked a few thousand. The wealth of information transformed our understanding of the galaxy, with astronomer Federico Sestito from the Strasbourg Astronomical Observatory stating, “Gaia initiated a new revolution.”

As astronomers downloaded this dynamic star map, they made a flurry of groundbreaking discoveries. They found that certain sections of the disk appeared remarkably ancient and uncovered evidence of massive collisions that shaped the early tumultuous history of the Milky Way. This data also suggested that the galaxy continues to evolve in unexpected ways.

Taken together, these findings have led to a new narrative about the Milky Way's chaotic past and its ever-changing future. Michael Petersen, an astronomer at the University of Edinburgh, remarked, “Our perception of the Milky Way has transformed rapidly. The theme is that the Milky Way is not a static entity; changes are happening everywhere.”

To investigate the galaxy's formative years, astronomers are on the lookout for stars that date back to that era. These ancient stars were primarily composed of hydrogen and helium, the most fundamental elements in the universe. Fortunately, many of these smaller stars burn slowly, allowing them to continue shining even today.

After years of observations, researchers compiled a catalog of 42 ancient stars known as ultra metal-poor stars (to astronomers, any element heavier than helium is considered metallic). According to the conventional narrative of the Milky Way, these stars should predominantly populate the halo, the first structure to form. In contrast, stars in the disk, which was believed to have taken an additional billion years to flatten, should contain heavier elements like carbon and oxygen.

In late 2017, Sestito aimed to analyze the motion of this metal-poor star group by coding software to interpret the upcoming Gaia results. He hoped their spherical trajectories might provide insights into the halo's formation.

Following Gaia's data release, he isolated the 42 ancient stars from the complete dataset and monitored their movements. While most were found to be traveling through the halo as expected, approximately one in four were located in the galaxy's younger regions. This unexpected finding left Sestito perplexed, prompting him to ponder, “What’s happening here?”

Follow-up studies confirmed that these stars were indeed permanent residents of the disk, not just transient visitors. Sestito and his colleagues compiled a library of around 5,000 metal-poor stars from two recent surveys. A separate team examined about 500 stars identified in another survey, while yet another research group found stars of varying metallicities and ages. Lead author Paola Di Matteo from the Paris Observatory stated, “This is something entirely new.”

What explains the presence of these ancient stars in the disk? Sestito speculated that pockets of pristine gas might have evaded the metallic debris produced by supernovae for extended periods, eventually collapsing to form stars that appeared deceptively old. Alternatively, the disk may have begun forming concurrently with the halo, nearly a billion years earlier than previously thought.

To determine which scenario was more plausible, Sestito collaborated with Tobias Buck, a researcher at the Leibniz Institute for Astrophysics in Potsdam, Germany, who specializes in creating digital simulations of galaxies. Earlier attempts typically produced halos first and disks later, as expected, but these simulations had relatively low resolution.

Buck enhanced the clarity of his simulations by a factor of ten, requiring significant computational power. Even with access to the Leibniz Supercomputing Center in Germany, each simulation took three months to run. He conducted this process six times, and of those, five produced Milky Way analogs, with two showing a significant number of metal-poor stars in the disk.

How did these ancient stars end up in the disk? They were effectively stellar immigrants. Some originated in clouds that predated the Milky Way, while others were sourced from small "dwarf" galaxies that merged with the Milky Way, contributing stars that would eventually form part of the galactic disk.

The team's findings suggest that traditional models of galaxy formation were lacking. While gas clouds do collapse into spherical halos as expected, stars arriving at precise angles can simultaneously kick-start the formation of a disk. Buck noted, “The theorists weren’t wrong; they were missing part of the picture.”

The complications extend further. With Gaia's data, astronomers have uncovered direct evidence of cataclysmic collisions. Although it was presumed that the Milky Way had a tumultuous early life, Helmer Koppelman, currently at the Institute for Advanced Study in Princeton, New Jersey, utilized Gaia data to pinpoint specific debris from one of the most significant mergers.

Koppelman recalled that the Gaia data release on a Wednesday led to a rush that temporarily froze the website. He processed the data the following day, and by Friday, he realized he had stumbled upon something monumental. He observed a multitude of halo stars moving in a peculiar manner in the center of the Milky Way, indicating they originated from a single dwarf galaxy. By Sunday, Koppelman and his colleagues had a paper ready and followed it up with a publication that June.

The debris from this galactic collision was widespread. It is estimated that nearly half of all stars within the inner 60,000 light-years of the halo (which extends hundreds of thousands of light-years in every direction) originated from this singular event, potentially increasing the young Milky Way's mass by as much as 10 percent. Koppelman expressed, “This is a transformative finding for me; I anticipated many smaller objects.”

The incoming galaxy was named Gaia-Enceladus, after the Greek goddess Gaia and her Titan son Enceladus. Another group at the University of Cambridge dubbed the galaxy the "Sausage" due to its appearance in specific orbital charts.

When Gaia-Enceladus collided with the Milky Way around 10 billion years ago, it likely caused significant damage to the delicate disk structure. Astronomers continue to debate why our galactic disk appears to consist of two distinct parts: a thin disk and a thicker region where stars oscillate up and down as they orbit the galactic center. Current theories suggest that Gaia-Enceladus disrupted much of the disk, puffing it up during the merger. Koppelman stated, “The first ancient disk formed relatively quickly, and then we believe Gaia-Enceladus destroyed it.”

Additional mergers have been detected in clusters of stars known as globular clusters. Diederik Kruijssen from Heidelberg University in Germany trained a neural network to analyze the ages, compositions, and orbits of these clusters, allowing the program to reconstruct the events that formed the galaxies. After testing the neural network on data from the real Milky Way, Kruijssen's team managed to replicate known events like Gaia-Enceladus and identified an older, more significant merger they named Kraken.

In August, Kruijssen’s group published their findings about Kraken and its forming dwarf galaxies, predicting the existence of ten additional past collisions awaiting independent verification. “We haven't found the other ten yet,” Kruijssen noted, “but we will.”

These mergers have led some astronomers to propose that the halo may predominantly consist of immigrant stars. Models from the 1960s and '70s predicted that most Milky Way halo stars should have formed in situ. However, as more stars have been identified as galactic newcomers, astronomers may no longer need to assume that many, if any, stars are native, according to Di Matteo.

While the Milky Way has experienced a relatively calm history in recent epochs, new arrivals continue to flow in. Stargazers in the Southern Hemisphere can easily spot a pair of dwarf galaxies, the Large and Small Magellanic Clouds, with the naked eye. These galaxies were traditionally thought to be stable companions of the Milky Way, akin to moons.

However, a series of studies conducted between 2006 and 2013 revealed that these clouds act more like incoming projectiles. Nitya Kallivayalil from the University of Virginia observed that the clouds were moving at around 330 kilometers per second, nearly double the previously predicted speed.

A few years later, Jorge Peñarrubia from the Royal Observatory of Edinburgh led a team that concluded that these fast-moving clouds must be significantly more massive than originally estimated—possibly ten times larger.

Peñarrubia stated, “It’s been a surprise after surprise.” Several research groups have speculated that the unexpectedly hefty dwarf galaxies might be exerting an influence on the Milky Way's structure. This year, Peñarrubia collaborated with Petersen to seek evidence of this phenomenon.

The challenge of analyzing galaxy-wide motion stems from the chaotic nature of the Milky Way, where astronomers observe from within a swirling mass of stars. Peñarrubia and Petersen spent much of the lockdown devising methods to account for the movements of Earth and the sun, allowing them to focus on the outer halo stars as a stationary reference.

After calibrating their data in this manner, they discovered that the Earth, the sun, and the rest of the disk were shifting in one direction—not toward the current position of the Large Magellanic Cloud, but towards where it was located a billion years ago. This finding was recently published in Nature Astronomy.

The movement of the disk against the halo challenges a fundamental assumption: that the Milky Way is a stable system. While it may spin and glide through space, astronomers had long believed that, after billions of years, the mature disk and halo had settled into equilibrium.

Peñarrubia and Petersen’s analysis disproves this assumption. Even after 14 billion years, mergers continue to reshape the galaxy's structure. This realization marks yet another shift in our understanding of the Milky Way, the stunning river of stars that paints the night sky.

“Everything we thought we knew about the future and history of the Milky Way,” Petersen concluded, “now requires a new model to explain it.”

Chapter 2: Unraveling the Milky Way's New Story

The first video, titled "The Evolution of the Modern Milky Way Galaxy," explores the formation and ongoing changes within our galaxy, offering insights into the latest astronomical discoveries.

The second video, "The Incredible Story of the Milky Way [4K]," presents a visually stunning journey through the history and dynamics of the Milky Way, shedding light on its complex origins and evolution.