Our home galaxy, the Milky Way, grew to its current size in part by consuming smaller galaxies. Now, new data from NASA’s Hubble Space Telescope shows definitive evidence of a dwarf galaxy merging with the young Milky Way galaxy in the earliest phases of its evolution. This finding extends our knowledge of our galaxy’s history 1.8 billion years farther back in time than before.

The results published Monday in the journal Nature Astronomy.

The Milky Way today is a massive spiral galaxy home to hundreds of billions of stars. However, our galaxy wasn’t always so large; it has grown by forming new stars from its gas clouds as well as collecting stars, gas, and dark matter from other galaxies through mergers.

The most recent massive merger in our galaxy’s history took place with the Sagittarius dwarf galaxy, beginning over 6 billion years ago and still ongoing today. Looking back into the even more distant past, researchers learned that the Milky Way galaxy consumed another dwarf galaxy called Gaia-Sausage-Enceladus 10 billion years ago. This ancient merger greatly affected the structure of our galaxy’s disk of stars. Other, smaller mergers occurred between these two.

But our galaxy’s history doesn’t stop there. Both observations and simulations have suggested that another large merger preceded these two, though the specifics of the event have been heavily debated. Now, Hubble has uncovered definitive evidence of an earlier merger that occurred about 11.8 billion years ago, or just 2 billion years after the big bang.

“Our home is the Milky Way galaxy, but we do not know how our house was built,” said Davide Massari, lead author, Astrophysics and Space Science Observatory of Bologna in Italy. “In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH.”

Cosmic archaeological sites

Immense astronomical surveys and precision data from spacecraft like ESA’s (European Space Agency’s) Gaia mission have been instrumental in piecing together the history of our galaxy. The farther back into our galaxy’s history that scientists attempt to look, the more difficult it becomes to tell what happened. When our galaxy was young, it was smaller and much closer in size to the galaxies it clashed with. It was also more chaotic, and it’s possible that the signs of mergers have been erased over billions of years.

It’s into this murky past that Hubble peered. Researchers used Hubble to study some of the Milky Way galaxy’s globular clusters: immense, roughly spherical collections of tens of thousands to a few million stars. Globular clusters contain some of the oldest stars in our galaxy, and they can act as cosmic archaeological sites that preserve stars from other galaxies the Milky Way galaxy has collected.

“Thanks to the high resolution and depth of Hubble imaging, we could measure the age and the metal content of these clusters with unprecedented precision,” said Chiara Zerbinati, study co-author, University of Bologna in Italy. “Coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters that are different from the others. These are the clusters that were born in LKH, and they tell us when that galaxy was devoured by ours, and how massive it was.”

The team analyzed Hubble observations of 39 globular clusters in the inner 20,000 light-years of our galaxy, where evidence of the most ancient mergers should be preserved. They expected this sample to contain globular clusters that formed within the young Milky Way galaxy as well as those collected from the Gaia-Sausage-Enceladus dwarf galaxy about 10 billion years ago.

Using Hubble’s sensitive observations to determine each cluster’s precise age and associated metallicity — the abundance of elements heavier than helium — they determined there was a third population of globular clusters in the inner regions of our galaxy. The team found that these clusters are older than the group collected in the Gaia-Sausage-Enceladus merger, but younger than those born in the Milky Way, regardless of their metal content. These  clusters, therefore, came from a separate and even earlier merger — in which the Milky Way galaxy absorbed a dwarf galaxy containing roughly 500 million times the mass of the Sun in stars, a significant fraction of our galaxy’s mass at the time. They named this dwarf galaxy Low-energy-Kraken-Heracles, or LKH, in honor of three earlier research papers that championed the idea of a merger early in our galaxy’s history.

Such a large merger so early in the Milky Way galaxy’s formation has profound implications for the evolution of our galaxy.

“Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” says Massari. “Here, we have shown that stars born in external galaxies also need to be considered.”

The team plans to continue their work to unravel the history of the Milky Way galaxy by studying its globular clusters, aiming to characterize all the massive mergers that our galaxy has experienced across cosmic history.

“Hubble is observing globular clusters that have never been studied before, and this will help us characterize the merger events that are far back in time in the Milky Way galaxy’s history,” said Fernando Aguado-Agelet, co-author, University of Vigo and the University of La Laguna in Spain.

The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Related Images & Videos

LKH Milky Way Merger Illustration

About 12 billion years ago, a dwarf galaxy known as LKH collided with a young Milky Way and merged with it. This artist’s concept portrays that collision. NASA’s Hubble Space Telescope uncovered definitive evidence of this collision by studying globular star clusters.

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As humanity looks to the Moon and stars for future exploration, predicting space weather — conditions in space primarily driven by the Sun — is more important than ever. 

Now, a team of astrophysicists and data scientists with NASA’s COFFIES (Consequence Of Fields and Flows in the Interior and Exterior of the Sun) has developed a novel machine-learning model capable of predicting the emergence of active regions on the Sun up to 12 hours before they appear. 

The Sun is constantly churning. Intense concentrations of localized magnetic fields can suddenly break through the solar surface, forming sunspots. Space weather forecasters then collectively number and track sunspots since they are visible manifestations of active regions, which serve as the main engines behind severe space weather events such as solar flares and coronal mass ejections. These eruptions send waves of high-energy radiation and charged particles across space, creating storms that can threaten astronauts, disable satellites, and disrupt radio communications on Earth. 

By bridging expertise across different scientific institutions, COFFIES, a NASA DRIVE (Diversify, Realize, Integrate, Venture, Educate) Science Center, brought together a team of researchers from New Jersey Institute of Technology (NJIT), Princeton University, and NASA’s Ames Research Center in California’s Silicon Valley. The team turned to advanced artificial intelligence architectures — which dictate how data is processed and used to produce reliable predictions or actions — to capture subtle, time-based pattern changes on the solar surface before an active region took shape. By analyzing data captured by the agency’s Solar Dynamics Observatory and using NASA Ames’ supercomputing resources, this new approach, published in the Journal of Geophysical Research: Machine Learning and Computation, looks at fluctuations in acoustic waves caused by sunspot regions when the regions form beneath the solar surface and begin the journey upward to emerge on the surface. 

“We cannot directly see the magnetic structure while it is still rising through the solar interior. Instead, we must look for indirect effects — very small changes in the magnetic field and in the pattern of acoustic waves continually traveling through the Sun,” said Alexander Kosovichev, a COFFIES co-investigator at NJIT. “The developed technique identifies precursors associated with an emerging active region in slight changes of the Sun’s acoustic power — more like a slight change in rhythm within a very noisy orchestra.” 

To develop current operational forecasts, the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center and the United States Air Force monitor active regions that are already visible on the Sun to analyze the regions’ characteristics and estimate the probability of solar flares.

The COFFIES team aims to revolutionize this process. The AI model the team developed a specialized early detection system to handle very long sequences of data — called sliding-window transformer architecture — to use observations to find tiny reductions in the Sun’s acoustic activity and magnetic field, signals that scientists struggled to capture until now. These reductions form patterns that the AI model uses to predict active regions several hours before they become visible on the solar surface. Instead of looking at all activity on the solar surface at once, like earlier deep learning approaches have done, this new model moves a fixed-size “viewing window” across a long timeline of the Sun’s activity to focus on recent data while remembering overall patterns. This method allows forecasters the ability to predict approximate locations of emerging sunspots, rather than relying on counting already visible sunspots. 

This promising AI architecture shows how deep machine learning can contribute to heliophysics — the field studying the nature of the Sun and how it influences the very nature of space and the planets that exist there. While the model is not ready for operational real-time forecasting, the team plans to validate the approach across many more known solar events to fine-tune the model. 

NASA’s real-time space weather monitoring 

As NASA focuses on sending humans to explore the Moon with the Artemis missions and sending the first crewed missions to Mars, monitoring and forecasting space weather is important for ensuring the safety of our astronauts and the equipment they rely on. This predictive leap from the COFFIES team could prove vital for safeguarding technology and deep-space explorers from the volatile environment of our solar system.

Teams across NASA and NOAA collaborate to transition research capabilities into actual 360-degree space weather monitoring operational tools — including NASA’s Space Radiation Analysis Group, Moon to Mars Space Weather Analysis Office (M2M SWAO), and Community Coordinated Modeling Center as well as NOAA’s Space Weather Prediction Center. Sunspot region emergence prediction capabilities, especially of the Sun’s far side, could provide new information that supplements current models used by these teams.  

“The COFFIES AI model is exciting to our team because it could provide us with new capabilities towards predicting potential flaring locations ahead of time,” said Michelangelo Romano, M2M SWAO deputy director. “With this heads up, we can provide additional support to NASA missions.”

NASA’s COFFIES is one of three DRIVE Science Centers created to encourage collaborative science by establishing centers that are made of multidisciplinary teams from several institutions across the U.S. These pioneering facilities employ modelers, theoreticians, computer scientists, and observers to study important mysteries of our star and its influence, a branch of science known as heliophysics.  

The COFFIES team focuses on the interconnected processes behind the Sun’s activity. Understanding the Sun’s interior and magnetic variability is key to advancing our understanding of the Sun’s 11-year activity cycle and fine-tuning space weather forecasting tools.  

About the Author

Desiree Apodaca

NASA’s Heliophysics Missions Communications Lead

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Growing up in Grafton, West Virginia, engrossed in Star Wars and all things science fiction, Fletcher Newell had an early interest in space exploration. That interest only grew when, in 2013, his fourth-grade class took a field trip to a nearby NASA facility he had not yet heard of – NASA’s Katherine Johnson Independent Verification & Validation Facility, located in Fairmont, West Virginia. Thirteen years later, Newell and 48 of his colleagues at the facility were sworn in as NASA civil servants during a ceremony on Aug. 10.

“I had always admired NASA from afar,” said Newell, who started working at IV&V as a systems engineer contractor in early 2025. “To my surprise, the agency was doing impactful work in my backyard.”
 
During high school, he began cultivating programming skills that would lay the foundation for his future. A self-professed computer aficionado, Newell taught himself how to code. In algebra class, he discovered that his graphing calculator was programmable, leading him to create basic scripts. He quickly moved to popular software applications, learning how they were built in order to develop his own programs. Artificial intelligence was still on the brink of becoming mainstream, but he was already asking himself, “What are the more interesting things we can teach computers to do?” As a junior in 2019, the precocious programmer merged his talents with his passion for space after being accepted into IV&V’s high school internship program.
Newell created a database for engineering methods, processing hundreds of documents to facilitate the work of his colleagues from a procedural aspect.

“Working as a high school intern really elevated my fascination for NASA,” he said.
 
Following graduation in 2020, Newell headed west to Stanford University in Palo Alto, California, majoring in, naturally, computer science, with a focus on AI and machine learning when both were scaling rapidly across industries and in everyday usage.
 
Instead of pursuing internships in the neighboring Silicon Valley – widely considered the global center for technological innovation – he returned to West Virginia for three consecutive summers, cutting his teeth at the Fairmont facility.

He began researching AI safety as engineers learned how to give spacecraft more onboard autonomy and have them learn on their own. Later, he was on a team responsible for assessing the safety of Terrain Relative Navigation, a vision-based guidance technology that could enable spacecraft to land on planetary surfaces without GPS. During his final internship, generative AI – which creates content based on user prompts – was becoming widespread, and he worked on teams exploring how to responsibly integrate it into mission assurance.
 
Newell also performed research about autonomous space docking at Stanford’s Center for AI Safety, resulting in two publications. Following his undergraduate education, he remained at Stanford to earn a master’s in computer science.
 
When it came time to enter the workforce, one organization was atop his list. A contractor opportunity opened up at IV&V, and he leapt at the chance.
 
“I enjoyed the work at IV&V back in high school and college,” he said. “There’s nothing better than pursuing what aligns with your interests.”
 
As a systems engineer, he has worked primarily on mission safety and security, identifying and resolving system defects and vulnerabilities for such spacecraft as Space Reactor-1 (SR-1) Freedom, Orion, Gateway, and the Human Landing System, all while helping guide NASA’s responsible adoption and development of AI systems.
 
His colleagues took notice of their junior member’s contributions, resulting in Newell being named IV&V Engineer of the Year in 2025 less than a year into the job after identifying more than 70 issues in Gateway – and later SR-1 Freedom – with clear mission impact and, as noted in his award citation, developing a reputation for clearly articulating their implications.
 
“During his internships and now as a full-time engineer, Fletcher has consistently demonstrated exceptional talent, curiosity, and a passion for our mission,” said Wes Deadrick, IV&V director. “He could have gone almost anywhere after Stanford. The fact that he chose to come home to build his career supporting NASA through the IV&V Program makes me incredibly proud.”
 
The Aug. 10 ceremony for Newell and his colleagues was part of NASA’s workforce directive to bring core, mission-critical positions into the civil service, from early-career professionals to seasoned technical experts.
 
“It’s an investment in NASA’s future, giving us the opportunity to bring exceptionally talented people into the civil service while strengthening long-term technical capabilities that support our nation’s most challenging missions,” said Deadrick. “At IV&V, initiatives like this help ensure we continue providing independent expertise and mission assurance that our customers depend on.”
 
For Newell, as he takes this next step in his professional progression as a civil servant computer engineer, he looks forward to expanding on his responsibilities, especially to help NASA return to the Moon. He is currently helping identify safety and security issues, including ones that could impact crew safety and lead to the loss of spacecraft control, related to the agency’s lunar endeavors.
 
“At IV&V, teams are ensuring every major lunar vehicle and system will be ready to safely embark on their missions, not just for flying around the Moon, but also for putting American boots on the surface and eventually establishing and working on a Moon Base,” he said.
 
More than a decade after taking that field trip and now working on some of NASA’s high-priority missions, Newell readily admits he didn’t always envision staying in West Virginia.
 
“It was a little strange coming back, but I was continually getting to do amazing things at an amazing organization in a place I already know,” Newell said. “And that’s really cool.”