To astronomers, Segue 1 barely qualifies as a galaxy at first glance. It’s dim, fragile, and so sparsely populated with stars — only a few thousand compared to the Milky Way’s hundreds of billions — that researchers long assumed only dark matter could be holding it together.
But students in a joint astronomy course between The University of Texas at Austin and UTSA have now delivered a curveball: Segue 1 isn’t glued together by just dark matter after all. It’s anchored by a black hole so massive, it outweighs all the galaxy’s stars combined ten times over.
The finding — published in The Astrophysical Journal Letters — didn’t come from veteran astronomers alone. It came from students.
A galaxy too small to behave this way
Segue 1 sits just 75,000 light-years from the Milky Way, a dwarf spheroidal galaxy faint enough that most people would mistake it for empty sky. Typically speaking, galaxies this tiny rely heavily on dark matter halos to keep their stars from drifting into space, but the UT/UTSA team kept running into numbers that didn’t add up.
When they modeled Segue 1’s stellar motions, the stars near the center weren’t behaving like they were under the influence of dark matter. They were circling fast and tight — a motion that usually screams black hole.
“Our work may revolutionize the modeling of dwarf galaxies or star clusters to include supermassive black holes instead of just dark matter halos,” said Nathaniel Lujan, the UTSA graduate student who led the project.
The classroom assignment that turned into a discovery
The research began in a course co-taught by UT Austin’s Karl Gebhardt and UTSA’s Richard Anantua. The pitch was simple: model the gravity inside Segue 1. What students delivered was anything but simple.
“Initially, the project was to model the gravitational dynamics inside Segue 1. That it resulted in a significant discovery is a real testament to the talent of these students and their determined, hard work.” Gebhardt said.
Students ran hundreds of thousands of simulations on UT Austin’s Texas Advanced Computing Center. Each one tested a different mix of possibilities — dark matter, no dark matter, black hole sizes, orbital paths — to see which best fit observations from the W.M. Keck Observatory.
A black hole that shouldn’t exist
Once the team filtered out stars being pulled away by the Milky Way — a process called tidal stripping — the central pattern became impossible to ignore. The remaining stars were moving far too quickly and cleanly to be explained by a small galaxy with only a handful of stars.
The best-fitting models pointed to a monster: a black hole estimated at 450,000 solar masses, roughly ten times the mass of all stars in Segue 1 combined.
“There is a strong relation between the mass of the black hole and the mass of the host galaxy. The black hole in Segue 1 is significantly larger than what is expected,” Gebhardt explained. “If this large mass ratio is common among dwarf galaxies, we will have to rewrite how these systems evolve.”
What Segue 1 might really be
The research team sees two viable scenarios:
- A stripped-down remnant. Segue 1 may have once been a much larger galaxy until the Milky Way stripped most of its stars away.
- A nearby example of a rare class. It may resemble the so-called Little Red Dots — distant early-universe galaxies that formed huge black holes first and stars second. If such is the case, Segue 1 could offer a close-up look at that process.
Either way, the dwarf galaxy has just become one of the most intriguing in the Local Group, the cluster of galaxies that includes the Milky Way and its neighbors.
As Gebhardt puts it, the finding could force astronomers to rethink how the smallest galaxies form, live, and survive.
A big surprise in a very small package
In the end, students — not senior researchers — cracked open a long-standing mystery and found something no one expected to be lurking in one of the least impressive galaxies nearby.
Segue 1 may be tiny. But its black hole just changed the conversation.
And for UT and UTSA students, a classroom project just rewrote a chapter of galactic astronomy.
Correction notice – December 18, 2025
An earlier version contained imprecise wording: “It’s anchored by a black hole so massive it outweighs the entire galaxy ten times over.”
This has been clarified to: “It’s anchored by a black hole so massive it outweighs all the galaxy’s stars combined ten times over.”
The black hole’s mass (~450,000 solar masses) is approximately 10 times the total mass of Segue 1’s visible stars, not the galaxy as a whole.
Sources: AAS Nova, McDonald Observatory, IOPScience