Thousands of Black Holes Could Be Lurking in Our Backyard—and You Can Help Find Them
Our galaxy is, by all theoretical accounts, teeming with black holes. Stellar evolution models predict that the Milky Way should be home to somewhere between 100 million and one billion stellar-mass black holes — the collapsed remnants of massive stars that burned brightly for millions of years before dying in spectacular supernova explosions. Yet despite decades of searching, astronomers have confirmed the existence of only around 70 black holes within our galaxy. The gulf between theory and observation represents one of the most tantalizing unsolved problems in modern astrophysics, and a pioneering citizen science initiative is now enlisting the help of amateur astronomers around the world to close that gap.
The project, called Black Hole Hunters, is led by researchers at the University of Southampton and operates through the Zooniverse platform — the world's largest and most successful citizen science hub. With the help of tens of thousands of volunteer astronomers, the team hopes to dramatically expand the known census of black holes lurking in our cosmic backyard.
Why Are Black Holes So Difficult to Find?
The fundamental challenge in black hole detection lies in the very nature of these extraordinary objects. A stellar-mass black hole — typically between 5 and 100 times the mass of our Sun — is formed when a massive star exhausts its nuclear fuel and its core collapses under its own gravity, creating a region of spacetime curvature so extreme that not even light can escape. This defining characteristic makes direct observation essentially impossible.
Astronomers have historically relied on detecting black holes through their interactions with surrounding matter. When a black hole is part of an X-ray binary system, actively pulling gas from a companion star, the infalling material heats to tens of millions of degrees and blazes in X-ray and radio wavelengths — a dramatic beacon detectable across thousands of light-years. Facilities like NASA's Chandra X-ray Observatory have been instrumental in identifying such systems. However, the vast majority of stellar-mass black holes are thought to be quiescent — solitary wanderers drifting silently through the interstellar medium, consuming nothing, emitting nothing, and leaving virtually no trace of their existence through conventional observational methods.
"The black holes we know about represent just the tip of the iceberg. For every black hole we've found actively feeding on a companion, there could be thousands more simply drifting in the dark, invisible to our telescopes." — Dr. Adam McMaster, University of Southampton
Einstein's Legacy: Gravitational Lensing to the Rescue
The solution to this observational challenge lies not in detecting the black holes themselves, but in detecting the distortions they impose on the fabric of spacetime. Albert Einstein's General Theory of Relativity, published in 1915, predicted that massive objects warp the geometry of spacetime around them, causing light to follow curved paths in their vicinity. This phenomenon, known as gravitational lensing, was first confirmed observationally during the solar eclipse of 1919 by Sir Arthur Eddington — a watershed moment in the history of physics.
When a black hole passes directly in front of a background star from our line of sight, its gravitational field acts as a cosmic magnifying glass, bending and amplifying the star's light in a phenomenon called microlensing. This produces a characteristic, temporary brightening of the background star that can last anywhere from days to months. At the grandest cosmological scales, when the alignment is nearly perfect between a massive foreground object and a distant background galaxy, this same effect produces the breathtakingly beautiful phenomenon known as an Einstein ring — a complete circle of distorted light around the lensing object. NASA's Hubble Space Telescope has captured some of the most stunning examples of this phenomenon.
For isolated black holes, microlensing events are typically one-off occurrences. A black hole drifts across our line of sight to a distant star, briefly brightens it, and then the moment passes — never to repeat. Identifying such transient events requires monitoring millions of stars continuously, a monumental undertaking. However, there exists a particularly elegant subset of these events that offers a far more accessible detection pathway.
Gravitational Self-Lensing: A Repeating Cosmic Signal
The key phenomenon at the heart of the Black Hole Hunters project is gravitational self-lensing. In certain binary star systems, rather than actively consuming its companion, a black hole orbits in relative peace alongside a normal star. As the black hole periodically passes between its companion star and our line of sight during the course of its orbit, it briefly amplifies the star's light through gravitational lensing. Because this geometry repeats with every orbital cycle, the lensing signal recurs predictably — a periodic, repeating brightening in what astronomers call the star's light curve.
This repeating signal is both the project's greatest asset and its greatest challenge. Unlike a one-off microlensing event that might be dismissed as instrumental noise, a periodic lensing signature carries a distinctive, recognizable fingerprint. However, other astrophysical phenomena — including sunspot activity, pulsating variable stars, and eclipsing binaries — can produce superficially similar patterns in light curve data, making automated identification alone insufficient and human pattern recognition essential.
- Self-lensing signal duration: Typically a few hours to a few days per event, depending on orbital parameters
- Orbital periods: Can range from days to years for known black hole binary systems
- Brightness amplification: Generally modest — often only a few percent above baseline — making precision photometry essential
- Key differentiator from eclipsing binaries: Self-lensing produces a symmetric brightening rather than a dimming, with no secondary eclipse from the compact object
From SuperWASP to TESS: Upgrading the Data Pipeline
The Black Hole Hunters project was originally launched in 2021 by Dr. Adam McMaster, then a doctoral student at The Open University and now based at the University of Southampton. In its initial incarnation, the project analyzed photometry data from the Super Wide Angle Search for Planets (SuperWASP) survey — a ground-based network of cameras dedicated to monitoring stellar brightness. While SuperWASP provided a rich dataset of light curves, ground-based observations are inevitably degraded by atmospheric interference, weather interruptions, and the day-night cycle, introducing significant noise that complicated the identification of subtle self-lensing signals.
Recognizing these limitations, Dr. McMaster undertook a major overhaul of the project's data pipeline, transitioning to data from NASA's Transiting Exoplanet Survey Satellite (TESS). Launched in April 2018, TESS was designed primarily to search for exoplanets transiting nearby, bright stars, but its extraordinary capabilities make it an equally powerful tool for hunting black holes. Operating in space, far above the distorting effects of Earth's atmosphere, TESS monitors the brightness of hundreds of thousands of stars every few minutes, producing continuous, high-precision light curves of unparalleled quality. The satellite surveys large swaths of the sky in successive 27-day observation sectors, collectively building a photometric database of remarkable scope and depth.
This space-based advantage translates directly into cleaner, more reliable data — exactly what is needed to tease out the subtle, periodic brightness variations that betray the presence of a self-lensing black hole companion.
Artificial Intelligence Meets Human Intuition
The second major innovation in the relaunched Black Hole Hunters pipeline is the integration of artificial intelligence. The sheer volume of TESS data — encompassing millions of individual stellar light curves — makes manual inspection by human astronomers utterly impractical without computational assistance. To address this, Dr. McMaster and his colleagues developed a machine learning algorithm trained to classify light curves as either scientifically "interesting" (exhibiting potential self-lensing signatures) or "boring" (showing no compelling features of interest).
This AI preprocessing step is transformative in scale. What would have taken a dedicated team of researchers decades to sort through by hand can now be accomplished in a matter of days. The algorithm dramatically reduces the haystack before human volunteers are asked to search for the needle.
However, machine learning algorithms are not infallible. They can be fooled by astrophysical phenomena that superficially resemble self-lensing signatures — including starspot modulation from active stellar surfaces, pulsating variable stars such as Cepheids and RR Lyrae variables, and instrumental artifacts introduced by TESS itself, including scattered light and momentum dumps from the spacecraft's reaction wheels. These edge cases require the nuanced pattern recognition that human observers uniquely provide — even untrained ones.
"You don't need a degree in astronomy to help us find black holes. If you can recognize a pattern, you can contribute to genuinely cutting-edge science." — Grace Clarke, undergraduate volunteer, University of Southampton
As Grace Clarke, an undergraduate at the University of Southampton who serves as both a project contributor and volunteer, noted in a recent press release, the task requires only basic pattern recognition, not specialist astronomical training. Volunteers are shown pre-processed light curves and asked to identify those displaying the characteristic repeated brightening signature of gravitational self-lensing — a skill that can be developed within minutes of starting the project.
The Broader Scientific Stakes
The implications of this research extend well beyond filling in a numerical census. Discovering a significant population of quiescent black holes in binary systems would provide transformative insights into several of the most pressing questions in stellar physics and gravitational wave astronomy.
- Stellar evolution: Understanding how many massive stars end their lives as black holes versus neutron stars helps constrain models of stellar death and supernova physics.
- Gravitational wave source populations: Binary systems containing black holes are the progenitors of the compact binary mergers detected by facilities like LIGO and Virgo. Mapping the local population informs predictions about gravitational wave detection rates.
- Binary star evolution: Self-lensing systems provide rare laboratories for studying how black holes and companion stars interact and evolve together over millions of years.
- The black hole mass gap: Identifying new black hole systems could shed light on the theoretically predicted but poorly constrained mass gap between the heaviest neutron stars and the lightest black holes.
- Future telescope targets: Every candidate system identified by volunteers becomes a priority target for follow-up observations with next-generation instruments, including the Vera C. Rubin Observatory and the European Space Agency's PLATO mission.
How to Join the Hunt
The Black Hole Hunters project is currently approximately 43% complete, with nearly 23,000 volunteers having contributed close to 12 million individual classifications since its relaunch in October 2021. The team is conducting an active outreach campaign to recruit additional volunteers and push the project toward completion.
Participation requires nothing more than a computer and an internet connection. New volunteers can begin contributing meaningful classifications within minutes of creating a free Zooniverse account. The project's built-in tutorial guides users through the classification process, and a community forum allows participants to discuss ambiguous cases with fellow volunteers and professional astronomers alike. This collaborative aspect of citizen science is one of its most powerful features — and has historically led to unexpected discoveries when attentive volunteers notice anomalies that automated systems and even professional researchers initially overlooked.
The candidates identified through this process will not end the search — they will begin the next phase of it. Promising self-lensing targets identified by volunteers will be prioritized for follow-up observations using more powerful telescopes, where spectroscopic and radial velocity measurements can confirm the presence of a black hole companion and characterize its mass, orbital parameters, and evolutionary history.
If you are interested in contributing to one of modern astronomy's most exciting detective stories — and potentially putting your name in the acknowledgments of a peer-reviewed discovery paper — you can join the project directly through the Black Hole Hunters Zooniverse page.
Learn More
- University of Southampton – Scientists Enlist Armchair Astronomers to Find Thousands of Black Holes Hiding in Our Galaxy
- Black Hole Hunters – Zooniverse Project Page
- NASA – TESS: Transiting Exoplanet Survey Satellite
- NASA Chandra X-ray Observatory – Black Holes
- LIGO Scientific Collaboration – Gravitational Wave Astronomy
