Citizen scientists can help search for hidden black holes by inspecting graphs of stars whose brightness changes over time. In Zooniverse’s Black Hole Hunters project, volunteers flag brief brightening patterns in measurements from NASA’s TESS mission. Those patterns can be clues to gravitational lensing—but they are candidate signals, not direct views of black holes or confirmed discoveries.
How can light reveal an object that emits no light?
A black hole can still affect light from a more distant star. Its gravity bends the light’s path; when the alignment is close, the background star can appear temporarily brighter. Astronomers call this effect gravitational microlensing. The black hole is inferred from what its gravity does to the light, rather than seen directly.
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In a binary system, a black hole may pass in front of its stellar companion from our viewpoint. The resulting brief brightening is called self-lensing. It is a related but distinct setting from an isolated black hole lensing a background star.
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What do volunteers look for in Black Hole Hunters?
Black Hole Hunters, hosted on Zooniverse, presents brightness measurements from NASA’s Transiting Exoplanet Survey Satellite (TESS) as light curves: graphs showing how a star’s brightness changes over time. Volunteers inspect the curves and mark a short, peak-like increase that could match a self-lensing signal. The project describes the graphs and task on its research page.
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No telescope or special equipment is needed for this task; participants work with the supplied graphs in a web browser. The goal is visual triage: identify interesting sections for researchers to review, not diagnose a black hole from one graph.
How does a possible signal become a research lead?
- TESS measures changing brightness. The project makes stellar time-series measurements available as light curves.
- Volunteers flag possible peaks. Several people classify each item, so the project can combine independent judgments rather than rely on one person’s call.
- Researchers review a shorter candidate list. Repeated classifications help narrow the data to events worth closer examination.
- Follow-up observations test the interpretation. Additional measurements, including precise observations of stellar motion, can help determine whether a candidate is consistent with lensing and what kind of object caused it.
The signal is expected to be rare. The project FAQ says researchers expect roughly 100 self-lensing examples in TESS; that is an expectation, not a count of confirmed detections. The FAQ also describes simulated curves used to help volunteers recognize the expected signal shape and estimate the minimum detectable magnification. Because the FAQ is a dynamic project page, its figures and status can change.
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Why a bright peak is not proof of a black hole
Other kinds of stellar variability, including flares and pulsations, can also create changes in brightness. A peak may be worth investigating without being unique to gravitational lensing, and even a lensing signal does not by itself establish that the lens is a black hole. Researchers need further evidence to distinguish explanations.
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →The Black Hole Hunters FAQ, accessed October 7, 2026, states that “No conclusive examples of self-lensing by a black hole in a binary system have ever been found.” Read this as the project’s status statement on that date, not as a permanent conclusion about all black-hole lensing or a guarantee that the status has not since changed.
Photometric and astrometric microlensing answer different questions
Black Hole Hunters focuses on photometric microlensing: the apparent change in a star’s brightness. Another clue can come from astrometric microlensing, a tiny shift in the star’s apparent position as the foreground object’s gravity bends its light. NASA’s Roman Space Telescope explainer describes how that positional signal can help constrain a lens’s mass, distance, and motion.
A separate NASA/Hubble account illustrates the broader method with an isolated black-hole candidate: the background star’s brightening lasted about 270 days, and astronomers followed positional measurements over several years. This was not a Black Hole Hunters result. NASA’s Hubble account, last updated March 31, 2025, describes that event.
NASA’s Roman explainer also gives an estimate of about 100 million stellar-mass black holes in the Milky Way. That is an estimated population, not a direct census. The challenge is that many may be isolated and difficult to detect unless their gravity reveals them through lensing.
How Black Hole Hunters differs from Euclid’s Space Warps
Both projects enlist volunteers to find gravitational-lensing clues, but they examine different data and phenomena.
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| Project | Target | Data and signal | Scientific aim |
|---|---|---|---|
| Black Hole Hunters | Potential stellar-mass compact objects, including black holes | TESS time-series light curves; volunteers look for temporary brightening | Identify candidate self-lensing events for further investigation |
| Space Warps | Foreground galaxies acting as lenses | Euclid images; volunteers look for shapes such as arcs, rings, and multiple images | Study galaxy mass, dark matter, and dark energy |
ESA’s April 21, 2026, Space Warps launch article says the project will show volunteers about 300,000 AI-preselected images drawn from 72 million galaxies, and scientists expect the search to yield more than 10,000 new lenses. Those are project plans and expectations, not confirmed results. The article also reports 500 galaxy-galaxy strong lenses in the first 0.04% of Euclid data. These galaxy-scale searches are not the same as looking for brief brightness changes around possible stellar-mass black holes.
What citizen science contributes
Rare signals can be hard to find among large collections of noisy light curves. Volunteers provide many visual checks, and the project combines classifications to make a more manageable shortlist for researchers. The scientific value is in helping identify where to look next; confirmation depends on additional analysis and observations.
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