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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteCitizen scientists can help flag possible black holes by inspecting a star’s brightness over time for a brief brightening that gravity may have caused. On Zooniverse’s Black Hole Hunters project, volunteers examine TESS light curves and mark possible microlensing signals. They are identifying candidates—not seeing a black hole directly or confirming a discovery.
How can gravitational lensing reveal an invisible black hole?
A black hole need not emit light to affect what astronomers observe. Its gravity can bend light from a more distant star. If the alignment is close enough, the star can appear temporarily brighter from Earth: a form of gravitational microlensing. In a binary system, a black hole may pass in front of its stellar companion and magnify the companion’s light; this is called self-lensing.
The effect is indirect evidence. The measured light comes from the background star, while the proposed lens is inferred from how its gravity changes that light. NASA’s Roman Space Telescope explainer describes microlensing as one way to find otherwise hidden black holes. It cites an estimate that the Milky Way may contain about 100 million stellar-mass black holes; that is an estimate, not a census.
What do volunteers do in Black Hole Hunters?
NASA’s TESS mission measures changes in stars’ brightness. Black Hole Hunters, hosted on Zooniverse, presents measurements as light curves—graphs showing a star’s brightness over time. Volunteers inspect the graphs and flag sections with a brief, peak-like brightening that might fit a lensing event. The project explains that “These graphs are called light curves, and they show how bright a particular star was each time it was measured.”
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The task is browser-based: volunteers work with graphs supplied by the project, so no special equipment is needed. Multiple people classify each item. Combining their classifications helps reduce the effect of individual mistakes and narrows the data to candidates researchers can investigate further.
How does a possible signal become a candidate?
- Review a light curve. Look across the supplied brightness measurements for a temporary peak rather than assuming every change is a lensing event.
- Mark a possible event. Flag the relevant part of a curve when it resembles the project’s expected self-lensing signal.
- Combine classifications. Repeated reviews help create a shortlist from many individual assessments.
- Investigate candidates with further observations. Researchers need additional evidence; precise measurements of stellar motion can help test whether a proposed lens is a black hole.
Events are expected to be rare. The project FAQ says researchers expect roughly 100 self-lensing examples in TESS data; this is a project expectation, not a count of confirmed discoveries. The FAQ also describes simulated light curves as a way to help volunteers learn the expected signal shape and estimate the minimum detectable magnification.
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Why a bright peak is not proof of a black hole
A temporary rise in a star’s brightness is not unique to gravitational lensing. Stellar flares and pulsations can also change a light curve, so a peak is a reason to investigate—not a diagnosis. Repeated volunteer classifications help with triage, but they cannot establish the identity of the object causing the signal.
The Black Hole Hunters FAQ, accessed October 7, 2026, says that no conclusive example of self-lensing by a black hole in a binary system had yet been found. That is the FAQ’s status statement at the time it was accessed, not a timeless claim. The project’s aim is to help researchers identify promising signals for further study.
Photometric and astrometric microlensing measure different effects
Black Hole Hunters focuses on photometric microlensing: a change in a background star’s apparent brightness. A related method, astrometric microlensing, measures a tiny apparent shift in the star’s position as the foreground object’s gravity bends its light. NASA’s Roman explainer describes how positional measurements can help constrain a lens’s mass, distance, and motion.
NASA’s Hubble account of an isolated black-hole lensing event describes background-star brightening that lasted about 270 days. NASA reports that astronomers followed the event with positional measurements over several years. This illustrates the broader microlensing method; it was not a Black Hole Hunters result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How Black Hole Hunters differs from Euclid’s Space Warps
Both projects use citizen science and gravitational lensing, but they ask volunteers to look for different signals in different data.
| Project | Target and data | Signal volunteers look for | Scientific aim |
|---|---|---|---|
| Black Hole Hunters | Potential stellar-mass compact objects; TESS time-series light curves | Temporary brightening in a star’s measured brightness | Find candidate hidden black holes through microlensing |
| Euclid’s Space Warps | Foreground galaxies; Euclid images | Galaxy-scale lens shapes such as arcs, rings, and multiple images | Study galaxy mass, dark matter, and dark energy |
ESA’s April 21, 2026, Space Warps article reports 500 galaxy-galaxy strong lenses in the first 0.04% of Euclid data. It also describes a planned workflow showing volunteers about 300,000 AI-preselected images drawn from 72 million galaxies, with scientists expecting more than 10,000 new lenses from that search. Those latter figures are dataset and project expectations, not confirmed results from the planned search.
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