Black holes are among the strangest objects in the universe. Their gravity is so powerful that, beyond a boundary called the event horizon, not even light can escape. Yet astronomers have measured their masses, tracked stars orbiting them, detected collisions between them and even produced images of the dark regions surrounding them.
How is that possible?
Scientists detect black holes by measuring their effects on the matter, light and space-time around them. The evidence comes from several independent techniques, including stellar orbits, X-ray emissions, gravitational waves and the Event Horizon Telescope. Together, these observations provide compelling evidence that black holes are real astrophysical objects, rather than simply mathematical predictions.
NASA’s guide to how scientists detect black holes explains the central principle: even when an object cannot be observed directly, its influence on its surroundings can reveal its presence.
1. Stars Reveal the Gravity of an Invisible Object
One of the strongest ways to identify a black hole is to observe what happens to nearby stars.
Gravity governs the movement of stars, planets and galaxies. By measuring an object’s orbit, astronomers can estimate the mass of whatever is pulling on it. If a star follows a tight orbit around something invisible, and the inferred mass is too concentrated to be explained by an ordinary star cluster or another known object, a black hole becomes the leading explanation.
A famous example lies at the centre of our own galaxy, the Milky Way.
Astronomers have tracked stars orbiting an apparently empty region known as Sagittarius A*. Some of these stars travel at extraordinary speeds around an object containing approximately four million times the mass of the Sun.
One particularly important star, S2, completes its orbit in roughly 16 years. Observations of its motion, including its close approach to the galactic centre, have provided strong evidence for a compact, supermassive object.
The conclusion is difficult to explain without a black hole. An ordinary cluster containing millions of stars would have to occupy an incredibly small region and would behave differently under the observed gravitational conditions.
The research has helped establish Sagittarius A* as the supermassive black hole at the heart of the Milky Way.
NASA explains this method in its overview of black holes and their detection.
2. Hot Gas and X-Rays Expose Black Holes
Black holes themselves do not emit light that can escape from inside their event horizons. However, the material surrounding them can become extraordinarily bright.
When gas is pulled towards a black hole, it often forms a flattened, rotating structure called an accretion disk. As the material moves inward, gravitational energy is converted into heat. Gas close to the black hole can reach temperatures of millions of degrees, producing radiation that includes X-rays.
Space telescopes such as NASA’s Chandra X-ray Observatory can detect this high-energy radiation. Researchers analyse the brightness, spectrum and changes in the emission to investigate the extreme conditions around the suspected black hole.
Some black holes are discovered in binary systems, where a black hole and a normal star orbit a shared centre of mass. If the black hole pulls gas from its companion, the resulting accretion disk may emit intense X-rays.
The pattern of radiation can reveal how matter behaves near the black hole, while observations of the companion star help astronomers estimate the invisible object’s mass.
Not every black hole has a bright accretion disk. A black hole with little nearby material may emit very little detectable radiation and remain difficult to find. That is why astronomers combine X-ray observations with other techniques.
NASA’s explanation of black hole accretion disks describes how the surrounding material becomes a source of information about an otherwise dark object.
3. Gravitational Waves Reveal Black Hole Collisions
Some black holes are detected without relying on light at all.
When two black holes orbit one another, they lose orbital energy by producing gravitational waves. These waves are ripples in space-time that travel outward at the speed of light. As the black holes spiral closer together and merge, the waves carry information about the masses, spins and motion of the objects.
In September 2015, the Laser Interferometer Gravitational-Wave Observatory, known as LIGO, made the first direct detection of gravitational waves. The signal, announced in February 2016, came from the merger of two black holes approximately 1.3 billion light-years away.
The signal matched the pattern predicted by general relativity for two massive objects spiralling together and merging into a single black hole.
LIGO does not photograph these objects. Instead, its instruments measure extraordinarily small changes in the relative lengths of their perpendicular arms as a gravitational wave passes through Earth.
The shape of the detected signal allows scientists to reconstruct the event that produced it. Subsequent observations by LIGO and its partner observatories have revealed many more compact-object mergers, including black hole mergers.
This method is especially valuable because it can detect black holes that produce little or no observable light.
The LIGO Laboratory explains how gravitational waves provide a new way to study some of the universe’s most energetic events.
4. The Event Horizon Telescope Captures a Black Hole’s Shadow
In April 2019, the Event Horizon Telescope collaboration released the first image of a black hole’s shadow. The object was located in the galaxy Messier 87, commonly called M87, approximately 55 million light-years from Earth.
The image showed a bright, ring-like structure surrounding a dark central region. It was a landmark achievement, but there is an important distinction: the image was not a conventional photograph of the black hole itself.
The Event Horizon Telescope, or EHT, is a global network of radio observatories that work together using a technique called very-long-baseline interferometry. Combining observations from widely separated telescopes gives researchers the resolving power needed to examine extremely small structures on the sky.
In M87, the bright emission comes from hot material around the black hole. Its intense gravity bends the paths of light, while the event horizon prevents light from escaping from within it. Together, these effects produce the dark shadow visible against the surrounding emission.
In May 2022, the collaboration released an image of Sagittarius A*, the black hole at the centre of the Milky Way. Imaging it was particularly challenging because the gas surrounding it changes rapidly, making the observed structure more variable during the observation process.
These images offer a powerful test of predictions from Einstein’s theory of general relativity under extreme gravitational conditions.
The Event Horizon Telescope collaboration provides further information about the observations, imaging methods and scientific results.
5. Can Gravitational Lensing Reveal an Invisible Black Hole?
There is another method that can help astronomers find black holes: gravitational lensing.
According to Einstein’s general theory of relativity, gravity bends the path of light. When a massive object passes in front of a more distant star from our viewpoint, the foreground object’s gravity can magnify or distort the background star’s light.
If the foreground object is a black hole, astronomers may detect its presence through a temporary change in the brightness or apparent position of the background star, even if the black hole emits no detectable light of its own.
This technique is called gravitational microlensing. It can help researchers identify isolated compact objects that would otherwise be extremely difficult to detect.
However, a brightening event alone does not automatically establish that the lens is a black hole. Astronomers need additional measurements, such as the duration of the event, the lens’s estimated mass and its motion, to distinguish a black hole from other possible objects.
The method is particularly useful because not all black holes are surrounded by luminous disks or accompanied by visible stars.
What Makes the Evidence Convincing?
No single observation is responsible for establishing the existence of black holes. The case rests on multiple lines of evidence that agree with the same physical picture.
Stellar orbits reveal extremely massive, compact objects. X-ray telescopes observe the intense radiation produced by gas near black holes. Gravitational-wave detectors measure the signals generated by merging black holes. The Event Horizon Telescope resolves the shadows formed by light interacting with matter and gravity near their boundaries.
Each technique has limitations, and researchers carefully consider alternative explanations. But when independent measurements converge on the same conclusion, confidence in that conclusion grows substantially.
Black holes are therefore not accepted simply because they fit a mathematical theory. Their predicted effects have been observed repeatedly across different environments and with different instruments.
The Invisible Objects We Can Still Study
The apparent contradiction at the heart of black hole astronomy is that an object can be invisible and yet leave unmistakable evidence of its presence.
Astronomers cannot receive light or information from inside a black hole’s event horizon. They can, however, study its gravitational influence, the radiation from surrounding matter, the waves produced during mergers and the way it bends light.
These observations have transformed black holes from theoretical possibilities into measurable objects that scientists can investigate.
As observatories improve and gravitational-wave detectors become more sensitive, researchers will be able to study more black holes, including objects that have remained hidden from conventional telescopes.
The black hole itself may remain dark, but the universe continually reveals clues about what is happening around it. By learning to read those clues, scientists have found a way to investigate some of the most extreme objects in existence.
