A black hole is neither a tunnel through space nor a universal vacuum cleaner. It is a region where matter has been concentrated to such an extent that a boundary, called the event horizon, separates what can still communicate with the rest of the universe from what can no longer escape. This definition raises several questions: how does such a region form, what does the orange ring in the famous images really show, and how can we establish the existence of an object that does not reflect any light?
To answer this question, we must follow the evidence rather than spectacular images. Researchers combine the motions of stars, the light from nearby gas, radio measurements from observatories around the world, and gravitational waves. Each method reveals a part of the phenomenon and has its limitations. The media in this dossier are scientific papers or animations identified as such; no video claims to show in real time the interior of a black hole.
The boundary that defines a black hole
According to general relativity, gravity is a consequence of the curvature of spacetime. When a sufficiently massive object is located in a sufficiently compact region, an event horizon appears. For an outside observer, no signal originating from the inside can cross this boundary outward, even if it is traveling at the speed of light. The horizon is not a solid shell on which one could land. It is a causal boundary: beyond it, possible trajectories directed toward the future no longer lead back to the outside Universe.
This boundary must be distinguished from the "shadow" reconstructed on certain images. Gravity deflects light rays, and some photons are captured. The apparent dark region depends on this capture, the geometry of the emission, and the data processing. It does not simply delineate the physical edge of the horizon. Therefore, to say that the luminous ring is the surface of the black hole is inaccurate: we are mainly studying the light from the matter that is still outside.
How are they born?
A well-established path to a stellar-mass black hole begins with the end of the life of certain very massive stars. When their cores can no longer withstand their own gravity, they collapse. Depending on the mass of the core, its composition, and previous losses of matter, the remnant can become a neutron star or a black hole. Not all stars end this way: the Sun, for example, does not have the mass necessary to follow this scenario.
Supermassive black holes occupy the centres of many galaxies. Sagittarius A*, at the centre of the Milky Way, is about four million solar masses. M87*, in the galaxy Messier 87, is much more massive. Mergers and gas accretion contribute to the growth of these objects, but the very early formation of the giants observed in the distant Universe remains a subject of research. Intermediate-mass candidates could help reconstruct this history.
Why do we see light around a black object?
The matter approaching does not necessarily fall in a straight line. If it has motion around the black hole, it can form an accretion disk. The interactions in this gas heat it up; the emission can appear in many parts of the spectrum, from radio waves to X-rays. The orange color of the Event Horizon Telescope images is a visualization convention for radio measurements. It is not the color that a human eye would see on the scene.
Not all black holes have a bright disk at all times. Their environment and their fuel supply change. Some systems produce fast jets of matter. These jets are not matter coming out from inside the horizon: they form in the outer region, which is related to gas, magnetic fields, and rotation. Observing a jet may signal intense activity, but it is still necessary to link this activity to the specific system that is feeding it.
First proof: follow the stars
At the centre of the Milky Way, dust blocks the view in visible light. Observations in the infrared have allowed us to track stars over many years on tight orbits around a very compact region. Their velocities and trajectories reveal the mass of the central object, even though it emits almost no light. This work provides independent confirmation of the radio images, and thus allows us to measure the gravitational effect of a body without directly imaging it.
An orbit provides more than just a sense of motion. With positions recorded at different times, astronomers fit a dynamical model, estimate a mass, test the distance, and assess instrumental uncertainties. Stars passing very close to Sagittarius A* also allow for the testing of effects predicted by general relativity. The result comes from a series of repeated, compared, and reanalyzed measurements, rather than a single spectacular snapshot.
Second proof: a telescope the size of the Earth
The Event Horizon Telescope, or EHT, combines distant radio telescopes to achieve angular resolution that is impossible with a single antenna. Observatories record the same object with precise timing. Teams then combine the signals, calibrate the data, and reconstruct an image. This technique, very-long-baseline interferometry, exploits the distances between antennas; it does not physically fill all the spaces between them. The incomplete coverage requires careful analysis of models and uncertainties.
The first image of M87* was published in 2019, based on observations from 2017. The image of Sagittarius A* was presented in 2022, also based on data from 2017. The gas near Sagittarius A* is rapidly evolving on the scale of the observations, which complicates the reconstruction. The published image summarizes several compatible reconstructions: it does not represent a single snapshot. The EHT does not reveal the interior of the horizon nor a continuous video of the matter within it.
Sagittarius A*: What does the ring mean?
The image of Sagittarius A* shows a dark center and a bright structure. The radio measurements from the EHT network, and their processing, are consistent with models of a supermassive black hole surrounded by gas. The mass inferred from this image agrees with that obtained from stellar orbits. It is the convergence of independent methods that makes the conclusion particularly robust; the ring alone does not tell the whole story.
Light follows curved paths due to gravity. It can reach the observer through different paths, while some of it is captured. The details of the brightness also depend on the gas and magnetic fields. A clearer image is therefore not enough to explain each bright spot: the data must be compared to simulations and checked for which properties survive the reconstruction choices. The teams are publishing their methods and error margins along with the image.
Third evidence: gravitational waves
When two black holes orbit each other and then merge, their accelerated motion produces ripples in spacetime. Detectors like LIGO and Virgo look for tiny signals in their measurements that are consistent with these models. The first signal of two black holes merging was detected in 2015 and announced in 2016. Here, we are not looking at a ring of gas: we are measuring a transient distortion of spacetime, whose shape provides information about the masses and dynamics of the pair.
Gravitational waves complement the light-based observations. A merger of two black holes may not produce any easily identifiable electromagnetic emission. Detectors also do not provide a detailed image of the environment. Combining multiple instruments increases confidence and allows testing of physics in extreme regimes. This diversity of evidence is more informative than searching for a single “definitive picture” of all black holes.
What would happen if we approached?
At a great distance, a black hole attracts like any other body of the same mass. It does not magically suck in an entire galaxy. If the Sun were replaced, in a thought experiment, by an object of the same mass that had become a black hole, the Earth's orbit would remain approximately the same; the absence of sunlight would obviously be catastrophic. The situation becomes special for a traveler approaching the compact region.
The differences in gravity between the near and far sides of an object can stretch it: this is the tidal effect, also known as "spaghettification". Its intensity near the horizon depends, among other things, on the mass of the black hole. For a distant observer, the light from an object approaching the horizon appears to be increasingly redshifted and weakened. The time experienced by the object and the signal received from afar do not behave in the same way. The drawings showing a person frozen and clearly visible forever at the horizon simplify the situation too much.
What the images don't show
The observations strongly confirm the existence of compact objects consistent with black holes in general relativity. They do not provide a view of the inside of the horizon, nor do they prove the existence of tunnels to another universe. Hawking radiation is a major theoretical prediction, but no such radiation from an astrophysical black hole has been directly measured to date.
The question of information, the details of the central region, and the link between gravity and quantum physics remain open. Recognizing these limitations specifies what is observed, calculated, or hypothesized. To read a new scientific image, three questions are useful: which instrument recorded which signal? What steps transformed that signal into an image? What other independent measurement confirms the interpretation?
A survey open to young African scientists
Studying black holes doesn’t necessarily require a giant telescope. Published data, computational software, statistical analysis, visualization, and source verification open up opportunities for training and research. A classroom can compare stellar orbits and EHT images, then draw a clear boundary between measurement, modeling, and graphical representation. An academic team can contribute to signal analysis or modeling with the necessary skills and collaborations.
The case of ALMA shows that a discovery also relies on a discrete technical chain: antennas, clocks, synchronization, data transfer, processing, and control. These fields join skills useful in telecommunications, instrumentation, and scientific programming. The best entry point is to ask a verifiable question and document each step of the answer.
To remember
A black hole is defined by its event horizon; its media image mostly shows the matter and light near it. Orbiting stars, radio interferometry, and gravitational waves provide distinct evidence that reinforces each other. M87* and Sagittarius A* are two different objects. Their historical images are based on data from 2017 published in 2019 and 2022. The two videos here are ESO educational montages or animations, not continuous shots from inside a black hole.
