1. The Black Hole at the Center of the M87 Galaxy
M87 is a giant elliptical galaxy located near the center of the Virgo galaxy cluster.
At the center of the galaxy lies a supermassive black hole called M87*. Its mass has been calculated to be about 6.5 billion solar masses.
Although M87* is about 55 million light-years away from Earth, its enormous mass allows us to observe the effects it creates on the surrounding gas.
Since the black hole itself does not emit light, imaging it does not mean taking a direct photograph of the black hole's surface. What is actually observed is the radiation produced by the hot matter around the black hole and how this radiation is shaped by the black hole's gravity.
2. Why Can't a Black Hole Be Seen Directly?
The gravity of a black hole is so strong that it does not allow light to escape from within a certain boundary.
Therefore, it is not possible to observe visible light coming directly from the black hole itself.
However, black holes are not completely “inactive” or unobservable. The gas, dust, and other matter around them are affected by the black hole's strong gravity.
In particular, matter falling toward the black hole can reach extremely high temperatures and emit intense radiation.
This radiation is one of the main sources that allows us to observe the region where the black hole is located.
3. The Hot Gas Around the Black Hole
There is hot gas around M87* that is affected by the black hole's gravity.
This gas moves at extremely high speeds around the black hole. As some of the gas approaches the black hole, it gains rotational motion and forms a very hot structure.
The radiation emitted by the particles in this region constitutes a significant portion of the observable signals that form the bright ring around the black hole.
Therefore, the bright structure seen in the M87* image is not the black hole itself; it is the radiation coming from the extremely hot and dense environment around the black hole.
4. How Did the Event Horizon Telescope Work?
The system that enabled the acquisition of the M87* image is called the Event Horizon Telescope (EHT).
The EHT is not a single telescope. It is a global observation network created by simultaneously directing radio telescopes located in different parts of the world at the same celestial object and processing the data obtained together.
This method is known as very long baseline interferometry (VLBI).
When signals from different telescopes are combined, a much higher angular resolution is achieved than a single telescope could provide.
The resolution required to distinguish such a small region as M87* at such a great distance was only possible with a telescope network of this scale.
5. The Bright Ring in the M87* Image
The most striking feature of the M87* image published in 2019 is the bright orange ring and the darker region at its center.
This ring is the image formed by the radiation coming from the hot gas around the black hole.
One reason the ring does not appear as a perfectly regular circle is that the gas around it moves at different speeds in different regions and the light is bent by the black hole's strong gravity.
The radiation from gas moving toward the black hole can reach the observer at different brightness levels depending on the direction of motion.
6. The Dark Region in the Image
The dark region at the center of the M87* image is not an area where the black hole itself is seen.
This region is called the black hole shadow.
The black hole's strong gravity bends the paths of light passing nearby. Some light rays fall into the black hole and cannot escape.
As a result, a darker region appears at the center of the bright ring formed by the light that can reach the observer.
Therefore, although the image is commonly described as a “photograph” of the black hole, from a scientific perspective, what is actually seen is the shadow and the surrounding bright emission region created by the light around the black hole.
7. Bending of Light Around the Black Hole
According to general relativity, massive objects warp space-time.
Since the curvature of space-time around a black hole is extremely strong, the path of light can also change significantly.
Some of the light around M87* comes from gas behind the black hole. This light can reach the observer by being bent by the black hole's strong gravity.
Thus, the observer perceives not only the matter in front of the black hole, but also a complex image formed by light coming from different directions.
8. Why Were Radio Waves Used?
The EHT observed radiation at a wavelength of about 1.3 millimeters in its observations of M87*.
This wavelength allows the study of the radiation emitted by the hot gas immediately surrounding the black hole.
In addition, radio telescopes operating at longer wavelengths can study different physical processes than visible light telescopes.
The radiation observed by the EHT also carries information about the properties of the hot plasma around the black hole related to magnetic fields.
9. A Virtual Telescope the Size of Earth
M87* occupies an extremely small angular area in the sky.
It is not possible to distinguish such a small structure in detail with a single telescope.
By using telescopes on different continents together, the EHT increased the effective diameter of the observation network to nearly the size of Earth.
Therefore, although the EHT is not physically a single telescope, it functioned as a virtual telescope the size of Earth during observations.
The data from the telescopes were then matched with precise timing information and processed together.
10. Creating the Image
The data collected by the EHT was not obtained directly as a photo file.
Measurements from different observatories were processed using computational methods to reconstruct the distribution of radiation around M87*.
Different data processing and image reconstruction methods were used in this process.
The resulting image represents the distribution of light around the black hole as seen in the sky.
Therefore, the M87* image was created by combining a very large amount of observational data, unlike a photograph taken by an ordinary camera.
11. The Size of M87*'s Shadow
The shadow of M87* has a very large physical scale, linked to the mass of the black hole.
The fact that the black hole's mass is about 6.5 billion solar masses contributes to its shadow reaching an angular size wide enough to be observed from Earth.
In the image obtained by the EHT, the diameter of the ring structure is about 42 microarcseconds in scale.
This measurement is consistent with calculations made about the black hole's mass and the geometry of space-time around it.
12. The Jet Around M87*
The black hole at the center of M87 is associated not only with the motion of the gas around it, but also with a massive particle jet extending from the galaxy.
The jet emerging from the center of M87 extends to very great distances and can be observed in radio, visible light, and other wavelengths.
It is thought that the formation of the jet involves physical processes related to the matter around the black hole, magnetic fields, and the rotational motion of the black hole.
This jet is another important indicator of the extremely energetic environment around M87*.
13. Conclusion and Evaluation
The example of M87* shows how a black hole that does not emit light can be imaged using the physical effects around it.
The data collected from different radio telescopes around the world by the EHT were combined to map the radiation produced by the hot gas around M87*. The bright ring in the image represents the radiation from the hot matter around the black hole, while the dark region at the center represents the shadow created by the black hole's strong gravity.
The bending of light around the black hole, the high-speed motion of the hot gas, and the strong warping of space-time all contribute to the characteristic structure of the M87* image.
The M87 image is not so much a direct view of the black hole itself, but rather the result of an imaging method that reveals its existence and fundamental physical properties by observing how the light around the black hole is shaped.*