There are also different types of black holes. Stellar mass black holes are formed from the death of very massive stars, where the core of the star collapses under the force of its own gravity after it runs out of fuel, or sometimes from the collision of neutron stars. Supermassive black holes are often found in the center of galaxies and can range from 100,000 to several billion solar masses. We do not currently know how these black holes form, but it is an active area of research. Another active area of research is intermediate mass black holes. These live in between solar mass and supermassive black holes, however very few of them have been observed and we do not know how they form.
Since no light can escape from a black hole, they appear completely dark in the night sky which makes it impossible to directly observe them. However, there are several observations that suggest the existence of black holes.
The main way to observe a black hole is to observe the material around it. The material around a black hole, called an accretion disk, gives off a lot of radiation, especially x-ray radiation, which can be observed through telescopes. There has even been an image captured by the Event Horizon Telescope where you can see bright material with a dark spot in the center. This image was released in 2019 and is the center of the M87 galaxy.
This image is relatively new, so before an image like this could be captured there were other observations that led astronomers to believe in the existence of black holes. One of these observations is Tidal Destructive Events or TDE. These events occur when a star gets too close to a black hole and the tidal force of the black hole is larger than the force of gravity holding the star together and the star is ripped apart in a process called spaghettification. These appear as large flares of light that can be observed in the optical, ultraviolet, and x-ray spectra. Some of the material from the destroyed star gets added to the accretion disk allowing more visible material to be studied around the black hole, while the rest of the material escapes. The video below shows an animation of one of these events occurring.
Another way to observe black holes is to observe the motions of the stars around them. For example, in the video from ESO below you can see the star S2 orbit around what appears to be an invisible mass.
<iframe width="560" height="315" src="https://www.youtube.com/embed/495OIRMV-1c" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>Based on observations, black holes are believed to be extremely massive, spinning objects with incredibly strong gravity.
Following the movement of a star orbitting around an unseen object over time allows astronomers to measure the acceleration of the star and subsequently the mass of the object it is orbitting. Since the mass can be measured we know that there is some kind of matter there even though we can not see it. Using this calculated mass, astronomers are also able to calculate the escape velocity at different distances from the center of the object, and have found that since the object is so massive the escape velocity inside a certain radius, called the event horizon, is greater than the speed of light. This means that no photons can escape from within the event horizon and explains why there is something so massive we can see the effect of its gravity on the stars around it but not the object itself.
The equation for escape velocity is $ v_{\text{esc}} = \sqrt{\frac{2GM}{r}} $ where G is the gravitational constant, M is the mass of the object, and r is the distance from the object. If we know the mass of a given black hole, we can plug this into the equation and we can also plug the speed of light in as the escape velocity to solve for the radius where the escape velocity is the speed of light. Since r is in the denominator of the equation, we know that the escape velocity increases as r decreases, so anywhere where the radius is less than radius we previously solved for, the escape velocity is greater than the speed of light.
Additionally, by studying the matter around the black hole, we can conclude that black holes are spinning and pulling material into them. The accretion disk is very bright and peaks in the x-ray which means it is very hot. This heat is caused by the material moving very fast, which suggests that something is pulling on it causing it to move this fast. This along with how massive they are leads astronomers to conclude that black holes have very strong gravity and pull in the surrounding material towards the event horizon. This is also confirmed by TDEs, since the gravity is so strong it is able to completely destroy a star that gets too close. The light we get from the accretion disks also allows astronomers to study the motion of the disk and see that it "wobbles" over time. This leads astronomers to find how much the black hole affects the motion of the accretion disk and learn that black holes spin. Since they are so massive, as they spin they drag space-time with them which is what causes the "wobble" of the accretion disk.
I believe that based on the observations, the black hole model is very believable. Black holes explain both the motions of stars around unseen objects as well as the source of observed x-rays. They also explain why they are not visible to us. Additionally as our observational tools continue to improve and we are able to see the effect of these objects more clearly, the observations continue to match the model. For instance, the first image of a black hole was not released until 2019 but they were first considered in 1915, over a hundred years before. If the model was completely wrong I doubt it would have lasted this long, and there would have been some observations suggesting a different type of object or model. Although there are still some things we do not know about black holes, such as what is inside the event horizon, I believe the current model accurately explains the observational data we have that lacks any other explanation.
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