A black hole is an area of space with a gravitational field so strong that nothing, not even light, can escape it. That’s why black holes appear black. In some cases, black holes are former massive stars that have been crushed to an extreme density during supernova explosions. In other cases, black holes contain the mass of millions or billions of stars.
People often ask, if black holes are black – if light cannot escape them – how can we see them? The answer is that we see the effects black holes have on the space around themselves.
In his general theory of relativity, published in 1915, Albert Einstein was the first to suggest that our universe contains such strange, dense, massive objects. Black holes emerge from Einstein’s equations of general relativity, as a natural consequence of the death and collapse of massive stars. The first person to formulate black holes mathematically was German mathematician Karl Schwarzschild in 1916. Theoretical physicist John Wheeler first coined the name black hole many years later, in 1967.
Up until the 1970s, black holes were generally considered to be mathematical curiosities only. But, as observational techniques improved, they began to be taken seriously as real objects. The first physical black hole ever discovered – Cygnux X-1 – was confirmed in 1971.
Black holes are of two main types. The first is the so-called stellar-mass black hole. These are the remnants of huge stars. When, at the end of its life, a star with more than about five times the mass of our sun explodes as a supernova, its core is suddenly and violently compressed under gravity. Depending on the star’s mass, the collapse may halt and form a neutron star, but if its mass is sufficient the core’s collapse will – in theory – continue, forming a black hole. Stellar-mass black holes have mass ranging from a minimum of about five times the mass of our sun up to about 60 times the sun’s mass. Their diameter is typically between 10 and 30 miles.
The second type of black hole is the supermassive black hole. These can have masses many billions of times that of our sun. One example is at the center of the quasar known as TON 618; the central black hole is an estimated 66 billion solar masses. As they have simply too much mass to have formed from the death of individual stars, it is thought that supermassive black holes formed in the early history of the universe from huge collapsing clouds of interstellar hydrogen, although their exact origin is unclear and is an area of much active research. It is also possible that they have accumulated extra mass over the eons from mergers with other black holes.
Supermassive black holes can have diameters bigger than that of our solar system. Most galaxies have a supermassive black hole at their centers: the one at the center of our own Milky Way galaxy, Sagittarius A*, has some 4 million times our sun’s mass and is some 37 million miles in diameter.
What’s inside a black hole? By definition, we can’t observe what’s inside there, because no light – no information of any kind – can escape a black hole. But astrophysical theories suggest that, at the core of a black hole, all the black hole’s mass is concentrated into a tiny point of infinite density. This point is known as a singularity.
It is this point – this singularity – that generates the black hole’s incredibly strong gravitational field. Consider, however, that the singularity might not exist. That’s because all known physics breaks down under the extreme conditions at the center of a black hole, where quantum effects doubtless play a large part. As we do not yet possess a quantum theory of gravity, it is impossible to describe what actually exists at core of a black hole.
Meanwhile, here is something that we are certain exists: the boundary of a black hole, known as its event horizon. It is not a physical edge. It’s just a point in space beyond which it is impossible to escape the black hole’s gravity. Once anything falling into the black hole passes the event horizon, it can never leave the black hole again, and is drawn inexorably and inevitably towards the black hole’s center. Within the event horizon, any solid object is torn apart by the fierce gravity and reduced to its constituent subatomic particles. At the event horizon, the escape velocity of the black hole reaches the speed of light.
As black holes don’t emit any light or other detectable radiation, they can be observed only by their gravitational effects on objects in the space close to them. If there are stars or gas near the black hole, it may be actively “feeding” on them; that is, material from these nearby objects may be drawn into the hole. In this case, a black hole will possess an accretion disk, where material spirals inwards before it is consumed, like water down a drain. The accretion disk may rotate at significant percentages of the speed of light: friction between colliding particles in the disk raises its temperature to million of degrees, radiating huge quantities of x-rays which can be detected with special telescopes.
In April 2019, the Event Horizon Telescope project revealed the first-ever direct image of a black hole, the supermassive black hole at the center of the giant elliptical galaxy M87. The image had been acquired using a global array of radio telescopes. As well as demonstrating beyond reasonable doubt that black holes exist, this amazing achievement represents the birth of a new branch of observational astronomy and has allowed General Relativity’s models of black hole behavior to be tested directly. The M87 black hole complies perfectly with these models.
Bottom line: A black hole is an area of space with a gravitational field so strong that nothing, not even light, can escape it. Find basic information about black holes here.
Andy Briggs has been a science and technology communicator for more than 30 years, mainly in the fields of information technology, astronomy and astrophysics. He has been involved with many astronomy societies in the UK and is a frequent contributor to Astronomy Ireland magazine. He also lectures regularly on astrophysics-related themes such as gravitational waves and black holes. Andy lives in Catalonia, Spain, with his wife and daughter.