Black Holes
About the above image: It is a visualization of a black hole and its (hot, thin, rotating) accretion disk formed by the infalling matter spiraling toward the black hole. The disk is distorted because of gravitational lensing.
Image credit: NASA’s Goddard Space Flight Center.
Overview
Can you visualize a massive star, 10 times the mass of our Sun, getting crushed to the size of a point with no volume? This is hard to imagine. Right? This is what happens when a black hole forms.
Even stars with tens of solar masses can get crushed by gravity to a point, called singularity, to become an object of zero volume and, consequently, with infinite density. How could this happen? Is this something real?
Black holes have such strange properties that astronomers thought that they were mere mathematical fictions rather than a reality. The prediction of black holes was the consequence of Einstein’s (1915) general theory of relativity which is the modern theory of gravity. Even Einstein refused to accept the existence of astronomical objects with such bizarre properties.
We will see more on the history of black holes in a moment. Now let us explore what a black hole is, how it forms, and its structure.
What are black holes
Stellar black holes are one of the end points (corpses) of supermassive stars. When a star runs out of fuel in the core, it ends its life either as a white dwarf or a neutron star or a black hole. The mass of a star controls its end point.
As mentioned above, black holes have bizarre properties. They are so dense, and they are with such a great gravitational pull that not even light can escape from it. Astronomers say that black hole is the place where God successfully divides by zero!
A black hole is an ideal Black body. Any light that falls onto it can never come out. It is like a deep well. Therefore, a black hole is not visible. Then, how do astronomers observe and study black holes? They study by observing the effects on the stars surrounding a black hole.
Structure of a black hole
A black hole has two parts:
- A singularity (a point) at the center where the entire mass of the black hole resides.
- An event horizon, a boundary where the escape velocity from the black hole’s mass is the speed of light.
Singularity
Singularity is a point; therefore, with zero volume and it carries all the mass of the black hole. Consequently, the density (of whatever is there at singularity) is infinite.
For instance, if a star’s core with mass 3M☉ ( M☉ is the mass of our Sun) collapses to become a black hole, the density of its singularity becomes infinity as shown below.
Note: any number (other than zero) divided by zero is infinity.
At singularity, our known laws of physics apparently break down. What is really happening at singularity is not yet clear.
Event Horizon:
The region around singularity is an event horizon. It is not anything physical. It is a spherical region around the singularity. Anything entering this sphere can never turn back. Its gets sucked into singularity. Not even light can escape from it.
The radius of the event horizon is known as the Schwarzschild radius. As shown in the picture above, the radius depends on the mass of the black hole. For instance, if Sun were to become a black hole, its Schwarzschild radius would be 1 km. If our Earth were to become a black hole, its Schwarzschild radius would be just 1 cm.
How black holes form
There are two classes of black holes – stellar black holes and supermassive black holes.
Stellar black holes are those with masses about 5 to 20 times the mass of our Sun. A star, when it runs out of fuel and collapses on its own weight, ends up in one of the cold states – white dwarf, neutron star or stellar black hole.
As you can see in the diagram below, Sun like stars end their lives as white dwarfs (path A). Stars with masses 5 to 10 times the solar masses end their lives as neutron stars (path B). However, those stars with 10 times or more solar masses end their lives as stellar black holes (path C).
Supermassive black holes are with masses millions to billions of times that of our Sun. Almost every large galaxy has a supermassive black hole at the center of it. Astronomers believe that they are primordial in nature and that they must have formed at the time of galaxy formation.
Historic Facts
Prediction of dark stars
At the end of eighteenth century, the Reverend John Michell (of UK) and Pierre Simon Laplace (of France) predicted the existence of stars that we could not see. They called them dark stars (The usage of the term black hole started in 1967). They combined the ideas of finite speed of light, Newton’s corpuscular theory of light and Newton’s concept of escape velocity to predict the possibility of existence of ‘dark stars’ in the sky. Because corpuscular theory of light was later abandoned, their prediction also went in obscurity.
Chandrasekhar’s Prediction
Chandrasekhar, a Nobel laureate, is one of the greatest astrophysicists of the 20th century. He not only discovered the limiting mass (Chandra limit) for the white dwarfs but also, he mathematically predicted the existence of neutron stars and black holes.
In his paper on white dwarf submitted to RAS (Royal Astronomical Society) on January 11, 1935, he stated in the summary that “for a star of small mass the natural white dwarf stage is an initial step towards complete extinction. A star of large mass (greater than the upper limit for white dwarfs) cannot pass into the white dwarf stage, and one left speculating on other possibilities.”
As stated above in the summary, he discovered a mass limit, Chandra limit. He determined that star-relics of larger mass were not permitted to end their lives in the way everyone at that time believed was standard, as white dwarf. Only star-relics, having mass less than Chandra limit, would become a white dwarf. Anything greater than that limit would take other paths to their complete extinction. Now we know that the other paths are neutron stars and black holes.
Schwarzschild solution
In 1915, Einstein published his equation of General theory of Relativity. Shortly after the publication, the German physicist Karl Schwarzschild discovered a solution (for Einstein’s equation) to the gravitational field surrounding a sphere in vacuum.
Schwarzschild solution predicted a region, around a collapsed star, which is now known as event horizon. The distance from the center point to the event horizon is called Schwarzschild radius.
The Schwarzschild radius is nothing other than the critical size of a star below which the escape velocity from the surface reaches the velocity of light. Schwarzschild solution opened the door on Michell and Laplace’s forgotten idea of invisible stars.
For a long time, astronomers considered the magic circle (event horizon) a flaw in the theory of General Relativity. Astrophysicist Arthur Eddington of UK was the greatest defender of General Relativity. He was also the fiercest opponent of the idea of a star condensed within the Schwarzschild radius. He said, “I think there should be a law of Nature to prevent a star from behaving in this absurd way!”
Oppenheimer and Snyder
In spite of doubts on Schwarzschild radius and event horizon, the progress in this field continued. In 1939, thanks to Oppenheimer and Snyder, the theory of gravitational collapse was truly born. Using the equations of General Relativity, they calculated the gravitational collapse of a spherical mass below the Schwarzschild radius. They proved, as Schwarzschild solution predicted, that matter would collapse to form a region from which not even light could escape.
John Wheeler:
John Wheeler of Princeton University used the term black hole the first time, on 29 December 1967, during his lecture given in New York. Apparently, a student from the audience suggested the term ‘black hole’.
Conclusion
Black holes are real:
As have evidences for the presence of black holes, it is clear that black holes are real. In addition to a supermassive black hole at the center of our galaxy, Milky Way, stellar black holes are found to be scattered across our galaxy like any other star.
Here is the famous image of a black hole at the center of M87 galaxy. This was published in 2019 and was captured by Event Horizon Telescope (EHT) – a network of linked telescopes. A computer program made this image possible.
The M87 black hole, in the above image, is about 55 million light years from our Earth.
Interesting Video
The video below (from NASA GODDARD space flight center) shows what happens to a star that passes close to a massive black hole.
You can watch NASA’s full video here.
Takeaways
References:
- Black Holes, Cambridge University Press, by J-P Luminet
- https://www.nasa.gov/vision/universe/starsgalaxies/black_hole_description.html
- https://imagine.gsfc.nasa.gov/science/objects/black_holes1.html
- https://www.nytimes.com/2008/04/14/science/14wheeler.html?smid=url-share
- https://www.nasa.gov/feature/goddard/2019/nasa-visualization-shows-a-black-hole-s-warped-world