Why do black holes spin? Most people have curiosity for this question because a black hole is often imagined as a motionless object in space. But scientists observe that black holes spin at incredibly high speed. At that speed they also dragged space time along with the rotation. The spinning of the black holes is directly related to the formation of massive stars and conservation of angular momentum. Scientists are searching very carefully for why do black holes spin because it also helps them to understand how rotating black holes launch jets, consuming nearby materials and how they shape the evolution of the galaxy.
The short answer to why do black holes spin is that they inherit the rotation of the stars that created them. During their lifetime, every star rotates. But they are very massive in size so they rotate very slowly. Astronomers often have to use Doppler shifts to detect the rotation at all. They almost all posses some angular momentum, and when a star collapses under its own gravity, this angular momentum is maintained. This is called conserved angular momentum. This principle works same when an ice skater spins faster after pulling their arms.
A collapsed star’s core rotates in high speed. This happens because previously the core was millions of kilometer wide and after the star dies it shrinks and becomes only a few tens of kilometers wide. But the angular momentum is conserved. That’s why the rotation speed increases. This conserved angular momentum is the true reason “why do black holes spin.”
Angular Momentum Is the Key
The real reason why do black holes spin is one of the fundamental laws of physics, the conservation of angular momentum. It basically states, that it can neither be created, nor destroyed. Unless any external force removes it otherwise the angular momentum of an isolated system remains conserved.
So when a massive star dies, gravity overwhelms the outward pressure created by the fusion. Then stellar core collapses under its own weight at unimaginable speeds. While doing so, it sheds off its outer layers, and the core itself tightens greatly. Due to the conservation of angular momentum, the angular velocity rises sharply during the collapse. This same principle applies to the formation of white dwarfs and neutron stars, which are spinning much faster than the stars that formed them.
Why Do Stars Rotate?
The science behind the rotation of stars makes our understanding even better towards why do black holes spin. For this we need to turn our attention to the life cycle of the star.
To begin, stars are born in large interstellar clouds of gas and dust, which are far from being motionless. Due to some random fluctuations, turbulence, and gravity, any collapsing cloud gets a small amount of rotation. As the gravity pulls the cloud into a tighter configuration, it increases the rotation speed. Thus, all stars are born rotating. Some stars rotate faster than others, with the rate of rotation depending on the star’s mass. Stellar rotation is significant to astrophysics because it influences many processes, including the star’s magnetism, stellar wind, internal structure, and the way it evolves.
Thus, astronomers have a pretty good understanding of what gives rotation to black holes. The short answer is that they are born spinning because all stars are rotating massive celestial bodies. The initial angular momentum of the star provided the rotation for the newly formed black hole, which explains why do black holes spin.
Black Hole Spin Can Increase Even After Formation
The story does not end when a black hole is created. One of the most interesting discoveries in modern astronomy is that some black holes can continue to spin faster long after they were initially formed. Most black holes are binary, meaning that there is a companion star to the black hole, and the gravity pulls gas from the companion star forming a rapidly rotating accretion disk around the black hole.
This gas has angular momentum, and when it crosses the event horizon, some of that angular momentum gets added to the black hole. This means that over a period of several million years, the angular speed of the black hole can be increased dramatically. Due to these factors scientists believe that some black holes with extremely high spin rates got their speed not only from its parent star but also from matter that was added to it afterwards.
Modern astronomy thinks, this may also a reason why do black holes spin at such incredible speed and why different black holes have different spinning rates.
Some Black Holes Spin Almost at the Speed of Light
Not all black holes are spinning at same speeds. Some of them rotate very slowly, but others can spin at relativistic speeds, with their edges many times faster than the speed of light.
But why are some black holes much faster than others? Another question arises, and it is related to why do black holes spin. The reason for this can be seen in their formation and evolution. A fast-rotating star will lead to the creation of a fast-rotating black hole. But the speed not ends there. If the black hole continues to pull gas from its nearby star or merge with some other objects in space like white dwarf or neutron star, it would get some more angular momentum. Over millions of years, black hole can reach extremely high speed.
It is worth mentioning that scientists classify the speed of rotation of black holes with the help of the spin parameter, which is denoted by a*. For example, a value close to “0” represents a black hole that hardly rotates, but a value close to “1” indicates an extremely fast spinning black hole. Scientists have managed to find examples of stellar-mass and supermassive black holes that have a spin parameter close to 1, indicating extremely high-speed rotation.
How Do Scientists Measure Black Hole Spin?
One of the most challenging aspects of astronomy is the fact that black holes are impossible to observe directly. Therefore, astronomers are puzzled trying to find out why do black holes spin and how fast they rotate.
Now scientists don’t look at the black hole, instead they observe the accretion disk around the black hole. Gas orbiting a fast spinning black hole becomes extremely hot and starts emitting bright X-rays. By studying X-rays emitted from the gas astronomers can figure out the distance between the center of the black hole and the inner edge of the accretion disk. Astronomers discovered that a fast spinning black hole’s inner edge is closer to its center as compare to a slow spinning black hole. Scientists are using two main methods for calculating the spin of a black hole:
- The first method is X-ray reflection spectroscopy, which examine the X-ray spectrum and see how much the X-rays from the accretion disk have been scattered.
- The second method is Continuum-fitting, which measures the temperature and brightness of the disk.
NASA’s Chandra X-ray Observatory, NuSTAR, NICER and XMM-Newton from ESA have become powerful enough to measure the spin rates of dozens of black holes, helping to get the insight on why do black holes spin and how their rotation may affect the evolution of these objects.
Conclusion
The answer to the question why do black holes spin begins long before its formation.
A huge amount of stars are in motion while they exist and when their core dies but the conservation law of angular momentum states that when the core collapses it will force rotation to speed up tremendously due to the decrease in its radius. When the mass of the core is big enough the result of such a collapse will be a black hole that possesses the same rotation as the collapsed star had. The spin of a black hole can also be increased by capturing interstellar gases from the surrounding stars or other black holes. The reason why astronomers want to find out why do black holes spin has a tremendous value in astrophysics because the rotation plays an important role in the formation of relativistic jets, as well as, in the structure of the event horizon and galaxy formation. Modern astronomers and astrophysicists have found answers to most of the questions that were raised concerning black holes, their rotation, and the impact it makes on the environment around them but there are still many details that need to be clarified like the relation of magnetic fields, accretion, and mergers to the spin of the fastest rotating objects in our universe.
FAQ
1. Why do black holes spin?
Black holes spin because they inherited their angular momentum from parent star. The core of the star shrinks significantly during formation, which is why it begins to rotate much faster.
2. Can a black hole spin faster than the speed of light?
No, parts of the surrounding space-time may move extremely fast, but the black hole itself cannot violate Einstein’s theory of relativity.
3. Why do some black holes spin faster than others?
Different black holes form from different stars and experience different evolutionary histories. Their spin depends on the parent star’s rotation. But it’s not end here, black hole’s gravity constantly pulling gas from its nearby star and also merges from other objects. This gives some extra angular momentum. Over millions of years, black hole’s rotational speed becomes very high.
4. How do astronomers measure black hole spin?
Astronauts are able to estimate their spin rate by looking at X-ray emissions from the accretion disks that form around black holes, which is detected by space telescopes such as: Chandra, NuSTAR, NICER, and XMM-Newton.
5. Can a black hole stop spinning?
Yes, for a very long time, they can slow down their rotation rate. It would take millions or billions of years for them to stop completely, through the emission of jets of relativistic particles, merger and Hawking radiation.