What is a Black Hole?
A black hole is an object so dense that not even light can escape from its gravitational field. They originate from stars which have reached the end of their lifespan. Anything that passes close enough to a black hole will be pulled into it, with no hope of escape. Before we go on to discuss their features and formation in more detail, you can become familiar with their basic concepts by reading one of these introductions.
Black Hole Formation
Black holes begin life as stars. Each star goes through a life cycle known as stellar evolution, during which it converts the hydrogen in its core to helium. When all of the hydrogen fuel has been used up, the star no longer produces enough thermal pressure to withstand the gravitational forces that cause it to collapse in on itself. The biggest stars collapse so violently that they undergo an energetic implosion known as a supernova. They may then evolve into dense neutron stars or black holes. Learn more about stellar evolution and find out how the mass of a star determines its fate with these references.
Stars that are not quite massive enough to end up as black holes after undergoing supernova may instead result in neutron stars or quark stars. Quark stars have not yet been discovered but are theoretically predicted to exist.
Gravity is the force that drives black hole formation. When gravitational forces are very large, the usual Newtonian laws of gravity break down so that we must use Einstein's theory of General Relativity to describe black hole physics. Here is a simple introduction to the different theories of gravity and how they can be used to predict the formation of a black hole.
Structure and Types of Black Hole
Black holes are complex structures, consisting of many different parts such as the accretion disk and the event horizon. They can be classified either by their size or by their rotations and magnetic fields. Learn about the structure of each type of black hole.
Emissions from Black Holes
Even though light cannot escape from a black hole, they still make their presence known through radiation. Matter falling into a rotating black hole begins to swirl around it in an accretion disk. It is heated by frictional forces in the disk. Once the matter is sufficiently hot, it begins to radiate. It is this radiation originating from around black holes that allows astronomers to detect them. Learn more about accretion and the radiation it generates.
Hawking radiation is radiation that is emitted by matter as it crosses over the event horizon due to quantum effects occurring at the event horizon. The emission of Hawking radiation causes the hole to gradually lose mass and the energy in a process known as black hole evaporation. Learn more about the discovery of Hawking radiation and its relation to the end of the Universe.
Black Hole Theories
After they were predicted to exist from the equations of general of relativity by Karl Schwarzchild in 1816, it took a long time for the scientific community to accept black holes. Telescopes have since found evidence for their existence. Find out about that journey of discovery and acceptance.
There are many speculative theories about the nature of black holes. For example, the existence of stars made of dark energy is an alternative theory that accounts for many of the phenomena typically associated with black holes. White holes are the theoretical "exit points" for the matter and information that is pulled into a black hole, ejecting all the accumulated matter and energy into a different part of the Universe, or possibly even into another universe. Some physicists are also using string theory to explain black holes and Hawking radiation. Read more about these radical ideas with the articles listed below.
Black Hole Discoveries
Studies of the motions of stars led to the discovery of a massive black hole at the center of our own galaxy, the Milky Way. Find out about this process in more detail and learn more about the search for these mysterious objects.
- Stellar Motion and Black Holes
- Imaging a Black Hole
- How Do Astronomers Detect the Biggest Black Holes?
Black holes are often found in the centers of galaxies. Read more about the galaxies that harbor these giants in their cores:
Since 1985, astronomers have been studying galaxies that appear to emit huge amounts of energy and radiation—these are known as quasars. For a long time, the identity of quasars was a mystery. Eventually they were tracked down to the centers of massive galaxies, but the reason for their brightness was still unknown. It is now thought that the radiation is produced by matter falling onto a massive black hole in the center of the galaxy in a hugely energetic accretion process. Learn more about quasars with these articles.
- Active Galactic Nuclei and Quasars
- Facts About Quasars
- The Differences between Stars and Quasars
- Extragalactic Astronomy: Galaxies and Quasars
Black holes are fascinating astronomical objects. Physicists study them not only because of their quirky characteristics, but also because the extreme environments that exist in and around them are the perfect context in which to think about new theories of gravitation and quantum physics. Although the equations describing a black hole are complex, it is possible to gain an intuitive understanding of these fascinating objects without mathematical expertise. We hope that you enjoy browsing through the above articles. You are welcome to leave comments or questions on any of them in order to spark a discussion.