Black holes

Black Holes

A black hole is a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although the event horizon has an enormous effect on the fate and circumstances of an object crossing it, no locally detectable features appear to be observed. In many ways, a black hole acts like an ideal black body, as it reflects no light.

Formation

Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses (M☉) may form. There is consensus that supermassive black holes exist in the centers of most galaxies.

Properties

Event Horizon

The event horizon is the defining feature of a black hole. It is the "point of no return" around the black hole, beyond which nothing can escape. The Schwarzschild radius is the radius of the event horizon for a non-rotating black hole.

Singularity

At the center of a black hole, as described by general relativity, lies a gravitational singularity, a region where the spacetime curvature becomes infinite. The singularity is hidden by the event horizon, making it impossible to observe directly.

Hawking Radiation

Stephen Hawking theorized that black holes are not entirely black but emit small amounts of thermal radiation due to quantum effects near the event horizon. This radiation is known as Hawking radiation, and it implies that black holes can eventually evaporate over time.

Types of Black Holes

Stellar Black Holes

Stellar black holes form when massive stars collapse under their own gravity. They typically have masses between 3 and 10 solar masses.

Supermassive Black Holes

These black holes have masses ranging from hundreds of thousands to billions of solar masses and are found in the centers of most large galaxies, including our own Milky Way.

Intermediate Black Holes

Intermediate-mass black holes are hypothesized to exist with masses between stellar and supermassive black holes, but evidence for them is still being gathered.

Primordial Black Holes

Primordial black holes are hypothetical black holes that formed soon after the Big Bang. They could have a wide range of masses.

Observational Evidence

Black holes cannot be observed directly with telescopes that detect electromagnetic radiation such as light, X-rays, or radio waves. However, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light. For example, a black hole can be observed by tracking the movement of stars orbiting around it or by detecting X-ray emissions from material accreting onto the black hole.

See Also

• General relativity
• Gravitational wave
• Event horizon
• Singularity (physics)

References

• Hawking, S. W. (1974). "Black hole explosions?" Nature 248, 30–31.
• Misner, C. W., Thorne, K. S., & Wheeler, J. A. (1973). Gravitation. W. H. Freeman.

External Links

• NASA's Black Holes
• ESA - Black Holes