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Home >  Astronomy > Cosmology > What Is a Black Hole? Black Holes Explained – From Birth to Death

Published 16th November 2008 by

A black hole is a region of space that has so much mass concentrated in a small area that there is no way for a nearby object to escape its gravitational pull.
Astronomical Objects Series
  1. Guide to the Constellations and Mythology
  2. What are Asteroids, Meteors and Comets?
  3. Binary Stars and Double Stars
  4. Variable Stars
  5. Supernova and Supernovae
  6. Types of Nebula and Nebulae
  7. What Is a Black Hole? Black Holes Explained - From Birth to Death
  8. Quasars
  9. Pulsars - The Universe's Gift to Physics
  10. What is inside a Neutron Star?
  11. Gamma Ray Bursts
  12. Kuiper Belt
  13. What is an Exoplanet?
  14. Galaxy Types and Galaxy Formation
  15. The Messier Catalogue
  16. The Caldwell Catalogue
  17. 25 Stunning Sights Every Astronomer Should See

In order to escape from the Earth's gravitational pull, an object must accelerate away from the surface at least 11.2 km/s. This is called the escape velocity. Any slower and the object will fall back to the surface. The same thing is true for a black hole, however, the escape velocity is so great that not even light has sufficient speed to escape.

Black holes are so called because we cannot see them; no light can leave its surface (either emitted or reflected) therefore it remains invisible. We can only detect black holes when they pass in front of another object and we witness gravitational lensing. To date, no black holes have actually been detected and confirmed, although there are several possible candidates.

It should be noted that contrary to popular belief, black holes are not "vacuum cleaners" sucking in everything around them. A black hole with a mass equal to that of the Sun will have a radius about 3 km. At a distance of about 106km, this black hole has no more attraction than any other body of the same mass. For example, if the sun were replaced by a black hole of the same mass, the orbits of the planets would remain unchanged. Once you cross the event horizon, however, it is a different story.

Black Hole Formation

When a star reaches the end of its life, it cools down and contracts under gravity. As it contracts it becomes denser as the volume decreases. Some stars will explode (supernovae) while others become neutron stars or black holes. Imagine a star 10 times bigger than our Sun, collapsed in on itself to the size of about 30km. The resulting object is so dense, its gravitational pull is immense.

Black holes could also be formed through high energy collisions, such as colliding neutron stars in binary systems.

Schwarzschild Radius

The Schwarzschild radius is the radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface of the sphere would equal the speed of light. If a stellar remnant were to collapse to or below this radius, light could not escape and the object is no longer directly visible outside, thereby forming a black hole.

Schwarzschild radius
Equation 50 - Schwarzschild radius

Where G is the gravitational constant, M is the object mass and c is the speed of light.

Black Hole Evaporation

Stephen Hawking provided a theoretical argument for the evaporation of black holes in 1974. The Hawking radiation process reduces the mass of the black hole and is therefore also known as black hole evaporation. Because Hawking radiation allows black holes to lose mass, black holes that lose more matter than they gain through other means are expected to dissipate, shrink, and ultimately vanish. Smaller micro black holes are predicted to be larger net emitters of radiation than larger black holes and to shrink and dissipate faster.

Finding a Black hole

There are a number of theoretical techniques that can be used to locate black holes. Since a black hole cannot be directly observed, we must use indirect methods - looking for the effect of a black hole on its surroundings.

Accretion disks and gas jets

Massive, ultra-dense objects such as neutron stars and white dwarfs can cause accretion disks and gas jets to form, and it is believed that a black hole will behave in a similar way. We can see accretion disks and we can account for the central star, but they could identify where it might be worth looking for a black hole.

Extremely large accretion disks and gas jets may be good evidence for the presence of supermassive black holes because as far as we know any mass large enough to power these phenomena must be a black hole.

Strong radiation emissions

Intense, one-time gamma-ray bursts may signal the birth of a black hole because astronomers believe that GRBs are caused either by the gravitational collapse of giant stars or by collisions between neutron stars. Both types of event involve sufficient mass and pressure to produce black holes.

Gravitational lensing

A gravitational lens is formed when the light from a very distant, bright source is "bent" around a massive object which passes between the source object and the observer. The process is one of the predictions of the general theory of relativity. According to this theory, mass "warps" space-time to create gravitational fields and therefore bend light as a result.

A Black Hole Simulation

A Black Hole Simulation


This diagram illustrates how light can be bent around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.

How light can be bent around a massive object from a distant source

How light can be bent around a massive object from a distant source


In the image below you can see observed gravitational lensing as imaged by Hubble Space Telescope in the galaxy cluster Abell 1689.

This image shows the full overview of the galaxy cluster Abell 2218 and its gravitational lenses.

This image shows the full overview of the galaxy cluster Abell 2218 and its gravitational lenses.


Objects orbiting possible black holes

Objects orbiting black holes probe the gravitational field around the central object. An early example, discovered in the 1970s, is the accretion disk orbiting the putative black hole responsible for Cygnus X-1, a famous X-ray source. While the material itself cannot be seen directly, the X rays flicker on a millisecond timescale, as expected for hot clumpy material orbiting a ~10 solar mass black hole just prior to accretion. The X-ray spectrum exhibits the characteristic shape expected for a disk of orbiting relativistic material, with an iron line, emitted at ~6.4 keV, broadened to the red (on the receding side of the disk) and to the blue (on the approaching side).

A Chandra X-ray spectrum of Cygnus X-1

A Chandra X-ray spectrum of Cygnus X-1


Heart of the Milky Way Galaxy

The centre of the Milky Way Galaxy is hidden from us in optical wavelengths by dense dust and gas, but other wavelengths such as radio allow us to see through such obstacles. The Galaxy's exact centre is occupied by an intense radio source that seems to have no orbital motion. Less than a decade ago this object, called Sagittarius A* (pronounced A-star) was identified as a massive black hole.

Tutorial Series

This post is part of the series Astronomical Objects. Use the links below to advance to the next tutorial in the couse, or go back and see the previous in the tutorial series.

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