Astronomy - Black Holes

 

Black Holes

 

A black hole is a region of space-time exhibiting such strong gravitational effects that nothing not even particles and electromagnetic radiation such as light can escape from inside it.

The theory of general relativity predicts that a sufficiently compact mass can deform space-time 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.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace.

The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958.

Black holes were long considered a mathematical curiosity; it was during the 1960s that theoretical work showed they were a generic prediction of general relativity.

The discovery of neutron stars [ the collapsed core of a large (10–29 solar masses) star] sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

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 may form.

There is consensus that supermassive black holes exist in the centres of most galaxies.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as visible light.

Matter that falls onto a black hole can form an external accretion disk heated by friction, forming some of the brightest objects in the universe.

If there are other stars orbiting a black hole, their orbits can be used to determine the black hole's mass and location.

Such observations can be used to exclude possible alternatives such as neutron stars. In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the radio source known as Sagittarius A, at the core of our own Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

Far away from the black hole, a particle can move in any direction.

Closer to the black hole, space-time starts to deform. There are more paths going towards the black hole than paths moving away.

Inside of the event horizon, all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.

As predicted by general relativity, the presence of a mass deforms space-time in such a way that the paths taken by particles bend towards the mass.

At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.

To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.

Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow as it approaches the event horizon, taking an infinite time to reach it.

At the same time, all processes on this object slow down, from the view point of a fixed outside observer, causing any light emitted by the object to appear redder and dimmer, an effect known as gravitational redshift.

Eventually, the falling object becomes so dim that it can no longer be seen