User:Ancin/Sandbox

BLACK HOLE

Black hole is a region of space whose gravitational force is so strong that nothing can escape from it.A black hole is invisible because  it even traps light.The fundamental  description of black hole sre based on  the equations  in the theory of general relativity developed by Einstein.

The gravitational force is strong near a black hole because all the black hole's matter is concentrated at a single point in its centre called singularity.It is believed to be much smaller than atom's nucleus.

The surface of the black hole is known as the event horizon.This is not a normal surface that you can see or touch.At the event horizon,the pull of gravity becomes infinitely strong.Thus ,an object can exist there for only an instant as it plunges inward at  the speed of light.

Astronomers use the radius of the event horizon to specify the size of a black hole.The radius of a black hole measured in kilometres equals the three times the number of solar masses of material in the black hole. To.To prove that a compact object is a black hole, scientists would have to measure effects that only a black hole could produce.Two such effects would be a severe bending of a light beam and an extreme slowing of time.

According to general relativity ,a black hole can form when a massive star runs out of nuclear fuel and is crushed by its own gravitational force.While a star burns fuel,it creates an outward push that counters the inward pull of gravity.When no fuel remains,the star can no longer support its weight.As a result,the core of star collapses.If the mass of the core is 3 or more solar masses, the core collapses into a singularity in a fraction of a second.

Most astronomers believe that the milky way galaxy contains millions of black holes.These objects are in binary stars that give off X rays.A binary star is a pair of stars that orbit each other.In a binary system containing a black hole,that object and a normal,visible star orbit one another closely.As a result ,the black hole strips gas  from the normal star ,and the gas fall violently towards the black hole.Friction between the gas atoms heats the gas near the event horizon to several million degrees.Consequently energy radiates from the gas as X rays .Astronomers have detected this radiation with X ray telescopes.Astronomers believe that a number of binary stars system contain black holes for two reasons:1.each system is a source of intense and variable X rays.The existence of these rays proves that the system contains a compact star-either a black hole or a  less compact object called neutron star.2.the visible star orbits the compact object at such a high velocity that the object  must be more massive than three solar masses.

Scientists believe that most galaxies have a super massive black hole at the center .The mass of each of those objects is thought to be between 1 million and 1 billion solar masses.Astronomers suspect that super massive black holes formed several billion years ago from gas that accumulated in the center of the galaxies.There is strong evidence for a super massive black hole at the center of the Milky way.The clearest indication of its presence is that several dozens stars are moving very rapidly near the galactic center.The motion of the star leads astronomers to conclude that an unseen object weighing 2.6 million solar masses lies at the galactic center.Other evidence shows that the object's radius is less than 300 billion miles.The only object that could have that mass and be so compact is black hole.

source:world book.