The Black Hole: Facts and Basics


WHAT IS A BLACK HOLE?

It is an object with an exceptionally strong gravitational field.  Nothing can escape its gravitational pull. Black holes have infinite density. Which means that a large mass is concentrated in a small location.

HOW ARE THEY FORMED?

When supernova explosion of a massive star takes place then the gaseous matter of star gets scattered, but its core remains intact and shrinks due to enormous gravitational pull of itself. The contraction causes the core to become compact.  Thus causes a large mass to get concentrated in a single point. This is called neutron star. This neutron star pulls nearby objects  like gases, neighboring stars, radiations etc and grows in size.

WHAT IS ACCRETION DISK?

Black holes pull material objects towards itself. These materials move in a spiral around the center placed inside neutron star. All the objects spirally revolving around black hole forms a disk called accretion disk.

WHAT IS SCHWARZCHILD RADIUS?

It is the distance from black hole where the speed required to escape the gravitational pull (escape velocity) of black hole is equal to ‘c’ (speed of light). At this distance only light have ability to escape the black hole.Once anything is inside this distance, it cannot escape including light.

WHAT IS AN EVENT HORIZON?

Event horizon is a boundary in space-time beyond which events cannot affect an outside observer.Light emitted from beyond the horizon can never reach the observer. Likewise, any object approaching the horizon from the observer’s side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.(from Wikipedia)

WHAT IS SINGULARITY?

Singularity is point inside event horizon where gravity becomes infinite. Space-time becomes so much warped and distorted, that gravity becomes independent of the coordinate system.

WHAT IS CHANDRASEKHAR LIMIT?

It is the minimum mass required for a white dwarf to become stable. A white dwarf above Chandrasekhar limit mass will become unstable. This means that its internal pressure ( due to more nuclear fuel) will be more , causing white dwarf to explode to supernova and forming neutron star or black hole.  Chandrasekhar limit is 1.44 times the solar mass. A white  dwarf more than this will have more mass than required and subsequently more nuclear fuel to explode.

HOW CAN A BLACK HOLE DIE?

It is a thermal radiation with a black body spectrum predicted to be emitted by black holes. It is because Hawking radiation allows black holes to lose mass and energy, black holes that lose more matter than they gain through other means are expected to dissipate, shrink, and ultimately vanish.

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