What are Black Holes?

The Black holes are one of the strange and most fascinating objects found in outer space.  The Black holes are cold remnants of dead stars with extreme density with such a strong gravitational attraction that even light cannot escape from the Black Hole if it comes near it.



The famous scientist Albert Einstein first predicted black holes in the year 1916 with his general theory of relativity.  The word "black hole" was coined in the year 1967 by American Astronomer John Wheeler.  The first black hole was discovered in the year 1971.

There is a mathematically defined surface called Event Horizon around black hole which marks the point of no return.  The hole is called "black" because it absorbs all the light which hits the horizon and reflects nothing.  

Most of the stars end up as white dwarfs or neutron stars.  The black holes are the last evolutionary stage in the lifetime of big stars which had been at least 10 or 15 times bigger than our Sun.  When these giant stars reach their final stages of their life, they often detonate into what is known as supernovae.  This kind of explosion scatters most of the star into the void of space but leaves behind a large "cold" remnant on which fusion no longer takes place.  In younger stars, nuclear fusion creates energy and a constant outward pressure exists in balance with the inward pull of gravity caused by star's own mass.  But in dead remnants of stars, no force opposes gravity - so the star begins to collapse in upon itself.


When there is no force to check gravity, the black hole shrinks to zero volume - at which point it is infinitely dense.  Even the light from such a star is unable to escape due to the immense gravitational pull.  The own light of the star gets trapped in it and the dark star becomes known as black hole.

The black holes are very small in size.  For example, a black hole with a mass equal to that of our Sun would have a 2-mile radius.  As they are very small and dark at a far off distance, we can not observe them directly.  But extremely large black holes do exist at the center of some of the galaxies.  These large black holes have mass of 10 to 100 billion Suns. 

 

Types of Black holes

 

 You can find three types of black holes.  1. Stellar black holes,    2. Super massive black holes,   3.  Intermediate black holes.
 

 The Stellar black holes are small but deadly.  When a star burns through the last of its fuel, it may find itself collapsing. For smaller stars, up to about three times the sun's mass, the new core will be a neutron star or a white dwarf. But when a larger star collapses, it continues to fall in on itself to create a stellar black hole.  Black holes formed by the collapse of individual stars are (relatively) small, but incredibly dense. Such an object packs three times or more the mass of the sun into a city-size range. This leads to a crazy amount of gravitational force pulling on objects around it. Black holes consume the dust and gas from the galaxy around them, growing in size.

Small black holes populate the universe but their cousins, Super massive black holes dominate.  The super massive black holes are millions or even billions of times as massive as our Sun but have a radius similar to that of Earth's closest star.  These kind of black holes are thought of lying in the center of every galaxy including our Milky Way galaxy.  Scientists are not sure how much large such black holes are.  Once they are formed, they easily gather the mass from the dust and gas around them, the material which is plentiful at the center of galaxies allowing them to grow to large sizes.  The super massive black holes may be a result of merging together of hundreds of tiny black holes.  Large gas clouds may also be responsible collapsing together and rapidly  gathering mass.  A third option may be the collapse of a group of stars all falling together.


The intermediate black holes were recently found by scientists.  Earlier they felt that black holes existed in small and large sizes.  This kind of intermediate black holes can be formed when stars in a cluster collide in a chain reaction.  Several of these intermediate black holes forming in same region could fall together in the center of a galaxy to create a super massive black hole.

Here is a video showing how a black hole is formed...