<ul><li>But scientists know they exist because they rip apart stars that get too close to them and because they can send tremors through space.</li><li>It was a collision between two black holes more than a billion years ago that triggered what are called ‘gravitational waves’.</li><li>Albert Einstein even wrote a paper in 1939 claiming that stars could not collapse under gravity because matter could not be compressed beyond a certain point.</li><li>The principal exception was the American scientist John Wheeler,emphasized that many stars would eventually collapse, and pointed out the posed for theoretical physics.</li></ul>

Can‘t see , but how do we know their existence

<ul><li>if you take pressure out of the calculation,a uniform spherically symmetric star would contract to a single point of infinite density,I.e,Singularity.</li><li>A singularity is what you end up with when a giant star is compressed to an unimaginably small point.</li><li>It refers not only to the end of a star but also to a far more fundamental idea about the starting-point for the formation of the entire universe.</li><li>All our theories of space are formulated on the assumption that space-time is smooth and nearly flat,so they break down at the singularity.In fact, the singularity marks the end of time itself. </li></ul>

Singularity

<ul><li>All the black hole will remember is the total mass, the state of rotation and the electric charge. </li><li>A black hole has a boundary, called the event horizon. This is where gravity is just strong enough to drag light back and prevent it escaping.</li><li>Because nothing can travel faster than light, everything else will get dragged back also.</li><li>There is a black hole with a mass of about four million times that of the sun at the centre of our Milky Way galaxy.</li><li>Scientists believe that there are huge black holes at the centre of virtually all galaxies.</li></ul>

Information

<ol><li>It was already clear that a uniform spherical star would contract to a point of infinite density, a singularity.</li><li>Einstein’s Theory of General Relativity says that objects distort the space-time around them.</li><li>At this point of infinite density, one can’t predict the future, which in turn implies that something strange could happen whenever a star collapsed.</li></ol>

Einstein‘s View 

Black holes cannot be seen as they don't reflect any light.The centre of black hole is called <a href="https://en.wikipedia.org/wiki/Gravitational_singularity?wprov=sfla1">singularity</a> - a point of infinite density i.e the space curvature extends to infinity. According to scientists, billons of black holes exists of which millions are having the abode Milky way galaxy

What is a Black hole ?

<ul><li>During most of the life of a normal star, over many billions of years, it will support itself against its own gravity by thermal pressure, caused by nuclear processes which convert hydrogen into helium.</li><li>The explosive force of nuclear fusion inside them creates outward pressure which is constrained by gravity pulling everything inwards.</li><li>Eventually, however, the star will exhaust its nuclear fuel. The star will now contract.</li><li>For more detail , check out <a href="https://en.wikipedia.org/wiki/Chandrasekhar_limit?wprov=sfla1">Chandrashekhar limit</a> and <a href="https://www.google.com/url?sa=t&source=web&rct=j&url=https://en.m.wikipedia.org/wiki/Tolman%25E2%2580%2593Oppenheimer%25E2%2580%2593Volkoff_limit&ved=2ahUKEwjp1-6yt87wAhVT4XMBHfxMBxEQFjAAegQIBxAC&usg=AOvVaw0IhSKbCPi5JGOCeN81oPV0">Tolmann Volkoff Oppenheimer approximation</a></li></ul>

Formation of Black holes 

<ul><li>With no light escaping from the black hole, there is no way that anyone watching from a distance could actually witness your descent. In space, no one can hear you scream; and in a black hole, no one can see you disappear.</li></ul>

Properties

"It is said that fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamed up by science fiction writers." In 2016 Professor Stephen Hawking delivered the BBC Reith Lectures on a subject that has fascinated him for decades - black holes. In these flagship lectures the legendary physicist argues that if we could only understand black holes and how they challenge the very nature of space and time, we could unlock the secrets of the universe.

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&quot;It is said that fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamed up by science fiction writers.&quot; In 2016 Professor Stephen Hawking delivered the BBC Reith Lectures on a subject that has fascinated him for decades - black holes. In these flagship lectures the legendary physicist argues that if we could only understand black holes and how they challenge the very nature of space and time, we could unlock the secrets of the universe.

Stephen Hawking is an accomplished writer whose work has resonated with readers around the world. Stephen Hawking has published several books that explore a range of topics, from fiction to nonfiction, and has received critical acclaim for the ability to weave compelling narratives. Stephen Hawking's writing style is known for its depth and authenticity, and has earned quite a loyal following of readers.

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