NEUTRON STARS.

 

All stars undergo evolution. In the process average stars like the Sun after becoming a Red Giant and exhausting all their nuclear fuel becomes unstable and expel most of the outer shell in an explosion creating a planetary nebula. The core of the star is then known as a White Dwarf.  

The White Dwarf is very hot and has a surface temperature exceeding 100,000 degrees kelvin. The White Dwarf star would now have recurrent explosions for a period of thousands of years. Then the White Dwarf cools down in the next 1 billion years as it does not have any heat generating source and becomes a Black Dwarf or a dead star after losing all its luminosity and heat.  A typical White Dwarf has about half of suns mass but that is packed into an area just as big as the earth. The White Dwarf is 200,000 times as dense as the earth. A Black Dwarf cannot be seen at all as it has no luminosity. The Black Dwarf is nothing but the White Dwarf but without its heat. The only way to know the existence of a Black Dwarf is the gravitational attraction it exerts on the bodies nearer to it.

However, those stars that are more than 1.44 times the Suns mass do not become White Dwarfs that cool down and instead turn into Novae. This is known as the Chandrasekhar’s limit.

Nova is possible only in a binary system where 2 stars orbit a common centre of gravity. In a Nova, a White Dwarf circles a normal star which has turned into a Red Giant so close that a stream of matter flows between them. The gas piles up a layer on the surface of the White Dwarf until a flash point where it detonates and results in a runaway nuclear explosion. Astronomers estimate that 20 to 50 Nova explosions occur in our Galaxy every year.  

The White Dwarf remains intact even after the explosion despite the fact that the explosion releases as much energy as the Sun emits in 100,000 years.

When a star that is 7 to 19 times the mass of the Sun ends its life its core collapses while the outer layers are blown off in a Super Nova explosion. What is left behind after the explosion is the Neutron star.

The mass of a Neutron star is equal to that of the Sun, but all that is packed into just 20 Km in diameter. If a star ending its life is 20 times the mass of the Sun then it explodes as a super nova becomes a Black hole instead of a Neutron star.

A Super Nova explosion outshines even galaxies (that have 10,000 crore stars) for days or even months. That is the immense amount of energy that is released at a stroke.  However, Super Nova is not common and in a Galaxy like the Milky Way only about 2 to 3 Super nova occur in a century.

A near earth Super Nova is one that occurs close enough to the Earth to have an effect on the life on earth. That would roughly be about 10 to 300 parsecs from the Earth. (One parsec is 3.26 light years) or about 30 to 1000 light years from the earth.