Our ancients believed that the stars are
eternal and unchanging. But today we know
that's not true. Stars are 
born live their life and they die. The
way a star dies mostly depends on its
mass. A low mass star becomes a white
dwarf a high mass star becomes a black
hole but starts in between become
neutron stars. The life of a star is
dominated by two forces being in balance
Its own gravity which is trying to
collapse the star inward and the
radiation pressure due to nuclear fission
at its core. A star in its life  converts hydrogen
into helium in its core. The hydrogen gets
exhausted. If the star is massive enough
then the helium will fuse into carbon.
The core of a massive star becomes like
onion cells as heavier and heavier
elements built up in its core. Carbon
fused into Neon, Neon to oxygen, Oxygen to Silicon. Eventually the fution process
lead up to iron which cannot be fused
into another element. The fusion
stop and the Stellar evolution comes
to an end. The radiation pressure drops
rapidly and the core of the star collapse
under its own gravity. For a low mass star
the collapse stop when the electron
degeneracy pressure took over and
balance the gravity. This happens due to
a quantum mechanical principle that says
no two electrons can stay in same plece at same time. This is a white dwarf star.
For a white dwarf star mass is inversely proportional to the radius. If the mass of a white dwarf star
is less than 1.4 solar mass for then it
will remain as a white dwarf star. This mass
limit is known as Chandrasekhar's limit but if the mass of the star is more than 1.4
solar mass then the electron degeneracy 
pressure cannot stop the collapse this
happens because in those cases
gravity will be strong enough to fuse
electrons and protons into neutrons and
neutrinos. Neutrinos are relativistic
particles. So they will flay away and
neutron remains. This happens to almost
all the electrons and protons in the star.
The gravity collapse the star to about
20 kilometers in diameter. Star is now
is essentially a ball of neutrons with some
proton and electron left here and there
that survive the collapse. It has a very
thin crust of some normal but
extremely condensed matter. The star is
now essentially sustained by the neutron
degeneracy pressure and we now have a neutron star.
In many ways neutron stars are similar to an atom core. The main
difference is the atom core is held
together by strong force of interaction
but neutron stars by gravity. If you increase the
mass of the neutron star then after a
certain mass limit the neutron star will
collapse into a black hole this limit is
known as Tolman-Oppenheimer-Volkoff 
limit. The exact value of this limit is
still not known. But its somewhere between 1.5 to 3.0 solar masses.  When the core of
a star collapsed into a neutron star
the angular momentum is conserved. As the
radius of a neutron star is very very
small they rotate really fast. The
rotational period of a neutron star can
vary within few milliseconds to minutes
long. This extremely fast rotation speed
gives rise to an extremely strong
magnetic field. The magnetic field of a neutron star can easily be a trillion times
stronger than that of Sun's. This
extremely strong magnetic field can erase
your credit card from hundreds of thousands kilo-meter away. The neutron start are ridiculously
dense. A single cubic centi-meter
of neutron star's matter can have a
mass of around four hundred millions of
tons. If you can squeeze Mount Everest
in such high density then it will come
inside a teaspoon. The incredibly strong
magnetic fields of neutron star gives rise
to twin beams of light from its two
magnetic poles. Now just like earth the
magnetic field of the neutron stars does not
have to line up with the axis of
rotation. So the twin beams of light will
sweep across the sky and from earth we see pulses of increased brightness they are
known as pulsars. The first neutron star was discovered in 1965. But it was not
recognised as in a neutron star. Another is discovered two years later and this time
people recognise it as neutron star. But
then things got little wired. In 1967
Jocelyn Bell was a graduate student and
she was helping to build a radio
telescope. There was some persistent noise in their data. Bell studied that noise nights
after nights and figured out that this signal is coming from an actual source the
signal was pulsating. So Bell
discovered the first pulsar. At that
time nobody believed that any such astronomical object can generate
pulses in so timely manner. So the object
was subsequently given a name little
green man 1 or LGM 1. Today we
know there are more than thousands of
pulsars in our own galaxy. Some of the
neutron star's magnetic field are stronger than
usual maybe a quadrillion times that of
Sun's. This type of neutron stars are known as
magnetars. But they are relatively rare.
Around 10% of the neutron stars are
born as magnetars and their life time is
also very small in astronomical time-scale.
pulsars will pulsate for years but this
video will not repeat itself.  So it will be
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