The ancients believed the stars of the night sky were eternal and unchanging. Today we
know this is not true. Stars are born, live their lives, and then die. The way a star
dies depends largely on its mass. A low mass star ends as a white dwarf. A high mass star
becomes a black hole. But in between, a star becomes a neutron star.
Stars spend their lives fusing matter together. This process begins with the simplest of atoms:
hydrogen. Fusing hydrogen nuclei gives you helium and releases some energy. It’s this
energy which causes the stars to shine.
If the star is big enough, then it continues to evolve by fusing matter together to make
HEAVIER elements: helium, carbon, neon, oxygen… But at some point the star runs out of steam.
Fusion stops, stellar evolution comes to an end, and the star dies.
Smaller stars end their lives as a WHITE DWARF, a glowing ball of white hot matter which slowly
cools down over billions of years. Although fusion has stopped for white dwarfs, they
still shine because of their astronomically high temperature. This is the death that awaits
our sun.
For the really big stars, the end of fusion enables gravity to do some real damage. Unconstrained
by fusion, the gravity of the star breaks down particles and squeezes everything together
as tightly as nature will allow. The result is a BLACK HOLE. The gravity of a black hole
is so strong that anything that gets close enough is sucked inside - including light.
The danger zone is called the Schwarzschild radius.
In between white dwarfs and black holes are NEUTRON STARS. These stars are made primarily
of neutrons which are NEUTRAL particles. Ernest Rutherford predicted the existence of neutrons
in 1920, and a dozen years later, they were observed by James Chadwick. You can find neutrons
in the nucleus of most atoms. They can also be created in a process called “electron
capture.” With enough force, a proton and electron combine to form a neutron and a neutrino.
Neutrinos are super fast and elusive, so they just fly off. But the neutron stays behind.
This is the key to understanding how neutron stars are made.
Imagine you have a dying star about 50% more massive than our sun. The star’s gravity
is strong enough to squeeze the electrons and protons together to form neutrons and
neutrinos. The neutrinos dart off into space leaving behind a big sphere of neutrons. Gravity
continues to squeeze the neutrons together, but eventually hits a wall - the Pauli Exclusion
Principle. This says roughly that two particles cannot occupy the same place at the same time.
You now have a neutron star!
Let’s quantify the transition from white dwarf to neutron star to black hole. Suppose
we have a dead star, and an imaginary dial that lets us change its mass. We’ll set
the dial to 1 solar mass - the mass of our sun. This produces a white dwarf, a spinning
sphere of white hot matter about the size of the Earth. As we increase the mass by turning
the dial, gravity gets stronger, the white dwarf gets smaller, and it spins more quickly.
Once we turn the dial to 1.39 solar masses, gravity is strong enough to combine electrons
and protons to make neutrons and neutrinos. This value on the dial is called the Chandrasekhar
limit. The dead star is now a neutron star. It shrinks down to a sphere with a radius
of about 10 kilometers, and the spinning can be as fast as hundreds of times per second.
If we move the dial further, gravity eventually becomes strong enough to break down the neutrons,
and the neutron star collapses into a black hole. This point on the dial is called the
Tolman–Oppenheimer–Volkoff limit and while its exact value is not known, it ranges from
1.5 to 3.0 solar masses.
If you were to look at the ingredients of a neutron star, it wouldn’t be 100% neutrons.
The number one ingredient is definitely neutrons, but there are still some protons and electrons
in there, too. Because the rapidly spinning neutron star contains these charged particles,
there will be a massive magnetic field. Just like on Earth, the magnetic field doesn’t
have to line up with the axis of rotation. Like a stellar lighthouse, the magnetic field
sweeps across the sky emitting regular bursts of electromagnetic radiation. Because of this
pulsing signal, neutron stars are sometimes called pulsars.
Neutron stars, like the neutron, were predicted to exist before they were observed. Almost
as soon as the neutron was detected, astronomers Walter Baade and Fritz Zwicky predicted that
a supernova could produce neutron stars. And in 1967, a pulsating neutron star was first
observed. In the decades since many more have been discovered.
The universe is a pretty big place, and so is that subscribe button. I’m not going
to tell you to click it, because I’m certain you’ll do the right thing…… The right
thing is to click the button.