So, what is a neutron star?
Well, basically, it’s an atom…except this
atom is huge, for an atom, but small for a
star.
It’s kilometers wide, and very, very massive.
They’re so dense that a teaspoon of neutron
star would weigh over 6 billion tons.
To get into what a neutron star really is,
we first have to discuss how they’re formed.
Stars are like humans—they are all born
in essentially the same way, and they also
come in all different shapes, colors, and
sizes!
Unfortunately, also like humans, they all
die, and there are many ways in which this
occurs.
The stars that we’re talking about, the
type that will collapse into a neutron star,
are massive—any star with an initial mass
of over 8 times that of the sun has the potential
to become a neutron star.
These stars are big, which has big consequences.
Now, at any given moment a star, large or
small, is maintaining a delicate balance between
the gravity crushing in on it, and the outward
pressure generated by fusing hydrogen into
helium (nuclear fusion).
This can’t go on forever, of course, and
eventually one of two things will happen first
if the star is to go on burning.
The first thing that could happen is that
the electron degeneracy pressure becomes high
enough to counteract gravity.
Electron degeneracy pressure is the pressure
arising from the electrons in the star being
packed elbow-to-elbow, unable to be packed
together any tighter.
As you could guess, this compacting of electrons
produces immense amounts of pressure in the
same way compressing a gas produces pressure.
The second thing that could initially occur,
which is what happens in the massive stars
we’re concerned with, is that the core of
the star becomes hot enough to start fusing
helium into other elements to keep up the
outward pressure from nuclear fusion.
Helium will then fuse into carbon, and eventually,
the star will get hotter and hotter.
Heavier and heavier elements will be fused
(a process called nucleosynthesis) until silicon
fuses into iron-56, resulting in an “onion”
of a star, so called because it has many shells
of different elements.
Iron’s heavy, and difficult to fuse into
other elements, so above process accumulates
iron at the core of the star.
The mass of the star’s core could exceed
what’s called the Chandrasekhar limit—approximately
1.39 solar masses.
Remember the electron degeneracy pressure
we mentioned earlier?
It, along with nuclear fusion pressure, is
what’s been mainly counteracting gravity’s
immense crushing power thus far.
But now, the star’s core mass is too great,
and electron degeneracy pressure is no longer
sufficient in supporting the star’s weight
against the force of gravity.
The core of the star collapses in on itself
in a magnificent, catastrophic explosion.
While the star’s envelope turns into a supernova,
the core contracts.
Temperatures are so high that protons and
electrons fuse to form neutrons.
When the neutrons reach a certain density,
degeneracy pressure, but of neutrons, not
electrons—stops the core from contracting
further, and the neutron star has been formed.
It is incredibly tiny: the star that collapsed
to form it was anywhere from 10 to 29 solar
masses in size, but the largest neutron stars
don’t exceed 3 solar masses, or else they
themselves will collapse to form black holes.
An average neutron star can fit comfortably
inside the borders of Philadelphia.
Neutron stars also spin incredibly fast: the
fastest rotation rate for a neutron star is
a whopping 716 times per second.
They are ridiculously dense, and as a result,
their surface gravity is immense despite their
minute size—200 billion times stronger than
on Earth!
As far as composition goes, the “crust”
of the neutron star consists mostly of a lattice
of normal ions with electrons flowing through.
As you go deeper into the star, you’ll find
higher, increasing densities of neutrons and
less electrons, until you hit the superdense
core.
No one really knows what’s at the core of
the neutron star—our best guess is that
it’s almost completely neutrons.
Not all neutron stars are created equal, however—they
come in lots of different flavors depending
on the properties of the parent star and the
circumstances of its demise.
Magnetars, for example, have incredibly strong
magnetic fields but spin slowly in comparison
to other neutron stars, while pulsars emit
beams of electromagnetic radiation sometimes
visible from Earth.
There’s a lot more to neutron stars—this
was just a short introduction.
Neutron stars represent one of the most mind-defying
extremes of this universe, minute but violent
points of light.
They are beautiful, but represent the intensity
and splendor that result from destruction;
they are some of the loudest most eloquent
poetry that this universe has to offer in
its endless sea of awe-inspiring lyric and
prose.
Thank you for listening.
