22 | New Scientist | 2 November 2019
IN ABOUT 5 billion years, our
sun – a pretty average-sized yellow
star – will turn into a red giant,
its outer layers expanding and
consuming the solar system.
Eventually, as the gas is blown off,
it will become a planetary nebula
and leave behind a very faint, very
dense object called a white dwarf.
A teaspoon of white dwarf material
would weigh 5 tonnes here on
Earth. Only quantum pressure
between electrons stands between
a white dwarf and black hole status.
While this might seem exotic,
there is another, even more
exciting end-of-life possibility for
stars: the neutron star. These are
the densest stars that we have ever
seen, and are the last gravitational
stop before black hole territory. A
neutron star packs one-and-a-half
times the mass of the sun into
a space about the size of Los
Angeles. One forms when a star
at least 10 times bigger than the
sun collapses into a white dwarf.
Because of the extra mass of
the originating star, the quantum
pressure that keeps a white dwarf
stable isn’t powerful enough to
prevent further collapse. The
electrons and protons in the white
dwarf will be forced to merge and
become neutrons. Those neutrons
can create a stable object because
there is another quantum
pressure that can kick in – neutron
degeneracy pressure – and it too
will prevent black hole formation
within a certain mass range.
With both electron and
neutron degeneracy pressure,
the key is the Pauli exclusion
principle. This says that particles
that have a quantum rotation
number of ½, like electrons and
neutrons, can’t share the same
quantum state. What does this
have to do with stars? In the case
of the neutron star, it means that
the neutrons have an energy that
causes them to push against eachother when they get too close. This
pushing behaviour causes them
to counterbalance gravity, which
tries to force them closer together.
The consequences of this
simple quantum mechanical
principle are lovely. Neutron stars
are, in my view, the most exciting
objects made out of everyday
mass we have ever observed in
the universe. I might even say
that they are more fascinating
than black holes, but I don’t want
to upset anyone! Neutron stars
are sometimes described as giant
atomic nuclei, and they do indeed
have features in common with a
tiny atom because they are madeprimarily of the same particles
that exist in an atomic nucleus.
But neutron stars remain stable
because of gravity, while atomic
nuclei are held together by the
strong nuclear force, which is
described by a particle physics
theory called quantum
chromodynamics (QCD).
Neutron stars are incredibly
important laboratories for
studying QCD, which presents us
with problems on Earth because
the conditions needed to really
probe it are hard to come by. We
can’t make a neutron star because
the energy required is enormous.
Even if we were to make one, the
gravitational implications would
be disastrous for Earth.
We know that the innards
of neutron stars are comprised
of what we call “nuclear pasta”,
although maybe it should be
called “nuclear lasagne” to give a
clearer visual. At the surface, thereis a mix of electrons and ionised
atoms, beneath which there is
a layer of neutrons, electrons
and nuclei. Below that, there
are quantum liquids, and then
something we physicists refer
to as “quark soup”, more formally
known as quark-gluon plasma.
This plasma is difficult to study
in the lab, and neutron stars are
probably key to understanding
how well the particle physics
theory matches reality.
Neutron stars can also exhibit
unique astronomical behaviours
that allow us to gain insight into
their structure and the QCD
behaviours that may underlie it.
For example, some can have large
magnetic fields and rotate very
quickly. Together, these
phenomena translate into an
object that emits strong beams of
light repeatedly, just like a
lighthouse. We call these pulsars
and, among other things, they can
be used for calibrating clocks. They
can also be easier to find than other
types of neutron stars, due to the
beams calling out so stridently.
Because we can’t build a
neutron star in a lab, we have to
look carefully at those we can see
and come up with sophisticated
ways to analyse that data. In my
work with Anna Watts at the
University of Amsterdam in the
Netherlands, I give a particle
physicist’s perspective on a model
that Watts and her other
collaborators have been
developing of what we call the
neutron star’s “equation of state”.
This is the relationship between
how pressure and density vary in
the star, and it is highly dependent
on the quark soup we know so
little about. With time, we hope
that, using a combination of data
and increasingly sophisticated
models, we can gain insight both
into neutron star structure and
fundamental particle physics. ❚This column appears
monthly. Up next week:
Graham Lawton“ Neutron stars
are, in my view,
the most exciting
objects made out
of everyday mass we
have ever observed”Neutron lighthouse in the sky To learn more about the
mysteries of quantum chromodynamics, we are probing the
universe’s densest stars, writes Chanda Prescod-WeinsteinField notes from space-time
What I’m reading
I’m just finishing
Samantha Frost’s
Biocultural Creatures
for an academic reading
group I am in.What I’m watching
I am super into the new
television drama Evil.What I’m working on
As discussed in the
column, I’m trying to
keep up with Anna Watts!Chanda’s week
Chanda Prescod-Weinstein
is an assistant professor of
physics and astronomy, and
a core faculty member in
women’s studies at the
University of New Hampshire.
Her research in theoretical
physics focuses on cosmology,
neutron stars and particles
beyond the standard modelViews Columnist