Lesson 48: Neutron Stars
In this lesson, we will learn about how neutron stars are
formed, how they were discovered, and what happens when they develop in close
binary systems.
What is a neutron star?
A neutron star is a ball of neutrons created by the collapse of the iron core
in a massive star supernova. It is just a few kilometers in radius but has more
mass than the Sun. Basically, a neutron star is made almost entirely of
neutrons held together by overwhelmingly powerful gravity, making it like a
giant atomic nucleus.
We learned how white dwarfs resist the crush of gravity with electron
degeneracy pressure, which occurs when electrons are closely packed. In a
neutron star, closely packed neutrons result in neutron degeneracy pressure
that resists the crush of gravity.
While a neutron star could easily fit in your town,
its gravity would destroy the planet!
Evidence of neutron stars was first observed in 1967 when a graduate student
named Jocelyn Bell discovered a strange source of radio waves, pulsing on and
off at precise intervals. It was not until a year later, in 1968, that
astronomers discovered that these "pulsars" originated from the centers of two
supernova remnants, and thus were originating from neutron stars.
Neutron stars spin rapidly due to
the law of conservation of angular momentum - as the iron core of a star
collapses into a neutron star, its rotation rate increases until it is rotating
many times per second. The magnetic field gets much stronger as the core
compacts, and these magnetic fields direct beams of radiation along the
magnetic poles, which we detect as pulses of radiation if the beams sweep by
Earth.
Just like a white dwarf, a neutron star with a nearby companion star can pull
material from the other star with its strong force of gravity. However, since
the gravity is so much stronger in a neutron star, the accretion disk that
forms will be much hotter and denser than the one that forms around a white
dwarf.
The super high temperatures means the accretion disk emits high levels of
energy as X-rays. It is due to this intense X-ray emission that close binaries
that contain accreting neutron stars are called X-ray binaries.
The emissions from X-ray binaries pulse as the neutron star spins. While
pulsars from normal neutron stars slow down with time, pulsars from X-ray
binaries tend to accelerate, possibly because matter from the accretion disk
adds angular momentum to the neutron star as it accretes on to its surface.
While accreting white dwarfs occasionally erupt as novae (pronounced NO-vee),
accreting neutron stars sporadically erupt due to helium fusion on the surface,
generating a burst of energy known as an X-ray burst.
If two neutron stars closely orbit each other,
they can spiral and merge releasing more energy than even a massive star
supernova. These mergers may be the source of rare elements such as gold and
platinum.