A pulsar is a rotating, magnetic neutron star, or a ball of nearly pure
neutrons, packed together so tightly, they touch each other.

We know pulsars are magnetic, because their radio waves show a distinctive
continuum spectrum (called sychrotron radiation), which we can reproduce
on Earth, using electrons moving through magnetic fields.

- Still, why are we so sure that neutron stars exist?
And how do we know they're so small?

- Pulsars are the most precise clocks in the Universe: their pulses are so
repeatable, and the times between pulses are so regular, it's hard to
explain;  unless it's from something that rotates, then it's easy to
explain.

- They pulse so rapidly: some are as fast as thousands of times per
second.  To do this, and not have to travel faster than light (not allowed
by relativity), they must be small, no more than a few kilometers across.

- Any object rotating this fast would fly apart, unless its gravity were
very strong, and it were made of a very strong solid.

- From neutron stars in binary star systems, Kepler's Third Law shows us
that their masses are typically about 1.4 Suns.

- Since density = mass/volume, anything this massive and this small is as
dense as an atomic nucleus: therefore, a neutron star.

- A neutron star is so dense, there is no space between atoms or even
between nuclei. The protons and electrons in atoms are so compressed into
each other, they make a solid mass of neutrons, with no space between
them.


Still, even in this extreme state, neutron star matter isn't infinitely
strong. If a neutron star had a mass of greater than 3 solar masses, it
would gravitationally collapse