After
the explosion the neutron core remains, while all the
other supernova remnants are carried away by the shockwave.
If the original star had a mass more than 25 times the
mass of our Sun then the neutron core will collapse
to a density so great that no form of radiation can
escape it. This is a 'black hole' with an event horizon
of around 20 kilometres diameter. It emits no light,
but can be detected by its gravitational effect on light
that passes close to it.
If the original star was between 8 and 25 times the
mass of our Sun, the neutron core will remain as a neutron
star, with a mass up to three times the mass of our
Sun. Neutron stars have a diameter of around 30 kilometres
and are incredibly dense at 1018 kg/m3.
A teaspoonful of neutron star material would weigh as much
as a mountain!
These
neutron stars are spinning incredibly fast. The gigantic
stars from which they formed would have had rotational
periods similar to that of our Sun, which revolves about
once every 27 days. But these stars have now collapsed
into an incredibly dense object only 30 kilometres across.
When a rotating mass is moved closer to its centre of
rotation, the speed of rotation has to speed up to maintain
a quantity called angular momentum. A star that rotated
once a month can end up rotating once per second after
its collapse into a neutron star.
The
neutron stars have incredibly high magnetic fields of
strength up to a thousand million Tesla. These produce
strong radio signals from the star in two opposite directions.
As the star rotates these radio signals are swept around
the sky in a circle. This was the 'lighthouse' explanation
of the pulsars which Gold had proposed.
