(Submitted September 21, 2010)
I've read somewhere that some pulsar's are speeding up. They do this by
'eating' another star in a binary system. What would happen if you kept
adding matter? Would it eventually go faster than the speed of light
(which is not possible), or will it collapse into a black hole?
Yes, you are right! Very fast rotating pulsars, known as millisecond pulsars
(because their rotation period is about 1 millisecond! Imagine that-- the
star, whose radius is the size of Manhattan is spinning all the way around
in one thousandth of one second!) are thought to be neutron stars that
have accreted surrounding material (possibly its binary companion star)
and ended up spinning faster as more material have fallen onto the surface.
However, if the star accretes more mass than a critical mass, known as the
Tolman-Oppenheimer-Volkoff limit (http://en.wikipedia.org/wiki/Tolman-Oppenheimer-Volkoff_limit), the star will collapse into a singularity, a black hole.
This is because the pressure within the star can't support the star's own mass
against gravity. Just to clarify, it is the mass limit that causes the neutron
star to collapse, not that the rotation speed approaches the speed of light.
The rotation speed can't reach the speed of light because it takes infinite
energy for any material with mass to travel the speed of light.
-Antara & Kevin
for "Ask an Astrophysicist"