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The Question

(Submitted January 25, 1998)

What is the difference between pulsars and neutron stars? If a pulsar is a neutron star, is every neutron star a pulsar?

The Answer

A pulsar is a rotating neutron star that can produce radiation by spinning its powerful magnetic field through space. (There are also 'accreting pulsars' which funnel matter from a companion star onto their magnetic polar caps as they rotate.)

A neutron star uses up a lot of its rotational energy moving its magnetic field around this way, and and so it gradually slows down. When it slows down enough, it no longer radiates very much energy, and so it is no longer considered a pulsar. This usually happens within a few million years. If a neutron star had only a weak magnetic field, it would also not be a pulsar.

Most neutron stars in the Universe are old enough and tired enough that they are no longer pulsars. A recent paper estimates a thousand million old neutron stars in our Galaxy, even though the number of known pulsars is about a thousand.

David Palmer
for Ask an Astrophysicist

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