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Pulsars and other neutron stars may be composed of a ‘new’ form of matter
which forms spaghetti-like strands and lasagna-like sheets, according to
researchers based in Denmark. The possibility depends on the existence of
droplets of ‘quark matter’ inside neutron stars, a theoretical prediction.

Quark matter would consist of equal numbers of the three quarks – up,
down and strange – found in the protons and neutrons of ordinary matter.
Theorists first suggested in the mid-1970s that the stuff of protons and
neutrons might be squeezed into a quark matter state under extreme conditions
inside neutron stars. Since then, many calculations have addressed the
effects of a core of quark stuff inside a neutron star. But the new variation
on this theme deals with droplets of quark stuff intermingling with nuclear
matter.

A team from the Niels Bohr Institute and the NORDITA research centre,
both in Copenhagen, calculated the range of conditions under which such
a mixture could exist (Physical Review Letters, vol 70, p 1355). The researchers
found that the transition from nuclear matter to the droplet phase occurs
at a lower density than the transition to a quark core, so ‘a large portion
of a neutron star consists of matter in the droplet phase’.

This would affect the way pulsars change their spin. In particular,
the team points to the possibility of spaghetti-like and lasagna-like
structures being formed inside the neutron star by the mixture of materials,
‘rigid to some shear strains but not others in much the same way as liquid
crystals’. This would affect the ‘starquakes’ that are thought to be important
in some pulsars, producing sudden changes, known as glitches, in their spin
rate.