Nobody knows what the surface of neutron stars are made of but
there’s no shortage of suggestions. We know that iron must play
a role because we can see its characteristics absorption lines
in the spectrum from these exotic objects. But we can’t tell
whether the iron forms a gaseous atmosphere, perhaps a meter or
so thick above an ultra-hard crust, or whether the iron itself
is solid. Now Felipe Llanes-Estrada and Gaspar Moreno Navarro at
the Universidad Complutense in Madrid Spain, say they know how
to tell the difference. Here’s how: If the iron is solid, then
it ought to form an extraordinary crystal, perhaps a few dozen
atomic layers thick, almost perfectly smooth and enveloping the
entire star. One way to examine crystalline solids is to use
x-ray crystallography. The new idea from Llanes-Estrada and
Navarro is to look for pairs of neutron stars in which one is an
x-ray pulsar and the other is dead with an iron crust. Then
x-rays from the pulsar should be diffracted from the surface of
the dead star and