Neutron stars are the densest objects in Nature. Their fundamental properties reflect the behavior of matter under conditions in which the details of the structure of hadrons and the fundamental interactions of quarks and gluons literally matter- none of which are known at these high densities. Probing the mechanical structure of neutron stars is one way to find out.

We have recently crossed a historic threshold: the high resolution X-ray spectrum of the neutron star in the Galactic Supernova remnant Puppis A shows evidence for identifiable atomic absorption features, indicating an oxygen/neon rich atmosphere. I will describe the spectrum and the fundamental information that can be derived from careful detailed spectroscopic study. In the course of this, we will come across probably the first and only application of the quadratic Zeeman effect you have ever seen outside your quantum mechanics textbook (if there).