The core question

Access through your institution Buy or subscribe The outer crust consists of a solid lattice of neutron-rich nuclei bathed in a sea of relativistic electrons. A few hundred metres below, immense pressure squeezes electrons and protons together to produce neutrons. At a depth of about half a kilometre comes the ‘neutron drip line’, where neutron-rich nuclei begin to leak neutrons and the inner crust becomes a rigid lattice permeated by a sea of superfluid neutrons.Deeper still, near the crust–core boundary, even stranger structures emerge. Competition between nuclear attraction and electrostatic repulsion forces nuclei to deform into complex, elongated shapes known as ‘nuclear pasta’. These phases include ‘spaghetti’ (long rods) and ‘lasagna’ (thin sheets). Further in, individual nuclei dissolve entirely into a somewhat mysterious superfluid soup. This superfluid provides the best explanation for the phenomenon known as pulsar glitches — sudden increases in a neutron star’s rotation speed. The idea is that the interior superfluid periodically ‘unpins’ from the solid crust, transferring angular momentum and briefly speeding up the star’s rotation. This is a preview of subscription content, access via your institution Access options Access through your institution Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any time Learn more Subscribe to this journal Receive 12 print issues and online access $259.00 per year only $21.58 per issue Learn more Buy this articlePurchase on SpringerLinkInstant access to the full article PDF.USD 39.95Prices may be subject to local taxes which are calculated during checkout Author informationAuthors and AffiliationsNature Physics https://www.nature.com/naturephysicsMark BuchananAuthorsMark BuchananView author publicationsSearch author on:PubMed Google ScholarCorresponding authorCorrespondence to Mark Buchanan.Rights and permissionsReprints and permissionsAbout this articleCite this articleBuchanan, M. The core question. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03249-9Download citationPublished: 16 April 2026Version of record: 16 April 2026DOI: https://doi.org/10.1038/s41567-026-03249-9Share this articleAnyone you share the following link with will be able to read this content:Get shareable linkSorry, a shareable link is not currently available for this article.Copy shareable link to clipboard Provided by the Springer Nature SharedIt content-sharing initiative