Neutron Stars: When Atoms No Longer Exist

Neutron Stars

Neutron stars are extreme astrophysical objects resulting from the gravitational collapse of massive stars after a supernova. At this stage, gravitational pressure far exceeds the capacity of electromagnetic forces that keep atoms intact. Thus, classical atomic matter disappears to make way for a state of matter dominated by degenerate neutrons.

Physics of Dense Matter

The structure of a neutron star can be approximated by several layers: a thin ionized plasma atmosphere, a crust consisting of highly compressed atomic nuclei, and a core where the matter is essentially composed of degenerate neutrons.
The typical density in the core is on the order of \(10^{17}\) kg/m\(^3\), roughly equivalent to the mass of the Sun compressed into a radius of about ten kilometers.

This density far exceeds the classical nuclear density (\(\rho_{\mathrm{nuclear}} \approx 2.8 \times 10^{17}\) kg/m\(^3\)) and implies neutron degeneracy. The degeneracy pressure, resulting from the Pauli exclusion principle, prevents total collapse into a black hole.

The Disappearance of Atoms

Under these extreme conditions, electrons are captured by protons to form neutrons via the reverse beta decay reaction: \( p + e^- \rightarrow n + \nu_e \)

Atoms no longer exist because there are no more electron shells around the nuclei: the matter is a super-dense neutron fluid. The atomic structure thus completely disappears, and the matter reaches a state where the distinction between individual particles becomes blurred.

Remarkable Physical Properties

• Pressure and density: The internal pressure is dominated by neutron degeneracy and strong nuclear interactions.
• Rotation: Some neutron stars are pulsars, rotating several hundred times per second, which is allowed by their extreme density and compactness.
• Magnetic field: Neutron stars possess some of the most intense magnetic fields in the universe, typically from \(10^8\) to \(10^{11}\) teslas.
• Tolman-Oppenheimer-Volkoff limit: The maximum mass of a stable neutron star is between 2 and 3 solar masses before collapsing into a black hole.

• Source: Lattimer & Prakash (2007), Physics Reports (new window).

Neutron stars: an extreme state of matter

Neutron stars represent an extreme state of matter where the classical physics of atoms no longer applies. The competition between gravity and neutron degeneracy quantum pressure shapes these compact and fascinating objects, gateways to complex and still widely studied astrophysical phenomena. The disappearance of atoms in favor of a neutron fluid perfectly illustrates the diversity of states of matter in the Universe.