Neutron star
A neutron star is the stellar remnant left behind by the supernova explosion of a massive star. Despite its name, it is not really a star—it no longer produces energy through nuclear fusion. It is an object of a density that is extremely difficult to imagine: the mass of the Sun compressed into a sphere approximately 20 kilometers in diameter.
Formation and Structure
When the core of a massive star collapses, electrons and protons are crushed together to form neutrons. Neutron degeneracy pressure—a quantum property that prevents two neutrons from occupying the same state— stops the collapse. The result is a sphere of nearly pure neutrons, with a density comparable to that of an atomic nucleus. A coffee spoonful of neutron star matter would weigh about one billion metric tons.
Pulsars
Many neutron stars manifest as pulsars: they emit beams of radiation from their magnetic poles and spin at high speeds. If Earth lies along the beam’s axis, we receive an extremely regular periodic signal—like a cosmic lighthouse. Some pulsars rotate several hundred times per second, with a regularity surpassing even the best atomic clocks.
Neutron Star Mergers
When two neutron stars orbit each other, they gradually lose éenergy by emitting gravitational waves and spiral toward each other. Their merger produces an extraordinary event called a kilonova, which emits enormous quantities of heavy elements— gold, platinum, and uranium. In 2017, theevent GW170817 was the first to be detected simultaneously via gravitational waves and visible light.
Did you know?
The magnetic field of a neutron star can be billions of times stronger than anything we can produce on Earth. Some ultra-magnetic neutron stars, called magnetars, have the most powerful magnetic fields in the known universe.
Frequently Asked Questions
What is the difference between a neutron star and a black hole?
A neutron star is held back from collapsing by the pressure of neutron degeneracy. If the mass exceeds about 3 solar masses, even this pressure is no longer sufficient, and the object collapses into a black hole. A black hole has no physical surface, unlike a neutron star.
Can we see a neutron star?
It’s difficult to see them directly: they are tiny and emit very little visible light. They are mainly detected as pulsars (periodic radio or X-ray emissions) or through their gravitational effects in binary systems.
Do all supernovae produce a neutron star?
No. Intermediate-mass stars (8 to about 20 solar masses) leave behind a neutron star. The most massive stars can collapse directly into black holes, sometimes without a visible supernova explosion, or after a very specific type of supernova.