Glossary · Astrophysics
Neutron Star
A neutron star is the ultra-dense remnant of a supernova: 20 km across yet 1.4 solar masses. Seen as pulsars or magnetars, they sit just below the threshold of becoming a black hole.
Categorie
Objet compact · Résidu stellaire
Rayon Typique
10-12 km
Masse Typique
1,4 M☉ (limite haute ~2,2-2,5 M☉)
Densite Centrale
~4 × 10¹⁷ kg/m³ (proche du noyau atomique)
Premiere Prediction
1934 (Baade & Zwicky)
Premiere Detection
1967 (pulsar PSR B1919+21, Jocelyn Bell Burnell)
Premiere Fusion Detectee
2017 (GW170817, LIGO/Virgo)
Full definition
Take a mass comparable to the Sun's, squeeze it into a sphere the size of a mid-sized city, and you get a neutron star. A teaspoon of its material would weigh several billion tons on Earth. After black holes, this is the densest known object in the Universe — literally at the edge of what matter can sustain before collapsing under its own gravity.
A neutron star forms at the end of the life of a star between roughly 8 and 25 M☉. When its iron core can no longer produce energy through fusion, it collapses in a fraction of a second. Under the enormous pressure, electrons are forced to merge with protons into neutrons (electron capture), and the star contracts until the quantum pressure of the neutron gas — neutron degeneracy pressure — halts the collapse. The outer layers bounce off this core and explode as a supernova.
The result is a star about 20 km in diameter, typically 1.4 M☉, spinning from a few milliseconds to a few seconds, with a magnetic field of 10⁸ to 10¹⁵ gauss. Surface gravity exceeds Earth's by more than 10¹¹ times; a mountain there stands no taller than a millimeter. It is a natural laboratory for extreme physics — and a recurring protagonist in gravitational-wave and gamma-ray-burst scenarios.
Physical structure and key numbers
From outside inward, a neutron star has several layers:
• Atmosphere (a few cm): hydrogen or helium plasma. • Outer crust (~300 m): crystalline lattice of neutron-rich ions bathed in an electron gas. • Inner crust (~1 km): even more neutron-rich nuclei + a superfluid of free neutrons. • Outer core (~9 km): neutron matter, a few % of protons, electrons, muons. • Inner core: state of matter still debated (deconfined quarks? hyperons? pion condensates?).
Some striking orders of magnitude: central density ≈ 4 × 10¹⁷ kg/m³ (that of an atomic nucleus), surface gravity ≈ 2 × 10¹² m/s², escape velocity ≈ 0.4 c. The most massive observed is PSR J0952-0607 (~2.35 M☉, 2022). The theoretical upper limit (Tolman-Oppenheimer-Volkoff) sits near 2.2-2.5 M☉; beyond that, collapse into a black hole is inevitable.
Varieties of neutron stars
Not all neutron stars look alike.
Classical pulsars. Young neutron stars (10⁴-10⁷ years) whose radio beam sweeps Earth at a regular period. Archetype: the Crab pulsar (PSR B0531+21, 33 ms), born from the supernova of 1054 recorded by Chinese astronomers.
Millisecond pulsars. Neutron stars recycled through accretion from a companion, spun back up to 1-10 ms periods. PSR J1748-2446ad holds the record at 716 Hz (716 rotations per second).
Magnetars. Extreme magnetic fields (10¹⁴-10¹⁵ gauss), responsible for soft gamma repeaters (SGRs) and possibly some fast radio bursts (FRBs). About 30 known in the Milky Way.
Accreting X-ray pulsars. Neutron stars in close binaries stripping gas from their companion, shining brightly in X-rays. Her X-1, Cen X-3, and GX 1+4 are the classics.
Isolated neutron stars. Cooled, weakly magnetized, detected through thermal X-ray emission (RX J1856.5-3754, the closest at 400 ly). Likely hundreds of millions exist in our Galaxy — we observe only about a thousand.
How do we observe them?
Neutron stars are tiny and often cool; we almost always spot them through their extreme manifestations.
Radio. The historic discovery by Jocelyn Bell Burnell in November 1967 (published February 1968) launched radio pulsar astronomy. Today Parkes, Arecibo (until 2020), FAST (China, 2016), and soon SKA have catalogued over 3,300 radio pulsars.
X-rays. Chandra (NASA, 1999) and XMM-Newton (ESA, 1999) monitor accreting neutron stars and thermal cooling. NICER, installed on the International Space Station since 2017, precisely measures radii and masses through pulse-profile analysis.
Gamma rays. Fermi-LAT (NASA, 2008) revealed a population of gamma-ray pulsars, sometimes without a radio counterpart.
Gravitational waves. On August 17, 2017, LIGO and Virgo caught GW170817: the first merger of two neutron stars, associated with a short gamma-ray burst and a kilonova tracked in the optical by 70 telescopes. This detection confirmed that neutron-star collisions produce the heavy elements (gold, platinum, rare earths) of our Universe.
What about amateur astronomy? You won't see a neutron star through an eyepiece, but their supernova remnants are accessible: M1 the Crab Nebula (magnitude 8.4, Taurus), the Veil Nebula in Cygnus, Supernova 1987A in the Large Magellanic Cloud. Our sky map tool helps you locate these targets.
Not to be confused with
Several compact objects live in a close conceptual neighborhood.
White dwarf. Remnant of lower-mass stars (< 8 M☉), supported by electron (not neutron) degeneracy pressure. A thousand times larger (~10⁴ km), a thousand times less dense. The Chandrasekhar limit (1.44 M☉) separates it from the neutron-star regime.
Black hole. The next step if the core mass exceeds ~2.5 M☉. A neutron star still has a surface, a magnetic field, an atmosphere; a black hole has only an event horizon. The observed boundary plays out in the 2-3 M☉ 'mass gap'.
Pulsar. A pulsar IS a neutron star, but not every neutron star is a pulsar: the beam must sweep Earth and rotation must stay fast enough.
Magnetar. Subclass of neutron star with an extreme magnetic field (10¹⁴-10¹⁵ gauss).
Quark star (hypothetical). Some theorists propose that beyond neutron-star density, matter decomposes into free quarks. No confirmed detection yet.
Frequently asked
Can a neutron star be seen with the naked eye?
No. An isolated neutron star is only 20 km across and radiates mostly in X-rays and radio: even Hubble sees only a point source for the closest ones. Their host supernova remnants, however, are within reach: the Crab Nebula (M1, magnitude 8.4) is visible in a small telescope, and the historic 1054 supernova that produced it was visible in daylight for 23 days.
How many neutron stars exist in our Galaxy?
Models estimate between 100 million and 1 billion neutron stars in the Milky Way, leftovers from every supernova in its history. We know only about 3,300 as radio pulsars, 30 as magnetars, and a few hundred in X-ray binaries. The vast majority are isolated, cool, non-pulsing neutron stars — invisible to current instruments.
What is the difference between a neutron star and a pulsar?
A pulsar is a neutron star viewed at the right angle. Any young, magnetized neutron star emits two radio beams from its magnetic poles. If the magnetic axis is tilted relative to the rotation axis AND the beam sweeps Earth, we detect a periodic signal: that's a pulsar. When the geometry doesn't favor us, the neutron star exists but stays silent to our telescopes.
What happens when two neutron stars merge?
They produce a short gamma-ray burst, a kilonova (an optical/infrared glow lasting a few days), and — depending on total mass — either a supramassive unstable neutron star or a direct collapse to a black hole. The GW170817 merger on August 17, 2017, at 130 million ly in NGC 4993, was the first detected in gravitational waves AND in light. It confirmed that such collisions forge the gold, platinum, and rare earths found in our Solar System.