About the image. Drawn as a tiny, blue-white glowing sphere with loops of magnetic field around it. Neutron stars are far too small to resolve; the field lines are a diagram of the field's shape, not something visible.
When a star of roughly 10 to 25 times the Sun’s mass runs out of fuel, its core collapses in a supernova. If the core is not quite heavy enough to become a black hole, it stops as a neutron star: about 1.4 times the mass of the Sun squeezed into a ball only about 20 kilometres across, the size of a city.
Dense beyond imagining
Inside, matter is packed as tightly as in an atomic nucleus. A teaspoon of it would weigh more than five billion tonnes, about 900 times the mass of the Great Pyramid of Giza. If all of Earth were compressed to that density, it would fit in a ball about 305 metres across. Gravity at the surface is more than a hundred billion times Earth’s, and anything trying to escape would need to travel at more than half the speed of light.
They also spin. Because the collapsing core keeps its rotation as it shrinks, newborn neutron stars turn very fast. The fastest known, PSR J1748−2446ad, rotates 716 times every second, and its equator moves at about a quarter of the speed of light. Their magnetic fields are hundreds of millions to thousands of trillions of times stronger than Earth’s.
Predicted, then found
In December 1933, less than two years after the neutron was discovered, Walter Baade and Fritz Zwicky proposed that supernovae turn ordinary stars into stars made of neutrons. For three decades it was only an idea. Then in 1967 Jocelyn Bell Burnell and Antony Hewish found radio pulses ticking with clock-like regularity: a pulsar, a spinning neutron star sweeping a beam past Earth.
How heavy can they get?
There is a limit. Above roughly 2.2 solar masses for a non-rotating star, a neutron star should collapse into a black hole, though exactly where the line falls depends on how matter behaves at these densities, which is still not settled. The heaviest one measured, PSR J0952−0607, weighs about 2.35 Suns.
The Milky Way is thought to hold around a billion neutron stars; about 3 200 are known. The nearest, RX J1856.5−3754, is about 400 light-years away and was the first isolated neutron star seen directly in visible light.
How long to get there?
Neutron star is about 400 light-years away (the nearest known, RX J1856.5−3754).
Light299 792 km/s400 years
Breakthrough Starshotplanned light sail, 20% of light speed2 000 years
Parker Solar Probefastest spacecraft so far, 192 km/s624 567 years
Voyager 1leaving the Solar System, 17 km/s7.1 million years
At Voyager 1's speed the trip would take about 88 174 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
A tiny bright star flanked by looping blue lines of a magnetic field, an impression of a neutron star. AI-generated painting.A small blue-white star inside a wide, glowing red ring of gas, an impression of a neutron star in a remnant of its supernova. AI-generated painting.Neutron star, 3D artist's impression, rendered from the interactive model.Neutron star, 3D artist's impression, from another angle, rendered from the interactive model.Neutron star, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
1933Walter Baade and Fritz Zwicky propose neutron stars.
1967The first pulsar is discovered.
1982The first millisecond pulsar is found.
2017Two neutron stars are seen colliding, in gravitational waves and light.
Questions people ask
How dense is a neutron star?
A teaspoon of it would weigh more than five billion tonnes.
How big is a neutron star?
About 20 kilometres across, the size of a city, with more mass than the Sun.