Black holes
Neutron Stars Explained Simply: a Sun inside a city
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Imagine pressing more than the entire mass of our Sun into a sphere the size of a city. That is exactly a neutron star, the densest visible object in the cosmos.
What a neutron star is
A neutron star is the compressed core of a massive star. Its matter is so dense that protons and electrons merge into neutrons.
As a result it consists almost only of neutrons, packed tightly together. This unusual matter cannot be observed anywhere else.
How it forms
At the end of its life, a massive star explodes as a supernova. The outer layers are flung into space.
The core, however, collapses in on itself. If it is not heavy enough for a black hole, the collapse stops at the neutron star. The enormous pressure of the neutrons holds it up.
How extremely dense it is
The density of a neutron star defies all everyday experience. A single teaspoon of its material would weigh billions of tons on Earth.
On its surface, gravity is also immense. It is many billions of times stronger than on Earth. Nothing solid could survive there.
Pulsars
Many neutron stars spin extremely fast, some hundreds of times per second. As they do, they emit beamed radiation from two poles.
When this beam sweeps across Earth, we see a regular blinking. Such objects are called pulsars. The first was discovered in 1967 and was briefly even considered a possible alien signal.
Magnetars
Some neutron stars have unimaginably strong magnetic fields. These specimens are called magnetars.
Their magnetic field is the strongest in the known universe. It can trigger enormous bursts of X-rays and gamma rays, measurable even at great distance.
Neutron star or black hole
Whether a stellar remnant becomes a neutron star or a black hole depends on its mass. There is an upper limit for neutron stars.
If it is exceeded, no pressure can hold out anymore. The core collapses further into a black hole, whose event horizon then encloses everything.
What collisions reveal
When two neutron stars merge, they emit gravitational waves and a bright flash of light. In the process, heavy elements such as gold are created.
Such events connect several branches of research at once. More on this is shown in the black holes section.
Frequently asked questions
How dense is a neutron star?
Extremely dense. A teaspoon of its material would weigh billions of tons on Earth. More than a solar mass sits in a sphere only about 20 kilometers across.
What is the difference from a black hole?
A neutron star still has a solid surface and emits radiation. If a star's remnant becomes too heavy, it collapses further into a black hole.
How does a neutron star form?
When a massive star dies, its core collapses in a supernova. What remains is an extremely dense sphere of neutrons, the neutron star.
What is a pulsar?
A pulsar is a fast-spinning neutron star that emits radiation in two cones. If a cone sweeps Earth, we see a regular pulsing, like a lighthouse.
How fast do neutron stars spin?
Very fast. Some rotate hundreds of times per second. Despite their enormous mass, they complete a rotation in fractions of a second.
Can neutron stars collide?
Yes. When two merge, gravitational waves and heavy elements like gold form. Such events have already been measured directly.
Sources and further reading
- Neutron Stars — NASA
- Pulsars — ESO
Update note (as of: 06/05/2026)
First publication of the neutron stars article.
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