Neutron Stars Explained: The Dead Star the Size of a City That Outweighs the Sun — and Spins 700 Times a Second

A dead star is sitting in our galaxy right now. It's the size of a city. It weighs more than the Sun. It's spinning 716 times every second — faster than a blender at full power.

That's not a metaphor. That's a neutron star. And it may be the strangest thing the universe has ever produced without crossing into a black hole.

What is a neutron star?

A neutron star is what's left after a massive star — at least 8 times the mass of our Sun — exhausts its fuel and explodes in a supernova. The outer layers blast outward with enough energy to briefly outshine an entire galaxy. The core implodes under its own gravity in less than a second. What survives is a sphere of pure neutrons, roughly 20 to 25 kilometres across, held in place not by chemistry or heat but by quantum physics itself.

It's not a black hole. The line between them is narrow. Push a neutron star past about 2.3 solar masses and quantum pressure loses the argument. It collapses completely. Same dramatic death — two very different endings.

How dense is a neutron star?

This is where human intuition stops working.

Neutron star material is roughly 400 trillion times denser than water. A teaspoon — about 5 millilitres — would weigh approximately one billion tonnes. That's more than the combined weight of every human being who has ever lived. In a teaspoon.

Here's another way to hold it. Take our Sun — 1.4 million kilometres across — and compress every single atom of it to neutron star density. The result would fit comfortably inside the city of London. The entire Sun. Inside the M25.

Surface gravity runs about 200 billion times stronger than Earth's. A 70 kg person standing on the surface — impossible for several reasons — would effectively weigh 14 trillion kilograms. The atmosphere, if there were one, would be roughly one centimetre thick.

What is a pulsar?

When a massive star collapses, it passes its rotation to the core — but compressed into something tiny, the way an ice skater spins faster by pulling their arms in. The result is a neutron star that can rotate dozens, hundreds, or even thousands of times per second.

The fastest known, PSR J1748-2446ad, completes 716 full rotations every second. If it were the size of Earth, its equator would be moving at a significant fraction of the speed of light.

As they spin, neutron stars beam radio waves from their magnetic poles. When those beams sweep past Earth with each rotation, we detect a regular pulse — like a lighthouse seen from the sea. That's a pulsar. And they keep time more precisely than any clock humanity has ever built. Some pulsars are accurate to within one second over 300 million years. Astronomers use them as reference points to calibrate everything from gravitational wave detectors to spacecraft navigation.

A dead star, more accurate than an atomic clock. The universe is strange. Explore what's orbiting overhead right now on the SkyLens live tracker — the same orbital physics pulsars originally helped refine.

What is a magnetar, and why is it terrifying?

Some neutron stars collapse so fast, spinning so violently, that they generate magnetic fields beyond anything else known to exist. These are magnetars.

Earth's magnetic field is about 0.5 Gauss. A magnetar's field reaches up to 10 quadrillion Gauss.

From 1,000 kilometres away, a magnetar's field would be strong enough to distort the electron shells of atoms — effectively ripping ordinary matter apart before it could get closer. This is not a theoretical concern. It is a physical consequence of the field strength.

The nearest known magnetar, SGR 1806-20, sits about 50,000 light-years away. In December 2004 it experienced a starquake — a sudden shift in its rigid neutron crust — and released more energy in 0.2 seconds than our Sun emits in roughly 100,000 years. The burst was energetic enough to briefly ionise Earth's upper atmosphere from across the galaxy. From 50,000 light-years away.

Have scientists ever seen two neutron stars collide?

Yes. Once. On August 17, 2017.

Gravitational wave detectors picked up two neutron stars spiralling into each other 130 million light-years away. The event — GW170817 — was the first cosmic event ever detected simultaneously in gravitational waves and light. A flash called a kilonova briefly outshone a billion stars.

When scientists analysed the debris, they found gold. Platinum. Uranium. Not traces — enough gold to outweigh the Sun several times over, created in an instant by the collision's extreme pressure.

The gold in a ring, a watch, a filling — it was forged in a neutron star collision billions of years ago. The debris eventually became part of the cloud that collapsed into our solar system. Then into rock, then into ore, then into your hand.

Is there any risk to Earth from neutron stars?

No known neutron star poses any threat to Earth on any timescale relevant to human civilisation. The nearest one is at least 200 to 400 light-years away, and neutron stars travel predictably.

However. A magnetar flare from within a few thousand light-years could in principle disrupt Earth's atmosphere significantly. Some researchers have suggested past mass extinction events might have involved nearby gamma-ray bursts — including those from neutron star mergers. The hypothesis has limited direct evidence. It's worth knowing. It's not cause for alarm.

For more on the extreme physics shaping the universe — from dead stars to colliding galaxies — read the SkyLens blog or browse the learning centre to understand the objects behind the orbits you can track tonight.