Neutron stars spin so fast because of conservation of angular momentum: when a massive star’s core collapses into an object roughly 10 to 20 kilometers across, all the rotation of that much larger core gets compressed into a tiny space, dramatically increasing the spin rate. The same physics explains why a figure skater spins faster when pulling their arms in. Some neutron stars rotate hundreds of times per second, and the fastest on record completes 716 full rotations every second.
How a Collapsing Core Creates Extreme Spin
A neutron star forms when a massive star runs out of fuel and its core collapses under gravity. Before the collapse, that core might be roughly the size of Earth. Afterward, it’s a ball of densely packed neutrons about 15 kilometers in radius, with a mass around 1.4 times that of the Sun. That’s an almost incomprehensible density: imagine compressing the Sun into something the size of a city.
The core was already rotating before the collapse, even if slowly. Angular momentum, the physical quantity that describes rotational motion, must be conserved. When the radius shrinks by a factor of thousands, the spin rate increases by a corresponding amount. A core rotating once every few hours can become a neutron star spinning multiple times per second. This initial birth spin is what creates young pulsars, the rapidly rotating neutron stars that sweep beams of radiation across space like cosmic lighthouses.
Why Some Neutron Stars Spin Even Faster
Birth spin alone doesn’t explain the most extreme rotators. A young neutron star actually slows down over millions of years, losing energy through radiation and magnetic braking. Left on its own, an old neutron star would eventually spin quite slowly. But many neutron stars aren’t alone.
If a neutron star orbits a companion star, it can get a second chance. When the companion expands as it ages, material spills off its surface and spirals onto the neutron star. This infalling matter doesn’t just add mass. It carries angular momentum, gradually spinning the neutron star back up to extraordinary speeds. Astronomers call these “recycled pulsars” because the process gives a dead, slow-spinning neutron star a new life as a fast rotator. In low-mass binary systems, where the companion transfers matter over a very long timescale, this recycling can spin a neutron star up to periods as short as a few milliseconds, meaning hundreds of rotations per second.
This is how millisecond pulsars are born. They represent the fastest-spinning neutron stars we know of, and nearly all of them are found in binary systems or show evidence of having had a companion in the past.
The Fastest Known Pulsar
The current record holder is PSR J1748-2446ad, spinning at 716 times per second. At that rate, a point on its equator moves at a significant fraction of the speed of light. It was discovered in a dense cluster of stars called Terzan 5, which is a rich hunting ground for millisecond pulsars because its tightly packed stars frequently form the binary systems needed for recycling.
Terzan 5 continues to yield discoveries. A recently identified pulsar in the same cluster, PSR J1748-2446ao, is a candidate double neutron star system. If confirmed, it would be the fastest-spinning pulsar known in that type of pairing, with a period of 2.27 milliseconds (about 440 rotations per second). The previous speed record of 642 rotations per second stood from 1982 until the 716 Hz pulsar dethroned it.
What Stops Them From Spinning Faster
There’s a theoretical ceiling. If a neutron star spun fast enough, the centrifugal force at its equator would overcome gravity and tear the star apart. This is called the breakup rate. But neutron stars never actually reach that limit. Observations show that no known neutron star spins faster than about half its breakup rate, and recent simulations explain why: the star’s rigid crust fails at roughly half the breakup speed. When the crust cracks under rotational stress, the resulting deformation causes the star to emit gravitational waves, which carry away angular momentum and prevent further spin-up. The crust essentially acts as a built-in speed limiter.
This is why, despite having an enormous reservoir of angular momentum available through accretion, millisecond pulsars seem to top out well below the theoretical maximum. The interplay between matter falling in (spinning the star up) and gravitational wave emission (slowing it down) creates a natural equilibrium.
How Extreme Spin Warps Space Itself
Neutron stars are already dense enough to severely curve spacetime around them. Add rapid rotation, and general relativity predicts an additional effect: the spinning mass drags the fabric of spacetime around with it, like a ball spinning in honey. This is called frame dragging. Anything orbiting near a fast-spinning neutron star doesn’t just follow a simple ellipse. Its orbital plane slowly wobbles, or precesses, because spacetime itself is being twisted by the rotation.
This effect is measurable. By studying how orbital paths precess around rapidly rotating neutron stars, astronomers can test predictions of general relativity in extreme conditions that can’t be replicated in any laboratory. The precession rate depends on how fast the star spins, how massive it is, and how its mass is distributed internally, making fast pulsars natural laboratories for fundamental physics.
Why Neutron Stars Hold Their Spin
Once a neutron star is spinning fast, very little can slow it down quickly. In the vacuum of space, there’s no air resistance. The main braking forces are magnetic: charged particles trapped in the star’s magnetic field radiate energy away, gradually sapping rotational energy. For young pulsars with strong magnetic fields, this braking is significant, and they measurably slow down over centuries. But millisecond pulsars tend to have much weaker magnetic fields (a consequence of the recycling process), so they lose energy extremely slowly. Some will maintain their rapid spin for billions of years.
This combination of factors, a violent birth that compresses rotation into a tiny volume, a recycling mechanism that can spin old stars back up, a crust that limits the maximum speed, and weak magnetic braking that preserves fast rotation over cosmic timescales, explains why neutron stars are the fastest-spinning macroscopic objects in the universe.