From stellar core to neutron star
A star much more massive than the Sun ends its life when its core runs out of fuel it can usefully fuse. The core can no longer support its own weight and collapses in a fraction of a second, while the outer layers are blasted away. If the surviving core is not too massive, the collapse halts when it becomes a ball of closely packed neutrons: a neutron star.
During the collapse, the extreme pressure squeezes electrons into protons, forming neutrons. What stops the compression is a quantum rule: neutrons, like electrons, cannot all occupy the same state, which produces a pressure that resists further squeezing. The details of this physics belong to quantum physics, and the behavior of matter at such density is not yet fully understood.
How dense is “extremely dense”?
A typical neutron star holds about one and a half times the mass of the Sun, but is only around 20 kilometers across, roughly the size of a city. This makes the material about as dense as an atomic nucleus. A common illustration is that a teaspoon of neutron star material would weigh billions of tons on Earth. That figure is a rough illustration rather than something that can be measured, since such material would not remain stable outside the star’s gravity.
The gravitational force at the surface is enormous, hundreds of billions of times stronger than on Earth, according to standard estimates. The star’s surface is thought to be extremely smooth, with any “mountains” at most a tiny fraction of a millimeter high, though this depends on the models used.
There is a limit to how heavy a neutron star can be. Beyond roughly two to three solar masses, the exact value being uncertain, the star is expected to collapse further, probably into one of the black holes known from general relativity. Finding the upper limit helps researchers understand how matter behaves under extreme pressure.
Spin and magnetism
Stars rotate, and magnetic fields are carried along with their gas. When a core shrinks from the size of a star to the size of a city, two things happen. Its spin speeds up, in the way that a skater spins faster when pulling in their arms, and its magnetic field is concentrated to enormous strength. Neutron stars can therefore spin from about once per second to hundreds of times per second, with a magnetic field vastly stronger than any magnet on Earth.
Pulsars
In 1967, a graduate student named Jocelyn Bell Burnell, working with a radio telescope in England, noticed a strangely regular series of radio pulses. The signal was so regular that the team briefly joked about whether it could be artificial, before concluding that it came from a natural source. The object became known as a pulsar, short for pulsating star. Pulsars were soon identified as rotating neutron stars.
The “pulsing” is thought to work like a lighthouse. A neutron star’s magnetic field channels charged particles, which emit beams of radiation from near its magnetic poles. If the magnetic axis is tilted relative to the spin axis, the beams sweep around the sky as the star rotates. When a beam happens to cross Earth, a telescope detects a pulse. The star itself does not blink; the beam just points at us once per rotation. Many neutron stars likely have beams that never cross our line of sight, so we cannot detect them as pulsars.
Pulsars can be extraordinarily regular. The most stable ones keep time well enough to compete with the best atomic clocks over long periods. Over millions of years, they gradually slow down as they lose energy, which is one way astronomers estimate their age.
Kinds of neutron stars
- Ordinary radio pulsars spin at rates of about a fraction of a second to several seconds per rotation. The Crab Pulsar, left behind by a stellar explosion recorded in the year 1054, spins about thirty times per second.
- Millisecond pulsars spin hundreds of times per second. They are generally thought to be old neutron stars that have been spun up by pulling in material from a companion star.
- Magnetars are neutron stars with extremely strong magnetic fields. They are linked to sudden bursts of X-rays and gamma rays.
Why scientists value them
Neutron stars are natural laboratories for physics that cannot be reproduced on Earth. Measuring their masses and sizes constrains theories of dense matter. Pulsar timing has also been used in the search for faint ripples in spacetime, connecting them to the study of gravitational waves. When two neutron stars merge, the collision can produce gravitational waves and bright flashes of light, and researchers think such events contribute to the creation of some heavy elements. Observations of such a merger in 2017 supported this picture, though the exact share of heavy elements made this way remains under study.
Neutron stars are also reminders of how stellar death creates new objects. For the broader story, the stellar life cycle explains where in a star’s life these remnants come from.