A neutron star is the collapsed core of a massive star that exploded as a supernova, the blast that ends the life of some large stars. It packs more mass than the Sun into a ball about 24–26 kilometers (15–16 miles) across. That width is its diameter, measured straight through its center.

Size

Scientists think a neutron star has a solid crust of heavy atoms under a thin atmosphere of hydrogen or helium. That gives it a surface whose size can be measured. Precise measurements come from NICER, an X-ray telescope on the International Space Station.

NICER studies pulsars, neutron stars that spin rapidly. Using its data, astronomers found that the pulsar PSR J0030+0451 is about 26 kilometers across and has about 1.4 times the Sun's mass. PSR J0740+6620, with about 2.1 times the Sun's mass, measured about 25 kilometers across in one team's analysis and about 27 in another's. In 2024 a third pulsar, PSR J0437−4715, was measured at about 23 kilometers across.

Each of these results is uncertain by a few kilometers. Combining the results for the first two pulsars with other observations, one team found in 2021 that a typical neutron star, with 1.4 times the Sun's mass, has a radius of 12.45 kilometers, give or take 0.65. That makes it about 24–26 kilometers across.

Size comparisons

• NASA compares the width of a neutron star with the length of Manhattan Island in New York City.
• Running once around the equator of a neutron star 24 kilometers across would cover about 75 kilometers (47 miles), the length of about 1.8 marathons.
• Phobos, the larger moon of Mars, is about 27 kilometers long, a little more than the width of a neutron star. Yet a typical neutron star has about 260 trillion times the mass of Phobos.
• The Sun is about 1.39 million kilometers (865,000 miles) across, 58,000 times the width of a 24-kilometer neutron star.
• If a neutron star were shrunk to the size of a basketball, the Sun at the same scale would be a ball about 14 kilometers (9 miles) across.

How a neutron star forms

At the end of its life, a star many times heavier than the Sun runs out of fuel in its core. The core collapses under its own weight, and the star explodes as a supernova. The heaviest of these stars leave behind black holes. Lighter ones leave neutron stars.

The collapse crushes the protons and electrons of the core together into neutrons, the particles that give the star its name. If the collapsing core has between about 1 and 3 times the Sun's mass, the neutrons can stop the collapse.

What is inside

Scientists think a neutron star is built in layers. Below the crust, the pressure breaks atoms apart into a sea of mostly neutrons. Deep inside, the neutrons are packed up to twice as tightly as the particles in an atomic nucleus. What form matter takes at the very center is still unknown.

On average, each cubic centimeter of a typical neutron star holds 300 million to 400 million metric tons. Measuring both the size and the mass of neutron stars allows physicists to work out how matter this dense behaves. PSR J0740+6620 has almost 50% more mass than PSR J0030+0451 but is essentially the same size. That result argues against some ideas in which the matter at the center is easier to squeeze.

Pulsars and their discovery

Walter Baade and Fritz Zwicky proposed in 1934 that neutron stars exist. Jocelyn Bell Burnell discovered the first pulsars in 1967. Most known neutron stars are seen as pulsars. Each sends out beams that sweep across the sky as it spins, like the light of a lighthouse, so it seems to flash on and off.

Pulsars can spin very fast. PSR J0740+6620 turns 346 times every second. NICER times the arrival of each X-ray from a pulsar to within 100 nanoseconds, or 100 billionths of a second. The changing brightness shows how strongly the star bends light, and from that and its mass, scientists can work out its size.

Sources

• NASA's NICER Probes the Squeezability of Neutron Stars. NASA, 2021.
• NICER. NASA Science.
• Neutron Stars. NASA Imagine the Universe, 2017.
• The Radius of PSR J0740+6620 from NICER and XMM-Newton Data. Miller et al., The Astrophysical Journal Letters, 2021.
• A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy. Riley et al., The Astrophysical Journal Letters, 2021.
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation. Riley et al., The Astrophysical Journal Letters, 2019.
• A NICER View of the Nearest and Brightest Millisecond Pulsar: PSR J0437–4715. Choudhury et al., The Astrophysical Journal Letters, 2024.
• Sun Fact Sheet. NASA, 2024.
• Mars Fact Sheet. NASA, 2025.
• Phobos. NASA Science, 2024.
• Marathon. World Athletics.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.