Neutron Star Size Comparison

A neutron star is what is left when a massive star explodes as a supernova and its core collapses. It holds more mass than the Sun in a ball only about 24 km across, about the size of a city. The visualization above puts that size next to familiar things, all at true scale: Mount Everest, a black hole, the Moon, a white dwarf, Earth and the Sun. Step through the objects, or press play for the full tour.

How big is a neutron star?

A typical neutron star has a radius of about 12 km, so it is roughly 24 km across, a little more than the length of Manhattan (21.6 km). Mount Everest, 8.8 km tall, would reach only a third of the way up its side.

That size is now measured, not just predicted. NASA’s NICER X-ray telescope on the International Space Station has mapped hot spots on spinning neutron stars. How strongly gravity bends their light shows how compact the stars are. Two pulsars give radii of about 12.7 km (PSR J0030+0451, about 1.4 solar masses) and 12.4 km (PSR J0740+6620, about 2.1 solar masses). Most estimates for neutron stars fall between about 11 and 14 km.

Oddly, a heavier neutron star is not much bigger, and can even be slightly smaller: more mass squeezes the matter harder.

How dense is a neutron star?

Packing 1.4 times the Sun’s mass into a 12 km ball gives:

• Density: about 4 × 10¹⁷ kg per cubic metre, around the density of an atomic nucleus. A teaspoon would weigh about 2 billion tonnes, more than all the people on Earth put together.
• Surface gravity: about 130 billion times Earth’s. Even without the extra boost relativity gives, a mountain on its surface would be at most millimetres tall.
• Escape speed: about 60% of the speed of light. A little more compression and nothing could escape at all: it would be a black hole.

Neutron star vs black hole size

You might expect a black hole to be smaller than a neutron star, and for the same mass it is: a black hole with 1.4 times the Sun’s mass would have an event horizon only about 4 km in radius. But the black holes left by collapsing stars are usually much heavier, and a black hole’s size grows with its mass. Cygnus X-1, the first black hole ever identified, has about 21 times the Sun’s mass and an event horizon about 63 km in radius, five times the neutron star’s.

Neutron star vs white dwarf

Stars too light to leave a neutron star end as white dwarfs. Sirius B, the companion of the brightest star in the night sky, holds about the Sun’s mass in a ball about 5,600 km in radius, a bit smaller than Earth. That makes it almost 500 times wider than a neutron star with a similar mass. In a white dwarf, electrons resist the squeeze. In a neutron star, gravity wins that fight: electrons and protons merge into neutrons, which pack almost as tightly as matter can.

Neutron star compared to Earth and the Sun

Earth is about 530 times wider than a neutron star, yet a neutron star holds about 470,000 times Earth’s mass. The Sun is about 58,000 times wider, but has less mass. At the scale of the Sun, the neutron star is far smaller than a pixel.

Why does it have beams?

Many neutron stars are pulsars. They have enormous magnetic fields, and they shine two beams of radio waves (and sometimes X-rays and gamma rays) out of their magnetic poles. The magnetic axis is tilted from the spin axis, so the beams sweep around like a lighthouse. If they cross Earth, we see a pulse every turn. The visualization shows this slowed down a great deal: newborn pulsars spin dozens of times a second, and the fastest known, PSR J1748−2446ad, spins 716 times a second.

About the visualization

Every object is drawn at its true radius: the neutron star at 12 km, black holes at their event horizon (the Schwarzschild radius, as in the Schwarzschild Radius Calculator). Each step zooms out to the next object, and the smaller ones stay as outlines so you can see them inside. The black hole bends the starlight behind it. The pulsar’s beams and field lines show its shape, not their true length. For the other end of the scale, see the TON 618 size comparison, which goes from the Sun to the largest black holes known.

Sources

• Neutron star radii: NICER results for PSR J0030+0451 (Riley et al. 2019; Miller et al. 2019) and PSR J0740+6620 (Riley et al. 2021; Miller et al. 2021).
• Cygnus X-1 mass: Miller-Jones et al., Science (2021).
• Sirius B: Bond et al., The Astrophysical Journal (2017).
• Fastest pulsar: Hessels et al., Science (2006), PSR J1748−2446ad.
• Moon, Earth, Sun: NASA planetary fact sheets; IAU nominal solar radius.