How Heavy Is a Neutron Star?
The short answer
One teaspoon of neutron star matter would weigh about a billion tons on Earth. That is more than every person alive put together. A whole neutron star typically holds about 1.4 times the Sun's mass, packed into a ball about 20 km (12 miles) across, the size of a city.
A neutron star is the collapsed core left behind when a star at least about 8 times the Sun's mass explodes as a supernova.
Where all that mass comes from
Atoms are mostly empty space. The nucleus holds more than 99.9% of an atom's mass, but it is only about 1/100,000 of the atom's width. When a giant star's core collapses, gravity squeezes that space out. Protons and electrons are crushed together into neutrons. The star ends up like one giant atomic nucleus, held together by gravity.
All the people on Earth together weigh about 390 million metric tons, so the teaspoon outweighs them by about 2.5 times. And 'a billion' is on the low side. Spread 1.4 Suns through a ball 20 to 26 km across, and a teaspoon (about 5 mL) holds roughly 1.5 to 3 billion tons.
Why you can't land on one
Gravity at the surface is over 100 billion times Earth's. Sources put it between 100 and 200 billion. Drop something from 1 m (about 3 ft) and it hits the ground at roughly 1,400 to 2,000 km per second (3 to 4.5 million mph), about a millionth of a second later.
Neutron stars also spin. The fastest known one, PSR Terzan 5ad, turns 716 times a second. Beams from its magnetic poles sweep across the sky like a lighthouse beam. Each time one crosses Earth, we see a blink. That's a pulsar.
Gold from a crash
On August 17, 2017, LIGO detected two neutron stars colliding about 130 million light years away. Telescopes then found freshly made gold and platinum in the glow. Estimates of the gold run from at least 10 Earth masses up to about 200. Some of the gold on Earth probably came from crashes like this. Collapsars and magnetar flares may make some too. Nobody knows the split yet.
Key numbers
| What | Number | Source |
|---|---|---|
| One teaspoon of neutron star matter, weighed on Earth | about 1 billion tons | NASA Goddard (NICER mission) |
| One teaspoon (about 5 mL) from the physics, for 1.4 Suns in a ball 20 to 26 km across | roughly 1.5 to 3 billion tons | Our arithmetic from University of Maryland (M. Coleman Miller) and NASA, NICER probes the squeezability of neutron stars, 2021 |
| All humans combined | about 390 million metric tons | Greenspoon et al. 2023, PNAS |
| How many times the teaspoon outweighs all humans | about 2.5 times | Our arithmetic from Greenspoon et al. 2023, PNAS |
| Typical neutron star mass | about 1.4 Suns | University of Maryland (M. Coleman Miller) |
| Typical neutron star width | about 20 km (12 miles) across | NASA Goddard (NICER mission) |
| Smallest star that leaves a neutron star | about 8 times the Sun's mass | NASA Science, Star types |
| Width of an atom's nucleus | about 1/100,000 of the atom's width | Chandra X ray Center, Field Guide: Neutron Stars |
| Surface gravity | over 100 billion times Earth's | Ohio State University (B. Ryden, Astronomy 162) |
| Speed after a 1 m (about 3 ft) fall | about 1,400 to 2,000 km/s (3 to 4.5 million mph) | Our arithmetic from Ohio State University (B. Ryden, Astronomy 162) and University of Illinois Physics Van |
| Fastest known pulsar spin | 716 times a second | Hessels et al. 2006, Science |
| Gold made by the 2017 crash | at least 10 Earth masses | The Conversation (Duncan Brown and Edo Berger) |
What the film simplifies
- 'Would weigh a billion tons on Earth' is a way to picture the density. The matter only stays that dense under the star's own gravity, so a real spoonful brought here would blow apart.
- 'Crushes everything into neutrons' rounds up. Most of the protons and electrons combine into neutrons, but not all of them. A neutron star is about 95% neutrons.
- 'The size of a city' uses NASA's round figure of 20 km (12 miles). NICER measurements in 2021 put two real neutron stars at about 25 to 27 km (15 to 17 miles) across.
- The rocket's 'Nope! Hovering!' is a joke. Hovering in that gravity would be just as impossible as landing.
- The gold line is hedged on purpose. Other events may also make gold, which is why the rocket's chain gets a 'maybe' sticker.
- The talking Gravity, rocket and little satellite, and the star's angry eye, are cartoon.
For teachers
Runtime 1 minute 24 seconds · Topics: Neutron stars, Density, Gravity, Pulsars · Full script below
Ask your class
All the people on Earth weigh about 390 million metric tons. A teaspoon of neutron star would weigh about a billion tons on Earth. About how many times heavier is the teaspoon?
Answer
About 2.5 times. A billion divided by 390 million is roughly 2.5.
The particles in a collapsing star don't get heavier. So how does a neutron star get so dense?
Answer
Atoms are mostly empty space. The nucleus holds over 99.9% of the mass but is only about 1/100,000 of the atom's width. The collapse squeezes out the space and crushes protons and electrons into neutrons, so the same mass fits into a tiny volume.
A pulsar seems to blink. Is the star switching on and off?
Answer
No. Beams from its magnetic poles sweep around as it spins, like a lighthouse. We see a flash each time a beam crosses Earth.
The script
Host: One teaspoon of this star would weigh a billion tons.
The Rocket: Tour stop two: a neutron star! Let's land on it!
Host: It's what's left after a giant star explodes.
Host: Its core collapses. More mass than the Sun, crushed into a ball the size of a city.
Gravity: My finest work. I crushed the atoms themselves.
Host: Atoms are mostly empty space. Here, gravity squeezes it out and crushes everything into neutrons.
Host: Packed that tight, one teaspoon outweighs every human on Earth. Combined.
The Rocket: Easy! I'll land and grab a spoonful!
Host: Bad idea. Its gravity is over a hundred billion times stronger than Earth's.
Gravity: Go on. Land. I dare you.
Host: Fall three feet there, and you'd hit the ground at millions of miles an hour.
The Rocket: Nope! Hovering!
Host: Oh, and it spins. The fastest one spins over seven hundred times a second.
Lil Sat: Why is it blinking at me?!
Host: Beams from its magnetic poles sweep past us like a lighthouse. That's a pulsar.
Host: Now the twist. Sometimes two neutron stars crash together.
Host: One crash we spotted made more gold than the mass of the whole Earth.
Host: Some of the gold on Earth probably came from crashes like this.
The Rocket: Wait. My gold chain came from a dead star?!
Host: Next stop: the speed limit nothing in the universe can break. Follow!
Sources
- A teaspoon of neutron star matter would weigh a billion tons.NASA Goddard (NICER mission)
- A typical neutron star packs 1.4 Suns into a city sized ball.University of Maryland (M. Coleman Miller)
- All humans together weigh about 390 million tonnes.Greenspoon et al. 2023, PNAS
- Surface gravity is roughly 100 billion times Earth's.Ohio State University (B. Ryden, Astronomy 162)
- The fastest known pulsar spins 716 times a second.Hessels et al. 2006, Science
- Pulsar beams sweep past Earth like a lighthouse.NRAO
- The 2017 neutron star crash made gold and platinum.LIGO Lab, Caltech
- That one crash made at least 10 Earths' worth of gold.The Conversation (Duncan Brown and Edo Berger)
- A teaspoon of neutron star would have a mass of about a trillion kilograms.ESA/Hubble, Neutron star glossary
- A neutron star forms from a star of about eight or more times the Sun's mass that explodes as a supernova.NASA Science, Star types
- Measured neutron stars are about 24.8 to 27.4 km (15.4 to 17 miles) across; J0030 is about 1.4 Suns and about 26 km (16 miles) across.NASA, NICER probes the squeezability of neutron stars, 2021
- The nucleus holds more than 99.9% of an atom's mass but is only 1/100,000 the diameter of the electron cloud.Chandra X ray Center, Field Guide: Neutron Stars
- The core collapse crushes protons and electrons together into neutrons; a pulsar is like a lighthouse.NASA Imagine the Universe, Neutron Stars
- Surface gravity is about 200 billion times Earth's (upper end of the range).University of Illinois Physics Van
- Neutron stars are about 95 percent neutrons.Symmetry magazine, Five extreme facts about neutron stars, 2017
- The 2017 crash may have made around 200 Earth masses of gold.UC Berkeley News, 2017
- Gold's origin is not solved; magnetar giant flares may contribute some heavy elements.NASA Science, Where Does Gold Come From? NASA Data Has Clues, 2025
- Collapsars may be a major source of the heaviest elements.Siegel, Barnes & Metzger 2019, Nature