Prediction and discovery
In 1934, only two years after James Chadwick discovered the neutron, Walter Baade and Fritz Zwicky boldly proposed that a supernova is the transition of an ordinary star into one made of neutrons. In 1939 Oppenheimer and Volkoff calculated the maximum mass, but evidence was lacking for almost 30 years. In 1967 Cambridge graduate student Jocelyn Bell Burnell spotted a signal in radio telescope charts that pulsed every 1.337 seconds. Jokingly labeled "LGM-1" for "little green men," it was a rapidly spinning neutron star sweeping a radio beam like a lighthouse: a pulsar. The 1974 Nobel Prize in Physics went to her supervisor Antony Hewish alone, a decision still debated, and in 2018 Bell Burnell donated her entire $3 million Breakthrough Prize to scholarships for under-represented physics students.
An extreme object
A neutron star squeezes about 1.4 solar masses into roughly 20 km, so a sugar cube of its material would weigh hundreds of millions of tons. Surface gravity is about 100 billion times Earth's, and the fastest spin more than 700 times a second. Magnetars carry magnetic fields a quadrillion times Earth's. Above roughly two to three solar masses, even neutron pressure can't resist gravity and the object is thought to become a black hole. The binary pulsar found by Russell Hulse and Joseph Taylor in 1974 showed an orbit shrinking exactly as expected from emitting gravitational waves, earning the 1993 Nobel Prize.
| Object | Rough size | Feature |
|---|---|---|
| White dwarf | Earth-sized | End state of Sun-like stars; held up by electron pressure |
| Neutron star | About 20 km | End state of massive stars; held up by neutron pressure |
| Black hole | Event horizon scales with mass | Not even light escapes |
Where gold comes from
On August 17, 2017, LIGO and Virgo caught gravitational waves, GW170817, from two neutron stars colliding about 130 million light-years away. A gamma-ray burst followed 1.7 seconds later, and some 70 telescopes worldwide then watched the glow of a "kilonova." Its spectrum showed heavy elements such as gold and platinum being forged in bulk, supporting the idea that much of the universe's gold comes from neutron-star mergers. The near-simultaneous arrival of light and gravitational waves also confirmed general relativity's prediction that gravity travels at the speed of light.
FAQ
- How heavy is a spoonful of neutron star?
- A teaspoon would weigh around a billion tons, comparable to Mount Everest. Brought to Earth, it would expand explosively once the pressure was released.
- Will the Sun become a neutron star?
- No. The Sun lacks the mass; it will become a red giant and then a white dwarf. Stars roughly eight or more times the Sun's mass leave neutron stars or black holes after a supernova.
Sources reviewed: 2026-10-26
References
- Baade, W., & Zwicky, F. (1934). On super-novae. Proceedings of the National Academy of Sciences, 20(5), 254–259.
- Hewish, A., Bell, S. J., Pilkington, J. D. H., Scott, P. F., & Collins, R. A. (1968). Observation of a rapidly pulsating radio source. Nature, 217, 709–713.
- Abbott, B. P., et al. (LIGO & Virgo) (2017). GW170817: Observation of gravitational waves from a binary neutron star inspiral. Physical Review Letters, 119, 161101.