A neutron-star teaspoon weighs millions to billions of tons

A teaspoon of neutron star material would weigh at least 10 million tons on Earth, according to the US Department of Energy, and NASA’s figures run far higher: about a billion tons for a single sugar cube’s worth, or a teaspoon as heavy as Mount Everest.

Those comparisons are illustrations, not measurements, and popular versions differ by a hundredfold or more, even between US government agencies:

Amount	Source	Weight on Earth
A teaspoon	US Department of Energy	at least 10 million tons
A sugar cube	NASA	about a billion tons
A teaspoon	NASA	as heavy as Mount Everest

What they agree on is the cause. Ordinary atoms are almost entirely empty: the nucleus holds over 99.9% of an atom’s mass in about 1/100,000 of its width. Chandra’s astronomers note that crushing the emptiness out of a rock the size of a football stadium would leave a grain of sand still weighing 4 million tons. In a neutron star, gravity has done exactly that, forcing electrons into protons to make neutrons until the star behaves like one gigantic nucleus.

These objects form when a massive star’s core collapses in a supernova. If the leftover core is between about 1 and 3 solar masses, the packed neutrons halt the collapse; heavier cores carry on into black holes. Surface gravity exceeds 100 billion times Earth’s, and magnetic fields are trillions of times Earth’s, with magnetars another thousand times stronger still.

The size estimates have recently become measurements. NASA’s NICER X-ray telescope on the International Space Station timed two pulsars: PSR J0030+0451, about 1.4 times the Sun’s mass, and PSR J0740+6620, about 2.1 times. Both came out roughly 16 miles (about 26 km) across, larger than the often-quoted 20 km.

The heavier star packs about 50% more mass into the same size, which tells physicists that neutron-star matter is harder to squeeze than some had predicted, a clue to what fills their cores.