NASA astronomers have calculated the diameter of a neutron star. These stars have about 1.5 times the mass of the Sun, but they are much smaller; it is estimated that the star they have observed is between 19 and 30 kilometers in diameter, probably 23 kilometers.
This data confirms that the star consists of neutrons and not other particles. In fact, the neutron star is the densest star you can see, it is not formed by atoms, but by neutrons. Neutrons are much more compact than atoms, which makes the mass large in a very small volume. According to astronomers, the matter taken with a spoon would weigh approximately three billion tons.
The size of this neutron star has been calculated indirectly. The star absorbs gas to another star next to it and this gas accumulates around the neutron star until it gets too hot and explodes. From the analysis of these explosions, astronomers have been able to verify that the star rotates 45 times in a second. And knowing the mass and movement of the star have been able to calculate its size.