A neutron star is primarily composed of tightly packed neutrons, hence its name. It forms when a massive star undergoes a supernova explosion and collapses under its own gravitational force. During the collapse, the core of the star becomes incredibly dense, and the electrons and protons in the atomic nuclei combine to form neutrons through a process called neutronization. As a result, the majority of the star's mass is packed into a compact object made up of densely packed neutrons.
While neutrons are the primary constituent of a neutron star, there are other particles and components present as well. The intense gravitational and magnetic fields of a neutron star give rise to a highly exotic environment. Within a neutron star, one can find a dense sea of neutrons, as well as a small fraction of protons, electrons, and other subatomic particles.
Regarding the weight of a teaspoon of neutron star material, it is indeed estimated to weigh billions of tons. This extreme density arises from the immense gravitational pressure compressing the matter within the star. It is not that new elements are formed within a neutron star; rather, the known elements are tightly compressed to a degree that is not observed under normal conditions. The compression is so intense that the density of a neutron star far exceeds that of any element or substance we encounter on Earth.
In summary, a neutron star is primarily composed of tightly packed neutrons, with other subatomic particles also present. Its immense density and weight are a result of the compression of known elements, rather than the formation of new elements.