What are neutron stare?

Answer 1

Do you mean neutron stars?

Any atom you can think of has the same number of electrons as protons in the nucleus. Some stars have collapsed under their own gravity so that the electrons have been squeezed into the nucleus. When they do that, they combine with the equal number of protons to form neutrons in every case. So instead of atoms with neutrons, protons and electrons, you are left with just neutrons, and neutrons are much much smaller than atoms. The densities and gravitational fields in the vicinity are enormous.

Answer 2

Neutron stars are the collapsed cores of massive stars that have gone through a supernova explosion. They are incredibly dense, with a mass greater than that of the Sun packed into a sphere about 10 kilometers in diameter. Neutron stars are composed mostly of neutrons and exhibit strong gravitational and magnetic fields.