The Chandrasekhar Limit
In our galaxy, 97% of stars do not exceed 10 solar masses. Such stars, during their evolution, transform into white dwarfs. The atoms inside them, under pressure, lose their electron shells, resulting in the matter turning into electron-nuclear plasma. Such stars slowly cool down over billions of years.
If we take a more massive star, its evolution can go further — where a white dwarf stopped at helium or oxygen synthesis, it will, thanks to increasing pressure and temperature, form new, heavier elements. And thus transform into a neutron star or even a black hole.
The masses of these stars are approximately equal to the mass of the Sun. But their sizes are orders of magnitude smaller, and their densities, correspondingly, higher. The typical radius of a neutron star is 10-20 km, while white dwarfs are hundreds of times smaller than the Sun's radius.
The Chandrasekhar limit is precisely that mass limit at which a star can exist as a white dwarf. If it is higher, then the star becomes a neutron star. It varies within the range of 1.38-1.44 solar masses.