Neutron star versus neutral star: On the 90th anniversary of Landau's publication in astrophysics.

In the late age of developing quantum mechanics, Lev Landau made great efforts to understand the nature of matter, even stellar matter, by applying the quantum theory. Ninety years ago, he published his idea of "neutron" star, which burst upon him during his visit over Europe in the previous year. The key point that motivated Landau to write the paper is to make a state with lower energy for "gigantic nucleus", avoiding extremely high kinematic energy of electron due to the new Fermi–Dirac statistics at that time. Landau had no alternative but to neutralize by "combining a proton and an electron" before the discovery of neutron. However, our understanding of the nature has fundamentally improved today, and another way (strangeonization) could also embody neutralization and thus a low‐energy state that Landau had in mind, which could further make unprecedented opportunities in this multi‐messenger era of astronomy. Strangeon matter in "old" physics may impact dramatically today's physics, from compact stars initiated by Landau, to cosmic rays and dark matter. In this essay, we are making briefly the origin and development of neutron star concept to reform radically, to remember Landau's substantial contribution to astrophysics and to recall those peculiar memories. [ABSTRACT FROM AUTHOR]