First-generation stars are objects formed in the early universe within a few hundred million years after the Big Bang. The signature of first-generation stars is imprinted in second-generation stars ‑ very old, low-mass, metal-poor stars with unique chemical abundance in our Milky Way Galaxy. High-resolution spectroscopic observation with the Subaru Telescope High Dispersion Spectrograph (HDS) has pioneered this field. Using HDS, astronomers successfully discovered second generation stars made of hydrogen and elements ejected from the explosion of first generation stars with masses of ten to over a hundred times that of the Sun. These results are crucial to understand nucleosynthesis and evolution of massive and very massive stars in the early universe.