I study the nature of the first stars in the Universe with high-resolution computer simulations and analytical models. Depending on their mass, the first stars eject heavy elements into the interstellar medium when they die as supernovae. We can observe this specific chemical fingerprint in the oldest stars in the Milky Way and constrain their progenitor masses. My simulations allow to correctly interpret observations, to optimise upcoming surveys, and to eventually constrain the characteristic masses of the first stars.