Comparison of circularity distribution at z = 0 and at birth. All central galaxies in TNG50 are included. The columns from left to right represent galaxies with increasing stellar masses, and the top and bottom rows represent stellar particles in the inner R_e and in the outer regions R_e < r < 5R_e, respectively. Within each panel, the solid black and green curve represent the present-day circularity distribution at z = 0 for in situ and ex situ stars, respectively, while the dashed black curve represents the birth circularity distribution of in situ stars. The shaded areas indicate the 1σ scatters among galaxies in each bin. Combining in situ and ex situ stars results in the overall present-day circularity distribution of stars in galaxies at z = 0. In lower-mass galaxies, the in situ stars at z = 0 maintain a circularity distribution similar to their birth conditions, with little or no significant heating across the whole galaxy. In higher-mass galaxies, the in situ stars are mostly born on cold orbits with peaking λ_z ~ 1. The stars in the inner R_e have much lower λ_z at z = 0 compared to their condition at birth, indicating that significant heating occurred in the inner regions. The stars in the outer regions R_e < r < 5R_e have circularity distributions at z = 0 similar to those at birth.