Gas surface density maps in different merger stages. The four panels show snapshots of the approaching stage (20 Myr, upper left), the first passage when the first SF peak appears (70 Myr, upper right), the first apocenter when the SFR exhibits a plateau (110 Myr, bottom left), and the second encounter when the second SF peak occurs (160 Myr, bottom right). The overset points represent young (< 40 Myr) and massive (> 8 M_⊙) stars with ages color-coded from violet (youngest) to red (oldest). A movie of the simulation is available at https://www.bilibili.com/video/BV18vKRzzEnj.