Hertzsprung-Russell diagram (HRD) for the stars in M 8. Left panel: PMS (black) and post-main-sequence (grey) tracks for stars of masses of 1, 2, 3, 5, 10, 20, 35, 70, and 100 M_⊙, respectively. Right panel: 0.1, 1, 3, and 5 Myr isochrones. The ellipses show the position of the observed stars. The dots show the position of previously classified OB stars reported in with 0.2 dex error ellipses representing the scatter about the ZAMS (Povich et al. 2017). The luminosity class IV and V stars are shown in red, the stars with luminosity class III are shown in green. In grey we show the late-K and M-type stars. The empty ellipses with crosses show the objects located below the reddening line in the NIR CCD, which have been excluded from the age determination. The age of M 8 is estimated to be between 1 and 3 Myr.