  The sigma Orionis cluster is to date the star-forming region with the largest
number of confirmed brown dwarfs and substellar objects below the deuterium
burning mass limit. The most massive star, sigma Ori Aa, just in the cluster
centre, is the \sim20Msol-mass O9.5V star that illuminates the Horsehead
Nebula, while the least massive object yet reported, S Ori 70, is only around 3
MJup. In the middle, there is a continuum of stars and substellar objects of
all types (including magnetically active B2Vp stars, Herbig-Haro objects, FU
Ori stars or T Tauri brown dwarfs) that makes the cluster a cornerstone in the
study of the initial mass function, disc presence, X-ray emission or accretion
at all mass domains. However, the derived masses strongly depend on the actual
heliocentric distance to the cluster. Gaia will solve the dilemma.

Solar and Stellar Astrophysics