The cluster contains at least a dozen
early O stars, of which at least three are
eclipsing binaries. All are hotter than 38,000 K and more luminous than . There are around 20 further O class stars in the cluster, all
main sequence objects implying a very young age for the cluster. (north is towards bottom right)|alt=A cluster of stars inside a large nebula. The clouds of gas and dust are predominantly bright red in colour and wispy, akin to flames. They are clumped in the bottom-left corner. Other clouds, deeper in the cluster behind many of the stars, appear pale pink. The stars are concentrated in the top half of the image and are mostly small, bright white and six-pointed. They cast blue light over the nebula. Other stars with very long spikes surrounding them lie in the foreground. Several
Wolf–Rayet stars are found in the vicinity of Westerlund 2, although not in the central core.
WR 20a, a binary consisting of two WR stars, and the single stars
WR 20aa,
WR 20b, and
WR 20c are all thought to be members of the cluster, although possibly now runaway members. All five Wolf Rayets are extremely young massive objects with OIf*/WN spectral types, amongst the most luminous stars in the galaxy. This composite spectral type indicates young very massive hydrogen-burning stars that are just starting to convect nitrogen and helium to the surface and develop denser
stellar winds so that they show the emission lines of a Wolf-Rayet star.
WR21a, itself a massive binary, lies in the same direction but is unlikely to be a member of Westerlund 2. Westerlund 2 also contains a large number of pre-
main sequence stars with masses below . These stars constrain the age of the cluster to near 2 Myr. ==Discovery==