The cluster contains a large number of rare, evolved, high-mass stars, including: six yellow hypergiants, four red supergiants, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB[e] star which may be the remnant of a recent stellar merger. In addition, X-ray observations have revealed the presence of a strange X-ray pulsar, a slow rotating neutron star that must have formed from a high-mass progenitor star. Westerlund 1 is believed to have formed in a single burst of star formation, so the stars have similar ages and compositions.