Westerlund 1 is the only known super star cluster in our Galaxy and thus a great laboratory for studying massive stars. It has a mass of ~60,000 solar masses, whereas the masses of open clusters range up to 10,000 solar masses. It contains 24 evolved Wolf-Rayet stars, several blue supergiants, red supergiants and yellow hypergiants, a B[e] star, a luminous blue variable and a magnetar. It is highly extincted by dust, however, it is still observable at optical wavelengths. I conducted an optical photometric survey in this paper (astro-ph/0702614), which aimed to discover massive ecipsing binaries, for which their masses and radii can be determined with subsequent spectroscopy. The survey resulted in the discovery of 129 new variable stars, including 4, possibly 5, eclipsing binaries that are cluster members. Among these are a Wolf-Rayet eclipsing binary and a detached eclipsing binary containing unevolved stars. The components of this detached binary are the first main sequence objects to be identified in Westerlund 1.
Below is a color image made by Joel Hartman (CfA), by combining the best seeing B, V and I frames taken with the 1 meter Swope telescope at the Las Campanas Observatory, Chile. They are 8.7 arcminutes on the side. Notice that the young hot cluster stars appear red instead of blue, because of the large amount of reddening (Av ~12 mag) caused by dust.