Yesterday’s apod is not shown in observed colour. Light from these giant stars is severely reddened by the intervening dust between us and the cluster. This redness has been artificially removed in the displayed image. The reddening by the Milky Way’s “zone of avoidance” explains why this cluster is not better known.

This spectacular cluster of massive stars is well worth keeping a watch on. Also well worth zooming in in further detail if at all possible, such as with the JWST.

“Explanation: Star cluster Westerlund 1 is home to some of the largest and most massive stars known. It is headlined by the star Westerlund 1-26, a red supergiant star so big that if placed in the center of our Solar System, it would extend out past the orbit of Jupiter. Additionally, the young star cluster is home to 3 other red supergiants, 6 yellow hypergiant stars, 24 Wolf-Rayet stars, and several even-more unusual stars that continue to be studied. Westerlund 1 is relatively close-by for a star cluster at a distance of 15,000 light years.”

The more unusual stars include “an unusual supergiant sgB star which has been proposed to be the remnant of a recent stellar merger, and an anomalous X-ray pulsar CXO J164710.2-455216.”

“The age of Wd1 is estimated at (a very young) 4–5 Myr from comparison of the population of evolved stars with models of stellar evolution. The presence of significant numbers of both Wolf–Rayet stars and red and yellow supergiants in Wd1 represents a strong constraint on the age: theory suggests that red supergiants will not form until around 4 Myr as the most massive stars do not go through a red supergiant phase, while the Wolf–Rayet population declines sharply after 5 Myr.”

“Wolf–Rayet stars are evolved, massive stars that have completely lost their outer hydrogen and are fusing helium or heavier elements in the core. Wolf Rayet stars are the central stars of planetary nebulae.”

This makes Wolf-Rayet stars the last stellar stage before a supernova, and finding 24 of them in one place means that Westerlund 1 contains a lot of supernova candidates. It could take 4,000 years before another one goes supernova, but that’s a short timescale for stellar evolution. The X-ray pulsar ought to already have a supernova remnant around it, and the planetary nebulae around the 24 Wolf-Rayett stars also ought to be well worth looking at because they will tell us how many years since the star transitioned from a supergiant to a Wolf-Rayett.