A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest, with just 20 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.
IRAS 17163-3907 is a yellow hypergiant that clearly shows the expelled material that probably surrounds all yellow hypergiants.
Yellow hypergiant HR 5171 A, seen as the bright yellow star at the center of the image.
Westerlund 1 is a compact young super star cluster about 3.8 kpc away from Earth. It is thought to be the most massive young star cluster in the Milky Way, and was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster.
Credit: ESO/VPHAS+ Survey/N. Wright
Images of Westerlund 1: left is visible light, with all stars appearing red due to interstellar absorption; right is X-ray wavelengths, with the magnetar marked
Artist's impression of the magnetar CXOU J164710.2-455216 in the star cluster Westerlund 1 (ESO/L. Calçada)
Comet-like stars in Westerlund 1