HD 179821 [SEP] constellation
HD 179821
HD 179821 is a yellow  supergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable and either a moderate-mass post-AGB star or distant massive yellow hypergiant among the largest known stars.
Discovery.
HD 179821 was first catalogued as an anonymous 8th magnitude star at the start of the 20th century. It was later listed as a spectral standard G4 0-Ia, indicating a highly luminous star type now known as a hypergiant.
It
was first considered notable for its infrared excess and double-peaked Spectral energy distribution in the infrared. These were considered to be indicators of surrounding dust and HD 179821 was identified as a possible proto-planetary nebula. Variability was also detected.
High resolution spectroscopic studies and modern space-based observations have revealed an unusual chemical makeup and a hollow spherical dust shell, but haven't fully resolved whether HD 179821 is a highly luminous yellow hypergiant or a more modest dying star.
Observations.
HD 179821 has a cold