Discovery
HD 179821 was first catalogued as an unremarkable 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 detached dust shell that has been studied with the help of the Hubble Space Telescope. The shell is approximately circular in shape, has an inner diameter of ~3".3 corresponding to 20,000 AU at 6,000 pc, and an outer diameter of 5".7 or more, with the star 0".35 from the centre of the shell. The current mass loss is low, but during the formation of the shell it is estimated to have been , an exceptionally high rate being comparable to that of the archetypical OH/IR red supergiant, VY Canis Majoris. Like its constellation neighbor and also hypergiant starVariability
HD 179821 is a semiregular variable star with the variable star designation V1427 Aquilae. Between 1899 and 1989, its photographic apparent magnitude varied erratically between about magnitudes 9 and 10, although coverage is not complete and some larger variations may have been missed. It then varied by no more than 0.1 magnitudes until 2009, at a visual magnitude around 8.1. The colour of the star changed noticeably so that the variability at different wavelengths is not consistent. In general, the star became bluer from 1899 until 1990, and then redder again. The colour changes most likely reflect changes in theChemical composition
The chemical composition of this star differs from that of other yellow supergiant stars. The star is moderately metal-deficient and the main elements present in the star (apart from hydrogen and helium) are oxygen, carbon and nitrogen. Molecules such as hydrogen isocyanide, sulfur monoxide and HCO+ have been detected in the circumstellar envelope of the star. These molecules may result from an active photochemistry, generated by UV photons emitted by the central star as it warms up, or can be produced in shocks.Controversy
While most authors consider HD 179821 to be a warm hypergiant star, others think it is actually a protoplanetary nebula or a smaller post- AGB star at a distance of 1 kiloparsec (3,200 light years). In that case the star's luminosity and radius would be much lower, around 16,000 times that of the Sun and 60 to , and its initial mass would be equal to the current mass of the Sun. This discrepancy arises because its distance was too great to be measured bySee also
* List of supernova candidatesNotes
References
Sources
* * {{DEFAULTSORT:HD 179821 Aquila (constellation) 179821 G-type hypergiants G-type supergiants K-type supergiants Post-asymptotic-giant-branch stars Protoplanetary nebulae Aquilae, V1427 094496 BD-00 3679 Semiregular variable stars J19135861+0007319