HD 179821 – a Yellow-white Main-sequence Star

HD 179821 – a Yellow-white Main-sequence Star

HD 179821 is a star located approximately 95 light years from Earth in the constellation Aquarius. It is a yellow-white main-sequence star with a spectral type of F7V, meaning it is similar in temperature and brightness to our own Sun, Sol. The star has a mass of about 1.35 times that of the Sun and is notable for having at least one confirmed exoplanet in its orbit.

It is a star located in the constellation Aquarius. It is located approximately 156 light-years away from Earth and is known to have at least one exoplanet in its orbit, called HD 179821 b. This exoplanet is a gas giant and was discovered using the radial velocity method. It is a yellow-white dwarf star with a spectral type of F5V. It is also known to have one exoplanet, HD 179821 b, which is a hot Jupiter-type planet.

V1427 Aquilae is a semiregular variable star with the designation HD 179821. Between 1899 and 1989, its photographic apparent magnitude fluctuated erratically between magnitudes 9 and 10, though coverage is incomplete and some larger variations may have been missed. It then fluctuated by no more than 0.1 magnitudes until 2009, when it reached a visual magnitude of around 8.1.

HD 179821

The star’s color changed noticeably, causing the variability at different wavelengths to be inconsistent. From 1899 to 1990, the star generally became bluer, then redder. The color changes are most likely due to changes in effective temperature and underlying evolutionary trends, with the star performing a blue loop between temperatures of 4,000 K and 8,000 K. For much of this time, pulses had a period of 100 to 150 days, but this increased to 250 days between 2010 and 2017, as expected for stars whose temperatures are decreasing. The spectral type has been recorded as K4 at its coolest, while it was classified as F7 near its hottest in 2007.


HD 179821 was discovered as an unremarkable 8th-magnitude star at the turn of the twentieth century. It was later classified as a spectral standard G4 0-Ia, indicating a highly luminous star type known as a hypergiant. It was first noted for its infrared excess and double-peaked spectral energy distribution in the infrared. These were thought to be indicators of surrounding dust, and HD 179821 was identified as a possible proto-planetary nebula. Variability was also detected.

It is estimated to be around 120 light-years away from the Earth and is known for having a giant planet in orbit around it, which is the first exoplanet to be discovered by the radial velocity method.

High resolution spectroscopic studies and modern space-based observations have revealed an unusual chemical makeup and a hollow spherical dust shell, but have not fully resolved whether HD 179821 is a highly luminous yellow hypergiant or a more modest dying star.