AZ Cygni

AZ Cygni (BD+45 3349) is a large red supergiant (M3 Iab) in the constellation of Cygnus. It is located 2090 pc from Earth. It has been studied by the CHARA array in order to understand the surface variations of red supergiants.

Observation history
AZ Cygni was first observed in the Bonner Durchmusterung catalogue, published by Friedrich Argelander in 1903. It has since then been included in many star surveys and catalogues, including the Two-Micron Sky Survey, 2MASS, Tycho-2 Catalogue and Gaia (DR2 and DR3 ).

From 2011 to 2016, it was observed using the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy Array (CHARA array)  to investigate the evolution of surface features on red supergiants. These observations were used to derive the fundamental stellar parameters of the star, reconstruct images of the star and test models of 3D radiative hydrodynamics in red supergiants.

Luminosity
In Gaia DR2 AZ Cygni's absolute bolometric magnitude was estimated at $10.897$, which correspond to a luminosity of approximately. Although the distance is potentially unreliable due to a very high amount of astrometric noise, rated at a significance of 47.4, anything above a rating of 2 is 'probably significant'.

The luminosity of AZ Cygni was calculated in a paper in 2019 derived from the Gaia distance with a bolometric magnitude of -7.58 which would correspond to a luminosity of around. Another paper in 2019 estimated three luminosity values of, and  with an average of. In a paper in 2021, the best fitting atmosphere models would correspond to luminosities of, and.

Radius
The radius of AZ Cygni was first determined to be around in a paper in 2019 based on the Gaia distance although it is likely unreliable due to a very high amount of astrometric noise. Using the angular diameter and Gaia parallax derived distance in the Mid-infrared stellar Diameters and Fluxes compilation Catalogue (MDFC), also from 2019, a radius of between and  can be derived. Another paper in 2019 estimated 5 different radii from observations lasting from 2011 to 2016 based on the angular diameter and Gaia parallax: (2011),  (2012),  (2014),  (2015) and  (2016). In the same paper the radius was estimated based on the model spectra where three radii of, and  were estimated with an average of.

The radius of AZ Cygni was estimated at based on the angular diameter and Gaia parallax in a 2021 study. In another paper in 2021, a radius of was calculated. This was an average using the angular diameter and Gaia parallaxes. Three different radii were calculated based on the best fitting atmosphere models:, and.

Temperature and spectral type
In a 1989 paper it was estimated that AZ Cygni would have spectral types of between M2Iab and M4Iab. A study in 2000 estimated that the spectral type of AZ Cygni is M3.1Iab. The spectral type of AZ Cygni was estimated at M3 Iab in a 2002 paper.

A paper in 2004 estimated that the effective temperature of AZ Cygni is 3,200 K with a spectral type of M3 Iab. AZ Cygni had 3 different effective temperature estimates in a paper in 2019 derived from model spectra: 4,000 K, 4,100 K and 3,867 K with an average of $6.191$ K. In another study in 2021 AZ Cygni would have three effective temperature estimates based on the best fitting atmosphere models: 4,000 K, 4,000 K and 3,972 K and also mentions that it is an M2–4.5 Iab star.

Mass
The mass of AZ Cygni was first determined based on the best fitting model spectra, which would correspond to three mass estimates:, and  with an average of. A paper in 2021 estimated three mass estimates equal to based on the best fitting atmosphere models.

Surface features
AZ Cygni has a complex surface, with large and small features that vary over different timescales. Patterns of large convection cells, varying over periods of more than a year, are combined with smaller hot granules of rising gas that vary over shorter timescales. The size of the larger surface features is in line with models of 3D radiative hydrodynamics in red supergiants.