PZ Cassiopeiae

PZ Cassiopeiae is a red supergiant star located in the Cassiopeia constellation, and a semi-regular variable star.
Characteristics.
PZ Cassiopeiae is a luminous red supergiant star, one of the largest stars currently known with a radius over 1,000 times the Sun's radius (), and also one of most luminous of its type, around 200,000 times more luminous than the Sun (). It is likely to be part of the Cas OB5 stellar association although apparently much younger than the other stars in the association. The star is losing mass at around and has also once been described as a hypergiant.
Its distance from Earth was initially estimated to be around 7,800 light-years (2.4 kiloparsecs). Subsequent studies of the star using the water masers that surround it have allowed to refine both the distance and the parameters of this star, deriving an accurate parallax of , corresponding to a distance of 9,160 light-years (2.81 kiloparsecs), that translates a luminosity for it around , and an initial mass of 20 to 25 times that of the Sun. These parameters are all similar to those estimated for the red hypergiant VY Canis Majoris. 
A less reliable Gaia Data Release 2 parallax of gives the star a luminosity below with a corresponding radius of .
PZ Cas is a slow semi-regular variable star with the period quoted as 925 days in the General Catalogue of Variable Stars, although periods of 850 and 3,195 days have been derived. The visual range is approximate magnitude 8-10, large for this type of variable.
Supergiant or AGB star.
PZ Cas has traditionally been treated as a massive supergiant, comparable to others such as VY CMa, but there is some evidence that it is a possible less massive O-rich S- or SC-type Asymptotic Giant Branch (AGB) or post-AGB star. It shows enrichment of Zr and Ba, but not Li as would be expected for a true supergiant.
Companion.
PZ Cas has a Cepheid variable companion, a 13th magnitude star 12" away.