S Persei is a red supergiant

Red supergiant

Red supergiants are supergiant stars of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive...

 or hypergiant

Hypergiant

A hypergiant is a star with a tremendous mass and luminosity, showing signs of a very high rate of mass loss.-Characteristics:...

 located in the Double Cluster

Double Cluster

The Double Cluster is the common name for the naked-eye open clusters NGC 884 and NGC 869 , which are close together in the constellation Perseus. NGC 884 and NGC 869 are at distances of 7600 and 6800 light-years away, respectively, so they are also close to one another in space...

 in Perseus, north of the cluster NGC 869

NGC 869

NGC 869 is an open cluster located 7600 light years away in the constellation of Perseus. The cluster is most likely around 13 million years old. It is the westernmost of the Double Cluster with NGC 884. Located in the Perseus OB1 association both clusters are located physically close to one...

. It is a representative of the semiregular variables, whose periods of variation are subject to more irregularities than those of Mira variable

Mira variable

Mira variables , named after the star Mira, are a class of pulsating variable stars characterized by very red colors, pulsation periods longer than 100 days, and light amplitudes greater than one magnitude in infrared and 2.5 magnitude in visual...

red supergiants.

Many of the visually bright (5th - 7th magnitude) variable stars belong to this class of semiregulars, as these stars are extremely large and luminous, and hence visible across long distances. S Persei is as much as 1000 times the sun's diameter.