31 Persei

31 Persei is a variable star located in the constellation Perseus. It is a red giant star of spectral type M4 II, indicating a cool surface temperature and advanced evolutionary stage. It is classified as a semiregular variable star, meaning its brightness fluctuates with a certain degree of periodicity, but the variations are not perfectly regular like those of Cepheid variables.

The star exhibits variations in its visual magnitude over a timescale of tens to hundreds of days. These brightness changes are due to pulsations within the star's atmosphere, a common characteristic of red giant stars. 31 Persei is significantly larger and more luminous than our Sun. Its distance from Earth is estimated to be several hundred light-years, making precise measurements of its properties challenging.

Due to its variability and relative brightness, 31 Persei is a popular target for amateur astronomers interested in monitoring variable stars. Its observations contribute to a better understanding of the behavior and evolution of red giant stars. Its designation "31 Persei" comes from the Flamsteed designation system, which cataloged stars within each constellation by order of right ascension.