T Ursae Minoris

T Ursae Minoris (T UMi) is a star in the constellation Ursa Minor, located 2'30" west-southwest of 3 Ursae Minoris toward the western border of the constellation with Draco.
Properties.
A red giant ranging between spectral types M4e and M6e and with a surface temperature of 3300 K, it is a long period Mira variable ranging from magnitude 7.8 to 15. These are highly evolved ageing stars that are on the asymptotic giant branch, their wide range in magnitude making them ideal targets for monitoring by amateur star observers.
T Ursae Minoris has been monitored closely since 1905. Up until 1979, its brightness had varied over a period of 310 to 315 days. However, from 1979 its period decreased suddenly to 274 days, and appeared to be decreasing by 2.75 days a cycle since. Variable star observers Janet Mattei and Grant Foster proposed that the star had just undergone a shell helium flash – a point at which "the helium shell around the dense core of the star reaches a critical mass and ignites", which "influences the star's pulsation via changes in surface luminosity and radius". By mid 2008, its period had decreased to 230 days (thus removing it by definition from the class of Mira variables), before changing to pulsation as a semiregular variable star, with a dominant period of 113.6 days. This has been the most dramatic change of any Mira variable. The lack of technetium in its spectrum indicates it is not as advanced in age as some other Mira-type stars, as it has either not yet dredged up this sort of material from its core or is of insufficient mass for this to occur.