T Ursae Minoris is (or was) a Mira variable, an asymptotic giant branch star pulsating with a period of about 300 days, and an amplitude of about five magnitudes. In the 1970s, the behavior of T UMi began to change, including a rapidly declining pulsation period, a decreasing amplitude, a decreasing mean luminosity, and the appearance of a shorter second period in addition to the primary period. This behavior has many of the hallmarks of the changes expected when an AGB star undergoes a thermal pulse, a brief period of thermonuclear helium burning in the core of the star. As the star readjusts to this energy release in the core, its physical characteristics change, leading to these observable changes over timescales of decades. T UMi no longer meets the technical definition of a Mira variable, and is more accurately classified as a semiregular variable now.
Prepared By: Matthew Templeton