T Ursae Minoris (T UMi) is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

in the constellation

Ursa Minor Ursa Minor (Latin: 'Lesser Bear', contrasting with Ursa Major), also known as the Little Bear, is a constellation located in the far northern celestial hemisphere, northern sky. As with the Great Bear, the tail of the Little Bear may also be see ...

, located west-southwest of 3 Ursae Minoris toward the western border of the constellation with Draco.

Properties

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

ranging between spectral types M4e and M6e and with a surface temperature of , T Ursae Minoris is a long period semiregular variable star ranging from magnitude 7.8 to 15. These are highly evolved ageing stars that are on the

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

, 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 and it was classified as a Mira variable. 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, 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. A model of T Ursae Minoris shows that following a thermal pulse, its properties are expected to change over a period of only a few years. The period would reduce gradually for about 35 years and then decrease dramatically from about 300 days to about 100 days as the pulsations switch from the

fundamental mode A normal mode of a dynamical system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The free motion described by the normal modes takes place at fixed frequencies ...

to the first

overtone An overtone is any resonant frequency above the fundamental frequency of a sound. (An overtone may or may not be a harmonic) In other words, overtones are all pitches higher than the lowest pitch within an individual sound; the fundamental i ...

. At the same time, the radius and luminosity would decrease from around and respectively to about and . The radius and luminosity would then increase slowly over the next few thousand years to become larger than before the thermal pulse. Multiple thermal pulses are expected to occur in stars at this stage of their lives, about 100,000 years apart, with the star become gradually larger and cooler until it eventually sheds its outer layers to become a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

. T Ursae Minoris is expected to reach its smallest size in about 50 years time, then begin to slowly increase in size and luminosity again.

References

{{DEFAULTSORT:T Ursae Minoris Ursa Minor (constellation) M-type giants Mira variables Ursae Minoris, T 118556 Emission-line stars J13344108+7325530