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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Metropolitan Ballroom,
We present stellar evolutionary calculations for the lowest mass stars, i.e. those stars with masses in the range between 0.08 Mødot and 0.25 Mødot. Our particular emphasis is on the post-main sequence evolution of these objects. We establish a hydrogen burning time scale of approximately ten trillion years for the minimum mass main sequence star. This time scale determines the duration over which the light of the galaxy will be dominated by a conventional stellar contribution. We find that for masses less than 0.25 Mødot, stars remain fully convective for a significant duration of their evolution. The maintenance of full convection precludes the development of large composition gradients, and allows the entire star to build up a large helium mass fraction. These effects combine to forbid the lowest mass stars from ever ascending the red giant branch. After becoming gradually brighter and bluer for trillions of years, the late M dwarfs of today will develop conductive-radiative cores and mild nuclear shell sources; these stars then end their lives as helium white dwarfs. We also discuss the ramifications of our work with respect to the general question of why stars become red giants. The fact that the lowest mass stars grow neither red nor giant as they evolve provides an important insight into this problem.
