Stars ranging in mass from 0.075 to 0.5 times the mass of the sun are categorized as red dwarf stars. Unlike the brown dwarf stars, these stars are massive enough to reliably sustain fusion of light element at its core. The low mass of these stars result in cores that are are much smaller and less active than larger stars like the sun. The slow rate of fusion in the cores of red dwarf stars allow large convective currents to form within the stars, cycling fused material out of the core and replenishing the core with fusible material. The fully convective nature of red dwarf stars gives them the longest lifespans of any other star type, allowing them to actively fuse material for more than 10 trillion years. To put this time in perspective, the universe is predicted to have only existed for 13.82 billion years, meaning that some of the oldest red dwarf stars are currently only around 0.1% through their supply of fusible material.
During their time of active fusion, red dwarf stars maintain a surface temperature around 4000k, around four times hotter than the surface of a brown dwarf star. As a result, red dwarf stars may be able to host habitable planets for trillions of years. Once all of the lighter elements in a red dwarf star are exhausted, the star will resemble a hot and incredibly high mass brown dwarf star. This final stage of life is similar to the leftover husks of larger stars, known as white dwarf stars, although the density of exhausted red dwarf stars would be much lower than that of white dwarf stars.