Introductory Astronomy: Red Dwarfs

Red Dwarf stars are main sequence stars with masses between 0.08 and 0.4 solar masses. They are totally convective stars. This means that the energy released by fusion in these stars is carried to the surface of the star by the circular motions of the star's material. This is also how water boils. In a pan of hot water (or a star), the water which is near the burner is hotter than the water at the top of the pan. This hotter water rises to the top of the pan and releases its heat. At the same time, the cooler water sinks to the bottom of the pan and begins to heat up. In this way, heat is carried from the bottom of the pan to the top. The same thing happens in a star. There, the stellar material which is closest to the core is hotter than the material at the stellar surface. As the hotter material rises and the cooler material sinks, energy is transported from the core to the surface of the star.

These circular motions do more than transport heat to the surface of the star, however. They also transport the stellar material itself, which means that the material throughout the whole star gets all mixed around. So, due to convection, the helium which is created by fusion in the core becomes distributed throughout the star. Because of this, there is no build-up of helium in the core. Without a helium core, the star is not able to reach the helium fusion phase (also known as the Red Giant Phase). Instead, as the hydrogen is used up by fusion, the star loses the balance of hydrostatic equilibrium and simply contracts and heats up due to gravity. At this point, it is no longer a MS star (since it is no longer fusing hydrogen), but becomes a White Dwarf.

The main sequence lifetime of a red dwarf is about 100 billion years. Since this is older than the age of the universe (on the order of 10 billion years), these stars have yet to reach the white dwarf stage.

An illustration of a Red Dwarf.