Why Study Low Mass Stars?
The rather abstract looking figure to the right is the so-called RECONS Marble Diagram of stars known to be within 32 light-years (10 parsecs) from Earth. The colors approximate real stellar colors and the size of the marbles are proportional to the radius for that type of star. The red dwarfs, or so-called M dwarfs, are represented by the three smallest sizes, corresponding to the three shades of red. These are stars that have masses anywhere from about half the mass of our Sun down to only 7.5 percent the mass of our Sun. They by far outnumber the other types of stars. In fact, they make up about 75 percent of all marbles in the diagram. Their large numbers mean that for some reason the stellar formation process heavily favors the formation of small stars.
Their sheer numbers alone is a powerful reason to study them. We would be leaving out about 75 percent of the Galaxy if we did not study these low mass stars. Red dwarfs are also known to frequently harbor rocky planets much like Earth, where we believe that life can develop. If we did not study low mass stars we would not be looking for life in 75 percent of the places where one may find it. We simply cannot do that.
But if these stars are so important how come we do not already know a lot about them? It turns out that low mass stars are the most abundant but least understood type of stars. The problem stems from their intrinsic faintness and their very complex atmospheric chemistry. Whereas these stars dominate the Solar Neighborhood by numbers, not a single one is visible to the naked eye, including the closest star to us outside the Solar System, Proxima Centauri. Over the past twenty years the advent of powerful digital sky surveys such as 2MASS, SDSS, WISE, and Gaia is generating comprehensive catalogs of faint (and bright!) celestial objects, thus making low mass stars much easier to find and study.
Understanding the outer layers of a star, commonly called the stellar atmosphere, is essential to understanding the star itself and its affect on the surrounding planetary environment because it is from these outer layers that the starlight and stellar energy in general emanates. By regulating how quickly energy leaves the star the stellar atmosphere's opacity serves as a sort of control for how hot the stellar interior is and what type of light bathes any planets in the star system. The opacity is determined by the chemical substances in the atmosphere as well as physical attributes such as temperature and pressure. Whereas hotter more massive stars tend to have only atomic species in their atmospheres low mass stars have cool enough atmospheres that simple molecules and even microscopic dust grains can form. Indeed when we use spectroscopy to study the light from a low mass stars we find clear signatures of molecules such at titanium oxide (THE spectroscopic hallmark of red dwarfs), Iron hydride, vanadium oxide, water vapor, and many many more molecular species. This complex atmospheric chemistry is much more difficult to understand than one dominated by atomic species, and as a result we still have some trouble understanding how all this plays into the bigger question of the overall structure and evolution of these cool stars. Having said all this, I note that recently there have been considerable advancements in the atmospheric modeling of the hotter subgroup of low mass stars. The problem remains mostly for objects with atmospheric temperatures below = 2,600 Kelvins (2,300 C, 4,200 F), when microscopic solid grains begin to form.
