New observations made it possible to find the temperature boundary separating red dwarfs and "failed stars" - brown dwarfs.
Distinguishing the smallest from the largest sub-stellar objects is not easy. It is difficult to say how hot a star must be to maintain stable emission. The lower temperature limit was found only recently. This is discussed in an article that will be published in the next issue of the Astrophysical Journal Supplement Series, and for now is presented in the open online preprint library arXiv.org.
"Minimal stars" - red dwarfs - are tens of times smaller than the Sun and heat up several times weaker. Nevertheless, weak thermonuclear reactions that create radiation still take place in their depths. In contrast, brown dwarfs are substellar bodies, and thermonuclear processes on them are unstable. Therefore, over time, brown dwarfs fade and cool down forever. But the red ones are able to exist and glow dimly for many billions, or even tens of billions of years, giving even good chances for the development of life in their "daughter" planets.
Distinguishing between brown and red dwarfs from afar is not so easy. However, astronomers from the University of California at San Diego decided to take advantage of the difference in their ages. The fact is that at birth such a body moves in an almost flat circular orbit around the center of the Galaxy. Later, under the influence of gravity from neighboring stars, clouds and clusters, the orbit is gradually extended. If red dwarfs are on average much older than brown ones, then their orbits should be more elliptical.
Therefore, astronomers looked at the orbits of 172 brown and red dwarfs and found that slightly hotter objects stand out in much more elongated orbits. The boundary corresponds to a surface temperature of 1200-1400 ° C: apparently, this is the minimum temperature possible on a star - a red dwarf. Below it, full-fledged thermonuclear reactions are impossible. For comparison, the surface temperature of the Sun is about 5500 ° C. However, the sample is still too small, and in the future the authors plan to expand the observations in order to draw the temperature boundary even more accurately and reliably.