red dwarf

History

The first red dwarfs were observed as early as the 19th century. In the early 20th century, astronomers identified them as a separate class of low-mass stars. For a long time, it was believed that they could not have planets with life, but modern discoveries have shown otherwise.

How it works

Inside a red dwarf, hydrogen turns into helium, releasing energy, but the reactions proceed slowly due to low core temperature and pressure. Unlike the Sun, the star's material is fully mixed: hydrogen from outer layers constantly flows into the core, and helium does not accumulate. Hence, the star shines stably for trillions of years.

History

The first red dwarfs were observed as early as the 19th century. In the early 20th century, astronomers identified them as a separate class of low-mass stars. For a long time, it was believed that they could not have planets with life, but modern discoveries have shown otherwise.

How it works

Inside a red dwarf, hydrogen turns into helium, releasing energy, but the reactions proceed slowly due to low core temperature and pressure. Unlike the Sun, the star's material is fully mixed: hydrogen from outer layers constantly flows into the core, and helium does not accumulate. Hence, the star shines stably for trillions of years.

History

Class M was identified in the Harvard spectral classification in the early 20th century. Henri Poincaré laid the foundations of the theory of convective stars, and later Subrahmanyan Chandrasekhar described the structure of low-mass stars in detail. In the 1990s, surveys showed that red dwarfs are the most numerous type of stars in the Galaxy. Interest in them grew with the search for exoplanets: the first Earth-like planets in the habitable zone were found around red dwarfs.

How it works

Thermonuclear fusion in red dwarfs proceeds via the proton-proton chain: the core temperature is insufficient for the CNO cycle. The key evolutionary parameter is mass. At masses less than 0.35 M☉, the star remains fully convective throughout its life; at higher masses, a radiative core eventually forms. Energy generation is described by the equation of state for degenerate or ideal gas depending on mass. Due to low luminosity, the main-sequence stage lasts trillions of years. After exhausting hydrogen, the star (theoretically) becomes a blue dwarf and then a white dwarf. Full convection, through the dynamo mechanism, generates strong magnetic fields that cause powerful flares and coronal mass ejections. The emission spectrum follows Planck's law with a peak in the near-infrared; class M is characterized by molecular bands of titanium and vanadium oxides (TiO, VO).

Open problems & fun facts

The red dwarf paradox: the observed number of low-mass stars is lower than theoretical star formation models predict.; Atmospheric erosion of exoplanets around active red dwarfs: can life survive under intense ultraviolet radiation and flares?; The exact mechanism of magnetic field generation in fully convective stars.

💡 If humanity survived until the time when the Sun becomes a red giant and fades, the nearest red dwarfs would still shine almost as brightly as today — their age at that point would be only 0.1% of their total lifetime.