Red Dwarf

Flare Star
Image from Steve Bowers
Young red dwarfs often emit bright flares of ultraviolet light, which can radically alter the environment of surrounding planets

A small cool faint main sequence star, spectroscopic class M. Red Dwarfs put out most of their luminosity in the form of infra-red light; for this reason they appear dimmer than G-Class stars when viewed from the habitable zone. However many red dwarfs emit frequent, bright stellar flares, which are accompanied by bright UV emissions. Due to these flares some water-bearing worlds around these stars may develop oxygen-rich atmospheres via the process of abiotic photodissociation. Any biosphere which exists on such worlds must be capable of tolerating sudden bursts of UV light and high oxygen levels.

The habitable zone of most red dwarfs is small compared to other stars, and tidal effects mean that any planet that orbits within this zone will be tidally locked, or rotate in a simple resonance with its own orbital period. Earth-like tidally locked planets of this kind form the Vesperian subclass in the NOLWOCS classification system.

Because they are so cool red dwarfs burn slowly and hence live for a very long time. As they live longer and are formed more frequently than larger bodies they are the most common stars; there are about two thousand per million cubic light-years of galactic space. They are located at the bottom right of the Hertzsprung-Russell diagram.

Red dwarf 1
Image from Steve Bowers
From a planet in the habitable zone, a red dwarf star will tend to look large in the sky; this is because of its low intrinsic brightness

As they get older, red dwarfs increase in luminosity but never become red giants or become white dwarfs. If left unmodified, many red dwarfs will eventually increase in brightness until they become as bright as a sun-like star; for smaller dwarfs, this era will not occur for a trillion years or so. Eventually the oldest dwarfs will cease to shine and become large, helium enriched black dwarfs, slowly radiating their heat and contracting.

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Development Notes
Text by M. Alan Kazlev
Additional material by Steve Bowers
Initially published on 22 December 2001.