BY Draconis variables are

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

s of late spectral types, usually K or M, and typically belong to the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

due to rotation of the

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

coupled with starspots, and other chromospheric activity. Resultant brightness fluctuations are generally less than 0.5 magnitudes.

Light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

s of BY Draconis variables are quasiperiodic. The period is close to the star's mean rotational rate. The light curve is irregular over the duration of the period and it changes slightly in shape from one period to the next. For the star BY Draconis the shape of the light curve over a period remained similar for a month. Nearby K and M stars that are BY Draconis variables include Barnard's Star, Kapteyn's Star, 61 Cygni, Ross 248,

Lacaille 8760 Lacaille 8760 (AX Microscopii) is a red dwarf star in the constellation Microscopium. It is one of the List of nearest stars, nearest stars to the Sun at about 12.9 light-years' distance, and the brightest Stellar classification#Class M ...

, Lalande 21185, Epsilon Eridani and Luyten 726-8. Ross 248 is the first discovered BY Draconis variable, the variability having been identified by Gerald Edward Kron in 1950. The variability of BY Draconis itself was discovered in 1966 and studied in detail by Pavel Fedorovich Chugainov over the period 1973–1976. Some of these stars may exhibit

flares A flare, also sometimes called a fusée, fusee, or bengala, bengalo in several European countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illuminatio ...

, resulting in additional variations of the UV Ceti type. Likewise, the spectra of BY Draconis variables (particularly in their H and K lines) are similar to RS CVn stars, which are another class of variable stars that have active chromospheres.

References

Further reading