BY Draconis variable

BY Draconis stars are red dwarfs with strong emission lines of the spectral DKE or sMe. They show low amplitude variability due to the rotation of starspots in its photosphere. The prototype is BY Draconis.


The variability has low amplitude of up to 0.5 mag and reaches usually not more than 0.1 mag. The period extending from fractions of a day up to 120 days. The cause of the changes in brightness are star spots (similar to sunspots ) that are periodically visible due to the rotation of the star and chromospheric activity, which manifests itself in the form of flares, torches ( hotter areas on the stellar surface ) and changes in the half-width of the emission lines. If a flare is detected in a BY Draconis star, the star also belongs to the class of UV Ceti stars. The star marks the BY Draconis stars and the flares of UV Ceti stars are two properties of magnetically active stars, which do not differ in their physical properties.

Star spots

The position and extent of the star spots can be derived by modeling their light curves. You can cover up to a fifth of the stellar surface, while sunspots reach only an extension of up to one percent of the sun's surface. Unlike the sun, many star spots hold on to the rotation poles of the star, while in the sun sunspots are observed in the range ± 30 degrees along the equator. The cause, however, for both types of stains, the breaching of the surface by bundled magnetic field lines and the resulting cooling due to the obstruction of energy transport from the stellar interior. There seems to be no difference in the physical parameters of stars that show a modulation of the brightness of star spots and stars with similar rotation velocities, temperatures and radii in the same cluster, whose brightness is constant. It is true that run through a Maunder Minimum, some of these inactive stars, but this can not apply to one-third of the potential BY Dra stars. There may in these stars before an approximately rotationally symmetric distribution of small star spots, whereby the brightness fluctuations would be below the detection limit.

Magnetic fields

The cause of all forms of stellar activity is in magnetic fields in the photosphere of late dwarfs. The high magnetic fields caused by a high rotational speed in combination with convection of electrically charged plasma. These movements cause electric currents through a dynamo effect to the formation of magnetic fields. The rotation is in binary star systems in the long term maintained by tidal forces due to a bound rotation. Therefore, single - BY Draconis stars are often young, while occurring in binary systems active stars can be several billion years old. The flux density of the magnetic fields achieved in accordance with high-resolution measurements of the spectrographic Zeeman effect up to 500 Gauss.


Compared to calm red dwarfs show magnetically active star rotation broadening of the spectral lines. The rotation is caused by broadening of the Doppler - effect due to the high rotational speed. In addition, emission lines occur on the calcium (H and K) and the Balmer series. The lines arise like the sun through a heated from below the chromosphere and corona. BY Draconis stars are detected as the sun in the field of X-ray radiation, as their coronas reach temperatures of several million degrees Kelvin.

Physical Properties

The radii of stars can be measured with great accuracy in eclipsing stars. Here, apparently, the radii of BY Draconis stars 3-12 % larger than expected theoretically. In addition, the temperatures in the photosphere seem to be around 3% below the expected values. These discrepancies are associated with the magnetic activity in the double stars. First, the cooler star spots on the surface lead to a reduced emission and the star responds with an expansion to restore the hydrodynamic equilibrium. Second, the bound rotation should lead to an enhancement of convection in the photosphere and thus also contribute to an expansion of the radius. The BY Draconis stars in close binary systems are therefore only partly representative of the physical properties of red dwarfs.