binary star form at the same time, and massive stars evolve much faster than the less massive stars, the more massive component Algol Aa1 is still in the main sequence, but the less massive Algol Aa2 is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can be seen. The gas flow between the primary and secondary stars in Algol has been imaged using Doppler Tomography. This system also exhibits x-ray and radio wave flares. The x-ray flares are thought to be caused by the magnetic fields of the A and B components interacting with the mass transfer. The radio-wave flares might be created by magnetic cycles similar to those of sunspots, but because the magnetic fields of these stars are up to ten times stronger than the field of the Sun, these radio flares are more powerful and more persistent. The secondary component was identified as the radio emitting source in Algol using Very-long-baseline interferometry by Lestrade and co-authors. Magnetic activity cycles in the chromospherically active secondary component induce changes in its radius of gyration that have been linked to recurrent orbital period variations on the order of ≈ via the Applegate mechanism. Mass transfer between the components is small in the Algol system but could be a significant source of period change in other Algol-type binaries. Algol is about 92.8 light-years from the Sun, but about 7.3 million years ago it passed within 9.8 light-years of the Solar System and its apparent magnitude was about −2.5, which is considerably brighter than the star Sirius is today. Because the total mass of the Algol system is about 5.8 solar masses, at the closest approach this might have given enough gravity to perturb the Oort cloud of the Solar System somewhat and hence increase the number of comets entering the inner Solar System. However, the actual increase in net cometary collisions is thought to have been quite small. Names Beta Persei is the star's Bayer designation. The name Algol derives from Arabic raʾs al-ghūl : head (raʾs) of the ogre (al-ghūl) (see "ghoul"). The English name Demon Star was taken from the Arabic