System
From the point of view of the Earth, Algol A and Algol B form an eclipsing binary because their orbital plane contains the line of sight to the Earth. To be more precise, Algol is a triple-star system: the eclipsing binary pair is separated by only 0.062 astronomical units (AU) from each other, whereas the third star in the system (Algol C) is at an average distance of 2.69 AU from the pair, and the mutual orbital period of the trio is 681 Earth days. The total mass of the system is about 5.8 solar masses, and the mass ratios of A, B and C are about 4.5 to 1 to 2.
| Components | Semimajor axis | Ellipticity | Period | Inclination |
|---|---|---|---|---|
| A—B | 0.00218″ | 0.00 | 2.87 days | 97.69° |
| (AB)—C | 0.09461″ | 0.225 | 680.05 days | 83.98° |
Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B 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.
This system also exhibits variable activities in the forms of x-ray and radio wave flares. The former is thought to be caused by the magnetic fields of the A and B components interacting with the mass transfer. The radio wave emissions might be created by magnetic cycles similar to those of sunspots, but since 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.
Algol is located 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.
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