The inflowing gas often forms an accretion disk around the accreting companion. Gas from the donor star is transferred to the companion and the mass transfer affects the evolution of the system. In a semidetached configuration one star fills its Roche lobe. The black line represents the inner critical Roche equipotential, made up of two Roche lobes that meet at the Lagrangian point L1. The filled regions represent the two stars. Schematic of a binary star system in a semidetached configuration with a mass ratio q=3, viewed at an inclination of 90 degrees (edge on). These are semidetached binary star systems consisting of a main sequence star and a cooler, more evolved secondary component that fills its Roche lobe. Algol variablesĪlgol is a prototype for a class of eclipsing variable stars known as Algol variables. Meanwhile, the companion has accreted the mass but stayed on the main sequence. The star that was originally more massive – now a class K subgiant – has filled its Roche lobe and transferred much of its mass onto the companion, all the while evolving and reaching the next stage faster. The paradox is explained by mass transfer, a common occurrence in close binary systems, in which the flow of material between the stars disrupts the normal process of evolution. In the case of Beta Persei, the less massive star – the secondary component – has already evolved into a subgiant, while the considerably more massive primary component is still on the main sequence. In a binary star system, components form at about the same time. A fundamental belief is that the more massive the star, the faster it evolves, leaves the main sequence and expands into a subgiant or giant. Observations of Algol have challenged the basic theories of stellar evolution. Five other faint stars are also listed as companions. The Washington Double Star Catalog lists two other companions, designated Beta Persei B and C, about one arcminute away. It has an effective temperature of 7,500 K and is 10 times more luminous than the Sun. of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090 (CC BY-SA 3.0)īeta Persei Aa1 and Aa2 are separated by only 0.062 astronomical units – that is only 6.2% of the distance between the Sun and Earth – while the third component, Beta Persei Ab, orbits the main pair with a period of 680.08 days at an average distance of 2.69 astronomical units.īeta Persei Ab has 1.76 solar masses and a radius 1.73 times that of the Sun. The form of Algol C, however, is an artifact. The elongated appearance of Algol B and the round appearance of Algol A are real. This is not an artistic representation, but rather is a true two-dimensional image with 1/2 milli-arcsecond resolution in the near-infrared H-band, reconstructed from data of the CHARA interferometer. The Algol system as it appeared on 12 August 2009. The secondary eclipses occur when the primary component eclipses the fainter star, but these are very shallow and, unlike primary eclipses, which can be seen by the unaided eye, they can only be detected photoelectrically. of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090 (CC BY-SA 3.0) Tidal distortions also result in “gravity darkening” effects, whereby in a significant number of images of Algol B, the edge or “limb” of the image is actually brighter than the center. ![]() (A milliarcsecond is about the size of a quarter atop the Eiffel Tower as seen from New York City.) Tidal distortions of Algol B giving it an elongated appearance are readily apparent. ![]() The images vary in quality, but the best have a resolution of 0.5 milliarcseconds, or approximately 200 times better than the Hubble Space Telescope. The phase of each image is indicated at the lower left. Because some phases are poorly covered, B jumps at some points along its path. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase. The eclipsing binary pair is separated by only 0.062 astronomical units (AU) from each other, so close in fact that Algol A is slowly consuming the less massive Algol B by continually stripping off Algol B’s outer layers. Algol (β Persei) is a triple-star system (Algol A, B, and C) in the constellation Perseus, in which the large and bright primary Algol A is regularly eclipsed by the dimmer Algol B every 2.87 days.
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