Astronomy Picture of the Day Index - Stars: Binary Stars

APOD: 1999 December 19 ?Accretion Disk Binary System
Explanation: Our Sun is unusual in that it is alone - most stars occur in multiple or binary systems. In a binary system, the higher mass star will evolve faster and will eventually become a compact object - either a white dwarf star, a neutron star, or black hole. When the lower mass star later evolves into an expansion phase, it may be so close to the compact star that its outer atmosphere actually falls onto the compact star. Such is the case diagrammed above. Here gas from a blue giant star is shown being stripped away into an accretion disk around its compact binary companion. Gas in the accretion disk swirls around, heats up, and eventually falls onto the compact star. Extreme conditions frequently occur on the surface of the compact star as gas falls in, many times causing detectable X-rays, gamma-rays, or even cataclysmic novae explosions. Studying the extreme conditions in these systems tells us about the inner properties of ordinary matter around us.

APOD: 2003 November 10 ?An Intermediate Polar Binary System
Explanation: How can two stars create such a strange and intricate structure? Most stars are members of multiple-star systems. Some stars are members of close binary systems where material from one star swirls around the other in an accretion disk. Only a handful of stars, however, are members of an intermediate polar, a system featuring a white dwarf star with a magnetic field that significantly pushes out the inner accretion disk, only allowing material to fall down its magnetic poles. Shown above is an artist's depiction of an intermediate polar system, also known as a DQ Hercules system. The foreground white dwarf is so close to the normal star that it strips away its outer atmosphere. As the white dwarf spins, the columns of infalling gas rotate with it. The name intermediate polar derives from observations of emitted light polarized at a level intermediate to non-disk binary systems known as polars. Intermediate polars are a type of cataclysmic variable star system.

APOD: 2005 August 30 - Albireo: A Bright and Beautiful Double
Explanation: Sometimes, even a small telescope can help unlock a hidden beauty of the heavens. Such is the case of the bright double star Albireo. Seen at even slight magnification, Albireo unfolds from a bright single point into a beautiful double star of strikingly different colors. At 380 light years distant, the two bright stars of Albireo are comparatively far from each other and take about 75,000 years to complete a single orbit. The brighter yellow star is itself a binary star system, but too close together to be resolved even with a telescope. Albireo, pictured above, is the fifth brightest star system toward the constellation of the Swan (Cygnus) and easily visible to the unaided eye.