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: 1997 February 19 - Mizar Binary Star
Explanation: Mizar (sounds like "My Czar") is a binary star. In fact, most stars are binary stars. In a binary star system, each star of the pair follows an elliptical orbital path. Mutual gravity causes the stellar companions to glide around their orbits as if tied to the ends of an elastic string passing through a balance point between them. The balance point is the system's "center of mass". Also known as Zeta Ursa Majoris, Mizar is the middle star in the handle of the Big Dipper and at a distance of 88 light years, was the first binary star system to be imaged telescopically. Spectroscopic observations of the Mizar system show periodic doppler shifts, revealing that both stars, Mizar A and Mizar B, are themselves binary stars! But, the companions are too close to be directly observed as separate stars, even by the largest telescopes. In developing a new optical interferometer capable of extremely high resolution while peering through the Earth's blurry atmosphere, U.S. Naval Observatory and Naval Research Lab astronomers have been able to detect the companion star to Mizar A. This composite image of their observations shows the daily and monthly relative orbital motion in the binary system. Binary stars are a boon to astronomers because these stars can be weighed -- their orbits providing a direct measurement of star masses.

APOD: 1999 November 20 - Small Star
Explanation: A dim double star system cataloged as Gliese 623 lies 25 light-years from Earth, in the constellation of Hercules. The individual stars of this binary system were distinguished for the first time when the Hubble Space Telescope's Faint Object Camera recorded this image in June 1994. They are separated by 200 million miles - about twice the Earth/Sun distance. On the right, the fainter Gliese 623b is 60,000 times less luminous than the Sun and approximately 10 times less massive. The fuzzy rings around its brighter companion, Gliese 623a, are image artifacts. The lowest mass stars are classified as red dwarf stars, but even red dwarfs are massive enough to trigger hydrogen fusion in their cores to sustain their feeble starlight. Slightly less massive objects, known as brown dwarfs, can shine only briefly as their central temperatures are too low to utilize hydrogen as nuclear fuel. The present estimates of the mass of Gliese 623b are right at this red dwarf/brown dwarf border but future observations should help clarify the nature of one of our Galaxy's small stars. Dim and difficult to detect, an abundance of objects like Gl623b has been proposed as a possible solution to the mystery of "Dark Matter" in the Universe.