The Necklace Nebula (PN G054.2-03.4) is located 15,000 light-years away in the constellation Sagitta. The Necklace Nebula is a the glowing remains of an ordinary, Sun-like star and consists of a bright ring, measuring 12 trillion miles across, dotted with dense, bright knots of gas that resemble diamonds in a necklace. The knots glow brightly due to absorption of ultraviolet light from the central stars. For bipolar nebulae, this material is funneled towards the poles of the aging star, creating the distinctive double-lobed structure.
About 10,000 years ago one a pair of aging stars ballooned to the point where it enveloped its companion star. This caused the larger star to spin so fast that much of its gaseous envelope expanded into space. Most of the gas escaped along the star's equator, producing a dense ring. The densest gas clumps in the ring show as bright knots, the beads of the necklace.
Photo : NASA HST, ESA, Hubble Heritage Team (STScI/AURA)