The "Necklace Nebula" is obviously not a necklace and, somewhat less obviously, it's not a nebula either. It's actually a planetary nebula, a not particularly accurate term for the remnants of an exploded star. Still...that's one expensive-looking nebula.
The Necklace Nebula was discovered relatively recently in the constellation Sagitta, some 15,000 light-years away. This image was taken by the Hubble Telescope on July 2 of this year, with the hydrogen gas colored in blue, the oxygen in green, and the nitrogen in red.
A NASA astronomer explains the origins of the Necklace Nebula:
The nebula consists of a bright ring, measuring 12 trillion miles wide, dotted with dense, bright knots of gas that resemble diamonds in a necklace. A pair of stars orbiting close together produced the nebula, also called PN G054.2-03.4. About 10,000 years ago one of the aging stars ballooned to the point where it engulfed its companion star. The smaller star continued orbiting inside its larger companion, increasing the giant's rotation rate.
The bloated companion star spun so fast that a large part of its gaseous envelope expanded into space. Due to centrifugal force, most of the gas escaped along the star's equator, producing a ring. The embedded bright knots are dense gas clumps in the ring. The pair is so close, only a few million miles apart, they appear as one bright dot in the center. The stars are furiously whirling around each other, completing an orbit in a little more than a day.