Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
The Necklace Nebula (PN G054.2-03.4) is a planetary nebula of almost 9 light-years long, located some 15,000 light-years away in the northern constellation of Sagitta, the Arrow (not be confused with the constellation Sagittarius, the Archer). It was only discovered in 2005 by a ground-based planetary nebula study of the North Galactic Plane.
Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose in the 18th century because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through small optical telescopes. The name has stuck even though modern telescopes make it obvious that these objects are not planets at all, but the glowing gassy outer layers thrown off by a hot dying Sun-like star which is collapsing from a red giant to a white dwarf.
The Necklace Nebula consists of a bright ring of about 2 light-years wide, dotted with dense, bright knots of gas that resemble diamonds in a necklace. It is produced by a pair of tightly orbiting binary stars with a period of only 1.2 days and a separation of about 5 times the radius of the Sun. The pair is so close that they appear as one bright dot in the center.
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 clumps of glowing hydrogen and oxygen gas in the ring. Each knot also dons a small tail pointing away from the central star.
The clumpy appearance of the ring may have been caused by density fluctuations in the shared material of the binary stars prior to the explosion, or possibly by magnetic field lines present in the giant star as it began to expand and shed off its outer layers. A fast, collimated outflow of nitrogen gas from the binary system has formed faint lobes and polar caps extending in the direction perpendicular to the ring.
This composite image is taken on July 2, 2011 with the Wide Field Planetary Camera 3 onboard the Hubble Space Telescope using three different color filters. It is showing hydrogen (blue), oxygen (green), and nitrogen (red).