This photo taken on July 2, 2011 by the Hubble Space Telescope shows a giant glowing cosmic necklace. The object is a recently discovered planetary nebula also called PN G054.2-03.4. It is located 15,000 light-years away in the constellation of Sagittarius.
The Necklace Nebula consists of a bright ring, measuring nearly 20 trillion kilometers across, with dense, bright knots of gas spread out in a way that resembles the diamonds in a necklace. The knots are not stars, they only glow due to the absorption of ultraviolet light from the two stars located in the center of the nebula.
This is a composite image where the glow of hydrogen is seen as blue, oxygen appears as green, and nitrogen is red.
The nebula was created by a pair of stars orbiting very close together. About 10,000 years ago one of the aging stars swelled to the point that it enveloped its companion star. This caused the larger star to gain momentum and spin so fast that much of its gaseous envelope was ejected into space along the star’s equator due to centrifugal force. This produced a dense ring-shaped nebula. The glowing knots we can clearly see on the photograph are the densest gas clumps of the ring.
Due to this violent event both stars survived albeit with reduced sizes. They are closely orbiting around each other, completing an orbit in a little more than a day. For comparison, Mercury, the closest planet to the Sun, takes 88 days to orbit the Sun.