Image Credit: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO).
The Helix Nebula (also known as The Eye of God or NGC 7293) is a 10 thousand years old planetary nebula of about 5.75 light-years across, located roughly 700 light-years away from Earth in the constellation of Aquarius (the Water-Bearer). It is one of the closest to the Earth of all the bright planetary nebulae.
When a star with a mass up to eight times that of the Sun runs out of fuel at the end of its life, it blows off its outer shells and begins to lose mass. This allows the hot, inner core of the star (collapsing from a red giant to a white dwarf) to radiate strongly, causing the outward-moving cocoon of gas to glow brightly. They are called “planetary” nebulae because early observers thought they looked like planets; but they don’t have anything to do with planets at all.
The Helix Nebula is one of the closest to the Earth of all the bright planetary nebulae. From our vantage point, it appears as if we are looking down a helix structure, hence its name. However, the nebula is shaped like a prolate spheroid with the outer-most ring flattened on one side due to its colliding with the interstellar medium.
More than 20,000 tadpole-like objects are found within the nebula, dubbed “cometary knots” because their glowing heads and gossamer tails resemble comets. Each gaseous head is at least twice the size of our Solar System; each tail stretches 100 billion miles (160 billion kilometers), about 1,000 times the Earth’s distance to the Sun.
The most visible gaseous fragments are embedded along the inner rim of the nebula, trillions of miles from the central star, which is a small, super-hot white dwarf called WD 2226-210. The comet-like tails that are pointing back toward the central star, form a radial pattern around the star like the spokes on a wagon wheel.
These gaseous knots are probably the results of a collision between gases. The doomed star spews the hot gas from its surface, which collides with the cooler gas that it had ejected 10,000 years before. The crash fragments the smooth cloud surrounding the star into smaller, denser finger-like droplets, like dripping paint. Eventually, the gaseous knots will dissipate into the cold blackness of interstellar space.
Seen also in the Ring Nebula, the Dumbbell Nebula and the Eskimo Nebula, the Helix Nebula was the first planetary nebula discovered to contain cometary knots.
The composite picture is a seamless blend of ultra-sharp images taken with the Advanced Camera for Surveys on the Hubble Space Telescope, combined with the wide view of the Mosaic Camera on the National Science Foundation’s 0.9-meter telescope at Kitt Peak National Observatory, part of the National Optical Astronomy Observatory, near Tucson, Ariz. Astronomers at the Space Telescope Science Institute assembled these images into a mosaic. The mosaic was then blended with a wider photograph taken by the Mosaic Camera. The colors correspond to glowing oxygen (blue) and hydrogen and nitrogen (red).