The Helix Nebula, also known as NGC 7293, is a large planetary nebula formed at the end of a star’s evolution. Located in the constellation Aquarius, it is discovered by Karl Ludwig Harding, around 1824. About 700 light-years from the Earth, it spans about 3 light-years across, and is similar in appearance to the Cat’s Eye Nebula and the Ring Nebula in its characteristics. At magnitude 7.3 the Helix Nebula is the brightest planetary nebula in the sky, with a 13.4 magnitude for its central star.
Gases from the star in the surrounding space appear as if we are looking down a helix structure. The remnant central stellar core, known as a planetary nebula nucleus is destined to become a white dwarf star. The observed glow of the central star is so energetic that it causes the previously expelled gases to brightly fluoresce. The rotational-vibrational temperature ranges from 1800 K in a cometary knot located in the inner region of the nebula, while in the outer region they are about 900 K.
In this infrared image from NASA’s Spitzer Space Telescope of the Helix nebula, the infrared light from the outer gaseous layers is represented in blues and greens. The white dwarf is visible as a tiny white dot in the center of the picture. The red color in the middle of the eye denotes the final layers of gas blown out when the star died. The brighter red circle in the very center is the glow of a dusty disk circling the white dwarf. Any inner planets in the system would have burned up or been swallowed as their dying star expanded. The composite picture is a seamless blend of ultra-sharp NASA Hubble Space Telescope (HST) images 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.
The blue-green glow in the centre of the Helix comes from oxygen atoms shining under effects of the intense ultraviolet radiation of the 120,000 degree Celsius central star and the hot gas. A careful look at the central part of this object reveals not only the knots, but also many remote galaxies seen right through the thinly spread glowing gas.
What causes unusual knots of gas and dust in planetary nebulas? Pictured here is a fascinating image of the Helix Nebula by the Hubble Space Telescope showing tremendous detail of its mysterious gaseous knots. These cometary knots have masses similar to the Earth but have radii typically several times the orbit of Pluto. They all extend away from the nucleus in a radial and symmetric direction, and each containing bright cusps and tails. There are more than 20,000 cometary knots estimated to be in the Helix Nebula. One hypothesis for the fragmentation and evolution of the knots includes existing gas being driven out by a less dense but highly energetic stellar wind of the central evolving star. The Helix Nebula is the closest example of a planetary nebula created at the end of the life of a Sun-like star.
Sources: Hubblesite; NASA; Space; Wikipedia.