Description and Basic Characteristics
The Helix Nebula (NGC 7293), also referred to as the 'Eye of God', is a planetary nebula in the Aquarius constellation that was discovered in the early 1800's by Karl Ludwig Harding. It lies at a distance of 700 light years, a fact that makes it one of the nearest planetary nebulae to Earth. The name 'Helix' denotes the strange helix-like shape of the nebula gases, a rather complicated structure that we are lucky to observe from our Earth's perspective.
The proximity of the Helix, and the fact that it covers an area of the sky of about a quarter of the full Moon, has allowed a more detailed analysis of its complicated structure. The nebula has a shape of prolate spheroid with the largest concentrations of material lying across an equatorial disk whose major axis is inclined about 21° to 37° from our vantage point. The simpler approach to describe its structure, suggests that it consists of an outer ring expanding at a rate of 40 km/s, and an inner disc with an expansion rate of about 32 km/s. Color enhanced images are used to depict the various details of the formation and make them visible to the human eye.
A rather important feature is the presence of small and highly symmetrical knots of gas and dust, described as comets with their tails extending away from the core. Each knot has the size of our solar system and given the fact that the nebula contains more than 20,000 of them, we can only imagine its enormous size. Although they have been extensively studied, their exact nature is still puzzling to the scientists. Some basic characteristics of the nebula are listed below:
Age: ~ 10,600 years, based solely upon its expansion rate of 31 km/s.
Distance: 700 light-years
Apparent magnitude: +13.5
Diameter of the inner disk: 8×19 arcmin (0.52 pc)
Diameter of the outer torus: 12×22 arcmin (0.77 pc)
Diameter of the outer-most ring: 25 arcmin (1.76 pc)
History and Evolution of the Nebula
At the center of the Helix lies a planetary nebula nucleus; a former star that was once very similar to our Sun. As the star evolved, it went through the red giant phase and eventually lost its outer layers via pulsations and strong stellar winds. These layers are actually the material that the nebula consists of. The outer layer was released about 12,000 years ago, while the inner layer almost 6,500 years ago.
As soon as the outer shells were released, the star passed to the white dwarf phase. No further fusion reactions take place now, although the core is still very hot and continues to emit radiation due to its temperature. Α sad truth is that Helix will no longer exist in about 10,000 years. The colorful assembly of gas and dust will eventually fade as the material drifts away, leaving a cool and inert white dwarf as the only remnant of a once magnificent spectacle. This fact will mark the last act of the star's death.
Recent studies performed with the aid of the Spitzer space observatory have shown that the stellar core is surrounded by a disk of dust and icy material circling at a distance of 35 to 150 astronomical units (AU). This was a rather unexpected discovery since any dust should have been blown away after the death of the star. A possible explanation involves comet and asteroid collisions taking place after their orbits having been disturbed by the star's expansion.
Another recent discovery indicates the emission of highly energetic X-rays from its area. The white dwarf has a temperature of 110,000° Kelvin, which is not sufficient to explain such a phenomenon. Astronomers suggest that material from the dust disk may be falling onto the star and triggering the X-ray outbursts.
Finally, X-ray observations have shown that the Helix star is not alone. It has a companion star and the pair are orbiting so closely together that they appear as a single image in optical telescopes. The existence of this companion may have affected the orientation of one disk while the orientation of the other one was determined by the dying star's spin axis or the axis of its magnetic field.
A Preview of What’s to Come
In about five billion years from now, our Sun will share a similar fate. A planetary nebula will be created as our star dies and evolves into a white dwarf. The expelled remnants of our dead star will eventually be used to create new stars.
- Clavin W. "Comets Clash at Heart of Helix Nebula", NASA
- Salisbury David F. "A new twist on an old nebula", Vanderbilt.edu
- Coffey J. "Eye of God", UniverseToday.Com
- "An Infrared Image of the Helix Nebula from the Spitzer Space Telescope" NASA, ESA, and C.R. O'Dell (Vanderbilt University)
- "A Close-Up of the Eye of God Nebula" and "Comet-like Filaments Along the Inner Rim of the Helix Nebula Gas Ring", NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO).