Tuesday, March 24, 2009

"The Eye of God" - The Helix Nebula.

This is an authentic photograph - or rather, a composite of photos - taken by NASA's Hubble Space Telescope at the Kitt Peak National Observatory in Arizona, USA. The image was featured on NASA's Website as an "Astronomy Picture of the Day" in May 2003 and thereafter posted on a number of Websites under the title "The Eye of God" (though I have found no evidence that NASA has ever referred to it as such). The awe-inspiring image has also been featured on magazine covers and in articles about space imagery.

What it actually depicts is the so-called "Helix Nebula," described by astronomers as "a trillion-mile-long tunnel of glowing gases." At its center is a dying star which has ejected masses of dust and gas to form tentacle-like filaments stretching toward an outer rim composed of the same material. Our own Sun may look like this in several billion years.

Will our Sun look like this one day?: The "Helix Nebula" is the closest example of a planetary nebula or 'planetary' formed/created at the end of the life of a Sun-like star (at the end of a star's evolution). Gases from the star in the surrounding space appear, from our vantage point, as if we are looking down a helix structure. The remnant central stellar core, known as a planetary nebula nucleus or PNN, 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 above picture is a composite of newly released images from the ACS instrument on the Hubble Space Telescope and wide-angle images from the Mosaic Camera on the WIYN 0.9-m Telescope at Kitt Peak National Observatory, part of the National Optical Astronomy Observatory, near Tucson, Arizona. A close-up of the inner edge of the Helix Nebula shows complex gas knots of unknown origin.

The "Helix Nebula": Also known as "The Helix" or "NGC 7293," it is a large planetary nebula (PN) located in the constellation of Aquarius. Discovered by Karl Ludwig Harding, probably before 1824, this object is one of the closest to the Earth of all the bright planetary nebulae. The estimated distance is about 215 parsecs or 700 light-years. It is similar in appearance to the "Ring Nebula," whose size, age, and physical characteristics are similar to the "Dumbbell Nebula," varying only in its relative proximity and the appearance from the equatorial viewing angle. The Helix has often been referred to as "the Eye of God" on the Internet, since about 2003.

Currently, the age is estimated to be 10,600+2,300−1,200 years, based solely upon a measured expansion rate of 31 km·s-1

Structure: The "Helix Nebula" is thought to be shaped like a prolate spheroid with strong density concentrations toward the filled disk along the equatorial plane, whose major axis is inclined about 21° to 37° from our vantage point. The size of the inner disk is 8×19 arcmin in diameter (0.52 pc); the outer torus is 12×22 arcmin in diameter (0.77 pc); and the outer-most ring is about 25 arcmin in diameter (1.76 pc). We see the outer-most ring as flattened on one side due to its colliding with the ambient interstellar medium.

Expansion of the whole planetary nebula structure is estimated to have occurred in the last 6,560 years, and 12,100 years for the inner disk. Spectroscopically, the outer ring's expansion rate is 40 km·s-1, and about 32 km·s-1 for the inner disk.

Knots: The "Helix Nebula" was the first planetary nebula discovered to contain knots. Its main ring contains knots of nebulosity, which have now been detected in many nearby planetaries. These knots are highly radially symmetric (from the PNN) and are described as "cometary", each containing bright cusps (local photoionization fronts) and tails. All extend away from the PNN in a radial direction. Excluding the tails, they are (very approximately) the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the (PNN). There are more than 20,000 cometary knots estimated to be in the "Helix Nebula."

The excitation temperature varies across the "Helix Nebula." The rotational-vibrational temperature ranges from 1800 K in a cometary knot located in the inner region of the nebula are about 2.5′ (arcmin) from the central PNN, calculated at about 900 K in the outer region at the distance of 5.6′.

General Information: While the image does indeed resemble a giant eye, there is no record of NASA actually referring to it as "The Eye of God". It is not clear who first called the image "The Eye of God", but the name appears to have "stuck" for obvious reasons. A number of non-NASA websites refer to the image by this name. In fact, other planetary nebulas have also been called "The Eye of God", including the "Hourglass Nebula, MyCn18."

The "Helix Nebula" is actually a vast tunnel of glowing gases a trillion kilometres long. Since Earth's position in relation to the nebula means we are looking more or less directly into the mouth of this tunnel, the Helix appears to us as an eye-like bubble rather than a cylinder.

The claim that this "event" only occurs once in 3,000 years is pure nonsense. The "Helix Nebula" is readily viewable by scientists all the time and can even be seen by amateur astronomers using telescopes or binoculars. Naturally, due to the composite nature of the image and the high-powered telescopes and photographic equipment used to create it, the nebula is unlikely to look as compelling or as "eye-like" from the ground as it does in the above image.

In spite of the inaccurate description, the picture is certainly a delight to behold, both for the scientists (Cosmologists, Astronomers, et al) as well as for the amateurs. Awesome, indeed! Gives anyone who views this picture - a feeling, that - "God loves you and watches over you every day." We bow before Mother Nature in all humility.....again and again and again......

Note: Image Credit - NASA, WIYN, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI), & T. A. Rector (NRAO). Information gathered from Wikipedia.
Photograph: "The Helix Nebula" - also known as "The Helix" or "NGC 7293," now commonly referred to as "The Eye of God."

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