Updated: Oct 7, 2020
The First Planetary Nebula where cometary knots were discovered
The Eye of God is a special kind of nebula. It is not a place where stars are born. Rather it has formed due to the death of a star. It was created due to the end of life of a star similar to the size of the Sun. When the star evolved and transformed into a red giant, it shed its outer layers and took the shape of something that looks strangely similar to that of an eye. Thus it gets its name The Eye Of God.
It is one of the most prominent examples of planetary nebula. It is not a place where stars are born. Rather it was formed due the death of a star. FYI the star was similar to the size of the sun.
The nebula appears as almost one-half the Moon’s diameter. The ring shape can only be resolved with large amateur telescopes, and the nebula’s radial streaks cannot be seen except through the largest ground-based telescopes..
The Helix Nebula (also known as NGC 7293 or Caldwell 63) is a planetary nebula (PN) located in the constellation 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 distance, measured by the Gaia mission, is 655±13 light-years. It is similar in appearance to the Cat's Eye Nebula and 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 Nebula has sometimes been referred to as the "Eye of God" in pop culture, as well as the "Eye of Sauron".
This is an infrared image from NASA's Spitzer Space Telescope.
This is a burning mass of gases mostly consisting of Hydrogen and Helium.
When the hydrogen fuel for the fusion reaction runs out, the star turns to helium for a fuel source, burning it into an even heavier mix of carbon, nitrogen and oxygen. Eventually, the helium will also be exhausted, and the star will die, puffing off its outer gaseous layers and leaving behind the tiny, hot, dense core, called a white dwarf. The white dwarf is about the size of Earth, but has a mass very close to that of the original star; in fact, a teaspoon of a white dwarf would weigh as much as a few elephants!
The intense ultraviolet radiation from the white dwarf heats up the expelled layers of gas, which shine brightly in the infrared. GALEX has picked out the ultraviolet light pouring out of this system, shown throughout the nebula in blue, while Spitzer has snagged the detailed infrared signature of the dust and gas in yellow. A portion of the extended field beyond the nebula, which was not observed by Spitzer, is from NASA's all-sky Wide-field Infrared Survey Explorer (WISE). The white dwarf star itself is a tiny white pinprick right at the center of the nebula.
The Eye Of God After Several Visible Filters
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, and about 32 km/s for the inner disk.
It was the first planetary nebula where knots were discovered.
GC 7293 was the first planetary nebula discovered to have cometary knots, which many planetary nebulae were later discovered to contain.
Each of these knots contains bright cusps and tails, and they all extend away from the central star in a radial direction. Most of the knots are believed to be roughly the size of the solar system.
The nebula is believed to contain about 20,000 cometary knots.
The Helix Nebula is also interesting because it appears really similar across a wide spectrum, from ultraviolet to infrared, and is easy to recognise at all these wavelengths even though there are noticeable differences in the nebula’s appearance.
Its main ring contains knots of nebulosity, which have now been detected in several nearby planetary nebulae, especially those with a molecular envelope like the Ring nebula and the Dumbbell Nebula. These knots are radially symmetric (from the CS) and are described as "cometary", each centered on a core of neutral molecular gas and containing bright local photoionization fronts or cusps towards the central star and tails away from it. All tails extend away from the Planetary Nebula Nucleus (PNN) in a radial direction. Excluding the tails, each knot is approximately the size of the Solar system, while each of the cusp knots are optically thick due to Lyc photons from the CS. There are about 40,000 cometary knots 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 CS, and is calculated at about 900 K in the outer region at the distance of 5.6'.
Zooming into the centre of the Eye Of God
While writing this blog, I just tried to imagine the eye of 'God'. I am not getting into the religious debate of his existence, but simply the general concept which is popularly referred to as 'God', the one behind the creation of all these wonders.
Take this for example: The Knots, which are just extended from the main centre, are individually the size of the entire solar system. Just imagine. Exactly how tiny are we? Exactly how much have we been able to know? And how much have we been able to learn?
The Hubble will be back in a few days. Stay Tuned!
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