The NASA Hubble Space Telescope has captured the sharpest view yet of the most famous of all planetary nebulae: Messier 57 or NGC 6720. Commonly known as the Ring Nebula, it was discovered in 1779 by astronomer Antoine Darquier de Pellepoix. This photo reveals elongated dark clumps of material embedded in the gas at the edge of the nebula, with the dying central star floating in a blue haze of hot gas. The central white dwarf or planetary nebula nucleus of 15.75 apparent magnitude has a mass of approximately 1.2 M (in solar masses). The nebula is about a light-year in diameter, and is located some 2,000 light-years from Earth in the direction of the northern constellation of Lyra. Blue emission gas indicates the very hot helium, which is located primarily close to the hot central star. Green represents ionized hydrogen and oxygen, which is located farther from the star, while orange and red shows ionized nitrogen and sulphur, which has radiated from the coolest gas, located farthest from the star. The gradations of colour illustrate how the gas glows because it is bathed in ultraviolet radiation from the remnant central star, whose surface temperature is white-hot at 216,000 degrees Fahrenheit (120,000 degrees Celsius), about 200 times more luminous than our Sun.
This image reveals an intricate structure only hinted at in previous observations, and has allowed scientists to construct a model of the nebula in 3D showing the true shape of this striking object. Formed by a star throwing off its outer layers as it runs out of fuel, the Ring Nebula is an archetypal planetary nebula. Despite the name, the phenomenon has nothing to do with planets – they take their title from their fuzzy circular appearance. NASA’s Spitzer Space Telescope’s infrared array camera detected this material expelled from the withering star. The outer regions are especially prominent in this new image because Spitzer sees the infrared light from hydrogen molecules. The molecules emit the infrared light having absorbed ultraviolet radiation from the star.
Astronomers have combined ground-based data with new observations using the NASA/ESA Hubble Space Telescope to observe the nebula, hunting for clues about its structure, evolution, physical conditions and motion. It turns out that the nebula is shaped like a distorted doughnut. We are gazing almost directly down one of the poles of this structure, with a brightly coloured barrel of material stretching away from us. Although the centre of this doughnut may look empty, it is actually full of lower-density material that stretches both towards and away from us, creating a shape similar to a rugby ball slotted into the doughnut’s central gap. The Nebula has an expansion rate of roughly 43,000 miles per hour, and will continue to expand for another 10,000 years, a short phase in the lifetime of the star. The nebula will become fainter and fainter until it merges with the interstellar medium.
The dark, irregular knots of dense gas embedded along the inner rim of the ring, which look like spokes in a bicycle wheel are gaseous tentacles formed when expanding hot gas pushed into cool gas ejected previously by the doomed star. The knots are more resistant to erosion by the wave of ultraviolet light unleashed by the star. The Hubble images have allowed astronomers to match up the knots with the spikes of light around the bright, main ring, which are a shadow effect.
Credits: ESA, Hubble, NASA, Wikipedia.