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The Strange Structures of the Saturn Nebula
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Name: NGC 7009, Saturn Nebula, Caldwell 55
Description: Planetary Nebula
Position (J2000): RA 21h 4m 10.81s Dec -11° 21' 48.60"
Constellation: Aquarius
Distance: 2000-4000 light-years
Visual magnitude: 8.0
Angular dimensions: 41×35 arcmin
Field of view: 1.21 x 1.23 arcminutes
Orientation: North is -0.0° left of vertical
Image Credit: ESO/J. Walsh
Release date: September 27, 2017

1997 Image:    N9738
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The spectacular planetary nebula NGC 7009, or the Saturn Nebula, emerges from the darkness like a series of oddly-shaped bubbles, lit up in glorious pinks and blues. This colorful image was captured by the powerful MUSE instrument on ESO's Very Large Telescope (VLT), as part of a study which mapped the dust inside a planetary nebula for the first time. The map - which reveals a wealth of intricate structures in the dust, including shells, a halo and a curious wave-like feature - will help astronomers understand how planetary nebulae develop their strange shapes and symmetries.

The Saturn Nebula is located approximately 5000 light years away in the constellation of Aquarius (The Water Bearer). Its name derives from its odd shape, which resembles everyone's favorite ringed planet seen edge-on.

But in fact, planetary nebulae have nothing to do with planets. The Saturn Nebula was originally a low-mass star, which expanded into a red giant at the end of its life and began to shed its outer layers. This material was blown out by strong stellar winds and energised by ultraviolet radiation from the hot stellar core left behind, creating a circumstellar nebula of dust and brightly-colored hot gas. At the heart of the Saturn Nebula lies the doomed star, visible in this image, which is in the process of becoming a white dwarf [1].

In order to better understand how planetary nebulae are molded into such odd shapes, an international team of astronomers led by Jeremy Walsh from ESO used the Multi Unit Spectroscopic Explorer (MUSE) to peer inside the dusty veils of the Saturn Nebula. MUSE is an instrument installed on one of the four Unit Telescopes of the Very Large Telescope at ESO's Paranal Observatory in Chile. It is so powerful because it doesn't just create an image, but also gathers information about the spectrum - or range of colors - of the light from the object at each point in the image.

The team used MUSE to produce the first detailed optical maps of the gas and dust distributed throughout a planetary nebula [2]. The resulting image of the Saturn Nebula reveals many intricate structures, including an elliptical inner shell, an outer shell, and a halo. It also shows two previously imaged streams extending from either end of the nebula's long axis, ending in bright ansae (Latin for "handles").

Intriguingly, the team also found a wave-like feature in the dust, which is not yet fully understood. Dust is distributed throughout the nebula, but there is a significant drop in the amount of dust at the rim of the inner shell, where it seems that it is being destroyed. There are several potential mechanisms for this destruction. The inner shell is essentially an expanding shock wave, so it may be smashing into the dust grains and obliterating them, or producing an extra heating effect that evaporates the dust.

Mapping the gas and dust structures within planetary nebulae will aid in understanding their role in the lives and deaths of low mass stars, and it will also help astronomers understand how planetary nebulae acquire their strange and complex shapes.

But MUSE's capabilities extend far beyond planetary nebulae. This sensitive instrument can also study the formation of stars and galaxies in the early Universe, as well as map the dark matter distribution in galaxy clusters in the nearby Universe. MUSE has also created the first 3D map of the Pillars of Creation in the Eagle Nebula (eso1518) and imaged a spectacular cosmic crash in a nearby galaxy (eso1437).


[1] Planetary nebulae are generally short-lived; the Saturn Nebula will last only a few tens of thousands of years before expanding and cooling to such an extent that it becomes invisible to us. The central star will then fade as it becomes a hot white dwarf.

[2] The NASA/ESA Hubble Space Telescope has previously provided a spectacular image of the Saturn Nebula - but, unlike MUSE, it cannot reveal the spectrum at each point over the whole nebula.

From Wikipedia:

The Saturn Nebula or NGC 7009 is a planetary nebula in the constellation Aquarius. It appears as a greenish-yellowish hue in a small amateur telescope. It was discovered by William Herschel on September 7, 1782, using a telescope of his own design in the garden at his home in Datchet, England, and was one of his earliest discoveries in his sky survey. The nebula was originally a low-mass star that ejected its layers into space, forming the nebula. The central star is now a bright white dwarf star of apparent magnitude 11.5. The Saturn Nebula gets its name from its superficial resemblance to the planet Saturn with its rings nearly edge-on to the observer. It was so named by Lord Rosse in the 1840s, when telescopes had improved to the point that its Saturn-like shape could be discerned. William Henry Smyth said that the Saturn Nebula is one of Struve's nine "Rare Celestial Objects."

The Saturn Nebula is a complex planetary nebula and contains many morphological and kinematic sub-systems in three dimensions. It includes a halo, jet-like streams, multiple shells, ansae ("handles"), and small-scale filaments and knots. The ansae are expanding non-radially from the central star. Although the ansae are most prominent in the Saturn Nebula, they are also visible in other planetary nebulae, including NGC 3242, NGC 6543 and NGC 2371-2.

The distance of the Saturn Nebula is not known precisely. Sabbadin et al. 2004 estimates the distance to be 5,200 light-years (1.6 kpc). In 1963 O'Dell estimated it to be 3,900 light-years (1.2 kpc), which gives an approximate diameter of 0.5 light years for the object as a whole.

The central star, a very hot bluish dwarf with a temperature of 55,000 K, from which the nebula is believed to originate, has an absolute magnitude of +1.5, which equates to a luminosity of about 20 solar luminosities and a visual magnitude of 11.5. This strong ultraviolet irradiation from the central star creates the characteristic fluorescent green tint of the nebula via the radiation of doubly ionized oxygen. The object overall has a visual magnitude of 8 and a radial velocity of 28 miles per second towards the Earth.

The nebula is 1 degree west of the star Nu Aquarii. The central portion measures 25? × 17?, while the outer shell extends to 41? × 35?. The object is on many "best of" observing lists.