Wednesday, September 27, 2017

The Strange Structures of the Saturn Nebula

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 colourful 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 favourite ringed planet seen edge-on.

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 colourful 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. This annotated version labels the features of this curious object.
Annotated image showing features in the Saturn Nebula
Credit: ESO/J. Walsh

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 energized 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].

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 colourful 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.
Credit: ESO

In order to better understand how planetary nebulae are moulded 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 Telescopeat 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.

This view shows how the MUSE instrument on ESO’s Very Large Telescope gives a three-dimensional depiction of the Saturn Nebula. For each part of this spectacular nebula, the light has been split up into its component colours — revealing in detail the chemical and physical properties of each pixel.  During the subsequent analysis the astronomer can move through the data and study different views of the object at different wavelengths, just like tuning a television to different channels at different frequencies.
Three-dimensional MUSE view of the Saturn Nebula
Credit: ESO/J. Walsh


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”).

This visualisation shows the three-dimensional MUSE data on the bright planetary nebula NGC 7009, also known as the Saturn Nebula. Each slice is presented in sequence shows a view of the object at a different wavelength. At certain wavelengths, corresponding to emission lines in the spectrum from hydrogen, oxygen and helium, the object glows brightly and strange structures are revealed. At the end these emission line views are combined to make a colour picture of the object.

Credit: ESO/J. Walsh/L.Cal├žada

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.

This zoom sequence starts from a very broad view of the sky and heads towards the constellation of Aquarius (The Water Bearer). After a while we see a bright tiny blue disc, which eventually turns into the spectacular planetary nebula NGC 7009, known as the Saturn Nebula because of its distinctive shape. The final image is from new data taken with the MUSE instrument on ESO's Very Large Telescope in Chile.

Credit: ESO/Digitized Sky Survey 2/N. Risinger (skysurvey.org). Music: Astral Electronic

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).
Notes

[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.



Contacts and sources:
Richard Hook
ESO

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