Saturn Nebula Has Strange Structures Other Than Its Unusual Shape

Even though planetary nebulae have nothing at all to do with planets, NGC 7009 or Saturn Nebula is so-named for the resemblance it bears to the ringed planet we all know from our solar system.

Located at a distance of between 1,500 and 5,000 light-years from Earth in the Aquarius constellation, this oddly-shaped nebula has been looked at once again by astronomers who are trying to understand why planetary nebulae have unusual shapes.

Using the Multi Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope (VLT) in Chile, an international team of astronomers led by Jeremy Walsh of the European Southern Observatory (ESO) looked beyond the veils of dust that usually obscure the view of what lies inside Saturn Nebula. The result was the production of “the first detailed optical maps of the gas and dust distributed throughout a planetary nebula,” according to an ESO statement Wednesday.

This image of the Saturn Nebula show 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”),” the statement said, adding that the heart of the nebula is made up of the star that was once a red giant and is now “in the process of becoming a white dwarf.”

Another structural feature that astronomers saw inside the nebula was “a wave-like feature in the dust” — a phenomenon they don’t yet properly understand. At the rim of the inner shell, there is a sharp drop in the amount of dust which is otherwise distributed throughout the nebula. The astronomers think the dust is being destroyed at the rim of the inner shell, which is, in its essence, an expanding shock wave. As the wave moves outward, it could be smashing dust particles against each other — an act that would create heat and perhaps evaporate the dust.

Planetary nebulae are formed when low-mass stars (by some estimates, still about 20 percent heavier than the sun) expand into red giants at the end of their regular lives — having lost too much mass to sustain themselves through fusion reactions in their cores — and the outer layers of the red giants start to shed. The relatively cool material on the outer edges is blown out by ultraviolet radiation that emanates from the hot stellar cores. That creates a circumstellar nebula like the Saturn Nebula, full of hot gas and dust.

The previous imaging of the streams that emerge outward from the axis of the nebula was done over 21 years ago by the Hubble Space Telescope. However, Hubble lacks the abilities of a spectroscope while MUSE “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” that helps reveal the spectrum at every point in the nebula.

The VLT is located at ESO’s Paranal Observatory in the Atacama Desert in northern Chile.