Anne’s Picture of the Day: The Saturn Nebula

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April 17, 2013

The Saturn Nebula, a planetary nebula in Aquarius

Image Credits: Bruce Balick (Univ. of Washington), Jason Alexander (Univ. of Washington), Arsen Hajian (U.S. Naval Observatory), Yervant Terzian (Cornell University), Mario Perinotto (Univ. of Florence), Patrizio Patriarchi (Arcetri Observatory), NASA

The Saturn Nebula (also known as NGC 7009) is a planetary nebula of roughly 0.6 light-year across, located somewhere between 2,000 and 4,000 light-years away from Earth in the constellation of Aquarius, while it is approaching us at about 46.6 kilometers per second. It has gotten its name from its resemblance to the planet Saturn with its rings nearly edge-on to the observer (through a small telescope).

Planetary nebulae represent the final brief stage in the life of a medium-sized star like our Sun. While consuming the last of the fuel in its core, the dying star expels a large portion of its outer envelope. This material then becomes heated by the radiation from the stellar remnant and radiates, producing glowing clouds of gas that can show complex structures, as the ejection of mass from the star is uneven in both time and direction.

Over the next several thousand years, the Saturn Nebula will gradually disperse into space, and then the white dwarf will cool and fade away for billions of years. Our own Sun is expected to undergo a similar fate, but fortunately this will not occur until some 5 billion years from now.

The Saturn Nebula is very complex, including a halo, jet-like streams, multiple shells, ansae (“handles”), and small-scale filaments and knots. The hot, blue star at the center is the old nuclear-burning core of what was once a Sun-like star, while the surrounding nebula is the remnant of the star’s lost envelope that has been structured by the remaining star’s hot wind.

The central star, a very hot bluish white dwarf with a temperature of 55,000 K, has a luminosity of about 20 Suns. It seems rather faint to us because of its distance and because the vast majority of its radiation comes out as energetic ultraviolet light. This strong ultraviolet irradiation is believed to create the characteristic fluorescent green tint of the nebula via the radiation of doubly ionized oxygen.

This bright star lies at the center of a dark cavity bounded by a football-shaped rim of dense, blue and red gas. The cavity and its rim are trapped inside smoothly-distributed greenish material in the shape of a barrel and comprised of the star’s former outer layers.

At larger distances, and lying along the long axis of the nebula, a pair of red “ansae”, or “handles” appears. Each ansa is joined to the tips of the cavity by a long greenish jet of material. The handles are clouds of low-density gas.

A computer model indicates that the central star of the Saturn Nebula first expelled the green gas which now confines stellar winds flowing from the central star, creating the jets which forms the ansae that appear in red at the tips. Much remains unknown, including why the gas has not become turbulent.

This image is taken on April 28, 1996 with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. The nebula appears as a greenish-yellowish hue in smaller amateur telescopes.

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