NGC 6881, a planetary nebula in Cygnus

 
NGC 6881, a planetary nebula in Cygnus

Image Credit: ESA/Hubble & NASA

NGC 6881 is a planetary nebula of about one light-year in length, located roughly some 8,000 light-years away from Earth in the constellation of Cygnus, the Swan. It is approaching us at approximately 15 kilometers per second.

Despite their name, planetary nebulae have nothing to do with planets. The name of planetary nebulae arose in the 18th century because of the visual similarity between some round planetary nebulae and the planets Uranus and Neptune when viewed through small optical telescopes.

Planetary nebulae represent the final 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 (collapsing from a red giant to a white dwarf) 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.

NGC 6881’s dying star has a temperature between 77,000 and 100,000 Kelvin, a mass of about 60% of the mass of our Sun and a luminosity of around 7000 Suns. It created this butterfly-shaped nebula that is formed of an inner nebula of about one fifth of a light-year across which is expanding at about 17 kilometers per second, and symmetrical “wings” that spread out about one light-year from one tip to the other.

It is an example of a quadrupolar planetary nebula, made from two pairs of bipolar lobes pointing in different directions, and consisting of four pairs of flat rings. There are also three rings in the center. The symmetry could be due to a binary star at the nebula’s center instead of one single star.

Planetary nebulae usually live for a few tens of thousands of years, a short phase in the lifetime of a star. So, over the next several thousand years, NGC 6881 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 image was taken with the Hubble Space Telescope through three filters which isolate the specific wavelength of light emitted by nitrogen (shown in red), hydrogen (shown in green) and oxygen (shown in blue).

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