NGC 6886, a planetary nebula in Sagitta

 
NGC 6886, a planetary nebula in Sagitta

Image Credit: ESA/Hubble & NASA

NGC 6886 is a planetary nebula with a size of only about 0.3 X 0.45 light-year, located some 7,000 to 11,000 light-years away from Earth in the northern constellation of Sagitta, while it is slowly moving toward us at approximately 36.4 kilometers per second. The nebula is expanding between 20 and 25 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.

These celestial objects signal the final death throes of mid-sized stars (up to about eight times the mass of the Sun); when such a star exhausts its supply of hydrogen fuel, the outer layers begin to expand and cool, which creates an envelope of gas and dust that surrouds the dying star. This allows the hot, inner core of the star (collapsing from a red giant to a white dwarf) to radiate strongly, causing this outward-moving cocoon of gas to glow brightly.

Over the next several thousand years, NGC 6886 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.

By studying the elements that are present in the nebula today, astronomers can determine the original chemical make-up of the star. Studies suggest that the star belonging to NGC 6886 may have originally been similar to the Sun, containing similar quantities of carbon, nitrogen and neon, although heavier elements, such as sulphur, were less plentiful.

Although this image is revealing complex detail in the “wings” that clearly surround the central structure, NGC 6886 remains something of a mystery thanks to a very uncertain distance and a hidden central star. The central star, with a substantial temperature of 168,000 Kelvin, is not seen thanks to the glare of the central structure. Analysis of the nebular spectrum indicates that the central star is quite faint, with most of its radiation in the invisible ultraviolet.

The star’s luminosity and condition depend strongly on the uncertain distance. At a distance of 10,000 light-years, a 0.58 solar mass star with a luminosity of 1250 suns (and what was once the core of a much more massive giant) has reached its maximum temperature and will shortly begin to cool and dim. (At the shorter distance, the cooling has already commenced.) Lack of chemical enrichment argues against a much higher mass.

This image was created by combining images taken with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. Filters that let through emission from ionized nitrogen gas (red), ionized oxygen (blue) and a broadband yellow filter (green, and also contributing to the blue) were used.

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