Sharpless 188, a planetary nebula in Cassiopeia

Sharpless 188, a planetary nebula in Cassiopeia

Image Credit: T.A. Rector/University of Alaska Anchorage, H. Schweiker/WIYN and NOAO/AURA/NSF

Sharpless 188 (SH2-188, Simeis 22 or the Dolphin Nebula) is an unusual planetary nebula in the galactic disk, approximately 850 light-years away from Earth in the northern constellation of Cassiopeia.

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. The name has stuck even though modern telescopes make it obvious that these objects are not planets at all.

When a star with a mass up to eight times that of the Sun runs out of fuel at the end of its life, it blows off its outer shells and begins to lose mass. 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.

Planetary nebulae last for only about 10,000 years, a fleeting episode in the 10-billion-year lifespan of Sun-like stars. So, over the next several thousand years, SH2-188 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 over about 5 billion years from now.

The expanding gas from SH2-188 has a nearly circular shape, but is much brighter to the southeast (lower left). This is because the rapidly moving central star – at about125 km/s – is s creating a bow shock in that direction in the surrounding interstellar medium. Faint wisps of gas can also be seen in the opposite direction, which will eventually dissipate away. SH2-188 is considered to be one of the best examples of interaction between a planetary nebula with the interstellar medium.

This image shows off the impressive imaging capabilities of the CCD detectors in the Mosaic 1.1 camera on the Mayall 4-meter telescope at Kitt Peak National Observatory. The image was generated with deep observations in the Hydrogen alpha filter (red) and the Oxygen [OIII] filter (cyan). In this image, North is up and East is to the left.

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