NGC 6818 in the constellation of Sagittarius



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This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. It is located in the constellation of Sagittarius (The Archer), roughly 6000 light-years away from us. The rich glow of the cloud is just over half a light-year across — humongous compared to its tiny central star — but still a little gem on a cosmic scale. When stars like the Sun enter retirement, they shed their outer layers into space to create glowing clouds of gas called planetary nebulae. This ejection of mass is uneven, and planetary nebulae can have very complex shapes. NGC 6818 shows knotty filament-like structures and distinct layers of material, with a bright and enclosed central bubble surrounded by a larger, more diffuse cloud. Scientists believe that the stellar wind from the central star propels the outflowing material, sculpting the elongated shape of NGC 6818. As this fast wind smashes through the slower-moving cloud it creates particularly bright blowouts at the bubble’s outer layers. Hubble previously imaged this nebula back in 1997 with its Wide Field Planetary Camera 2, using a mix of filters that highlighted emission from ionised oxygen and hydrogen (opo9811h). This image, while from the same camera, uses different filters to reveal a different view of the nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.

Credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

The NASA/ESA Hubble Space Telescope had imaged NGC 6818 before, but it took another look at this planetary nebula, with a new mix of colour filters, to display it in all its beauty. By showing off its stunning turquoise and rose quartz tones in this image, NGC 6818 lives up to its popular name: Little Gem Nebula.

This cloud of gas formed some 3500 years ago when a star like the Sun reached the end of its life and ejected its outer layers into space. As the layers of stellar material spread out from the nucleus – the white stellar remnant at the centre of the image – they ended up acquiring unusual shapes.

NGC 6818 features pinkish knotty filaments and two distinct turquoise layers: a bright, oval inner region and, draped over it like sheer fabric, a spherical outer region.

The central star has a faint stellar companion 150 astronomical units away, or five times the distance between the Sun and Neptune. You can just about make this out: if you zoom in to the centre, you’ll notice the white dot in the middle is not perfectly round, but rather two dots very close together.

With a diameter of just over half a light-year, the planetary nebula itself is about 250 times larger than the binary system. But the nebula material is still close enough to its parent star for the ultraviolet radiation the star releases to ionise the dusty gas and make it glow.

Scientists believe the star also releases a high-speed flow of particles – a stellar wind – that is responsible for the oval shape of the inner region of the nebula. The fast wind sweeps away the slowly moving dusty gas, piercing its inner bubble at the oval ends, seen at the lower left and top right corners of the image.

NGC 6818 is located in the constellation of Sagittarius and is about 6000 light-years from Earth. It was first imaged by the Hubble Space Telescope’s Wide Field Planetary Camera 2 in 1997, and again in 1998 and 2000 using different colour filters to highlight different gases in the nebula.

 

 


Source: European Space Agency

Posted in Space on November 7, 2016