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101 Must-See Cosmic Objects: NGC 6781

101 Must-See Cosmic Objects: 

Planetary nebula NGC 6781 lies in the constellation Aquila the Eagle. But while this star figure ranks 22nd in size among the 88 constellations, you won’t find any Messier objects or emission nebulae in it. In fact, although it sits squarely within the Milky Way, it contains few even reasonably bright star clusters.

NGC 6781 doesn’t make up for all that, but it is a fine target for amateurs with medium-sized telescopes. This interesting planetary lies some 3,500 light-years away and is about 2 light-years in diameter. Like all such objects, however, the bubble continues to expand and will do so until it’s too far from the central white dwarf for the star’s radiation to ionize it. At that point, it simply will fade from view.

ngc6781 potw - 101 Must-See Cosmic Objects: NGC 6781
Stars such as our Sun do not contain enough mass to finish their lives in the glorious explosions known as supernovae. However, they are still able to salute their imminent demise into dense, Earth-sized embers called white dwarfs by first expelling colourful shells of gas known as planetary nebulae. This misnomer comes from the similarity in appearance of these spherical mass expulsions to giant planets when seen through small telescopes. NGC 6781 is a nice representative of these cosmic bubbles. The planetary nebula lies a few thousand light-years away towards the constellation of Aquila (the Eagle) and is approximately two light-years across. Within NGC 6781, shells of gas blown off from the faint, but very hot, central star’s surface expand out into space. These shells shine under the harsh ultraviolet radiation from the progenitor star in intricate and beautiful patterns. The central star will steadily cool down and darken, eventually disappearing from view into cosmic oblivion. This image was captured with the ESO Faint Object Spectrograph and Camera (EFOSC2) through three wide band filters (B, V, R) and two narrow-band ones (H-alpha, OIII). EFOSC2 is attached to the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. EFOSC2 has a field of view of 4.1 x 4.1 arcminutes.

To find NGC 6781, aim your telescope 3.8° north-northwest of magnitude 3.4 Delta (δ) Aquilae. What you’ll see is an almost perfect bubble of gas cast off by a star that once generated energy like our Sun, but which has long since stopped fusing hydrogen into helium within its core.

Through a 6-inch telescope at 100x, magnitude 11.4 NGC 6781 stands out well against the rich, star-filled background of Aquila. Through larger instruments, you’ll see that the disk doesn’t have a sharp edge and is slightly oval-shaped. If the seeing (atmospheric steadiness) at your observing site is good, you’ll notice that the central region of the nebula is darker than its surroundings.

If you’re able to observe this planetary through a 14-inch or larger scope, you’ll see lots of structure in its thick ring. At high powers, you’ll spot small, dark blotches across its face. The planetary’s central star, now a slightly bluish white dwarf, glows weakly at magnitude 16.2, so don’t spend too much time searching for it.

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Scottish astronomer James Dunlop discovered NGC 5128 in 1826 using a 9-inch reflecting telescope in his observatory in Parramatta, New South..

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