NGC 6781 is a rather nice, round-looking planetary nebula located at a distance of 1500 light years (460 pc) [1] in the constellation Aquila. Like other planetary nebulae, NGC 6781 was created by a relatively small star when the star reached the end of its life. In the case of NGC 6781, that star was about 2.25 to 3 times the mass of the Sun [1]. After the star's core filled up with helium, it first expanded into a red giant, and then the fusion of helium into carbon and oxygen in the core was triggered, and then a bunch of other things happened until the core filled up with carbon and oxygen and the star blew away its outer layers to form the planetary nebula that we see today.
What makes NGC 6781 a little more interesting than the typical planetary nebula is its structure. As I have already said, it looks like a relatively round planetary nebula. However, take a closer look at an image of NGC 6781 and you will see that it isn't quite perfectly round. First of all, one of the edges of the nebula looks a little fuzzier or diffuse than the other. Second, the nebula almost looks like it contains two ring shapes instead of one, and the rings are offset slightly from each other. Well, NGC 6781 is not actually a circular planetary nebula; it just looks that way as seen from Earth.
Planetary nebulae come in many shapes, but quite a few are bipolar. In these nebulae, the gas flows outwards from the central star towards its poles, producing three-dimensional cylindrical or hourglass shapes. The reasons why some planetary nebula look spherical and some look bipolar is not entirely understood, but it looks likely that bipolar nebulae are produced when stars that form planetary nebulae are in binary star systems where something related to the interactions between the two stars and the outflowing gas produces the bipolar structure [2].
To make things more complicated, some bipolar planetary nebulae may be aligned in such a way that the outflowing gas is aimed almost directly towards the Earth. In this situation, the nebulae could look round even though they are cylindrical or hourglass-shaped, and that is because we are looking down the axes of these shapes. This seems to be what is happening with NGC 6781 [3, 4, 5]. One lobe of the planetary nebula is aimed towards us and forms one of the ring shapes that we see, while the other lobe is aimed away from us and forms a second ring slightly offset from the first one.
This bipolar structure was only discovered in the early 2000s. From what I can tell, this discovery would not have been possible without the digital imaging capabilities, including infrared imaging capabilities, that modern telescopes have as well as the advanced three-dimensional modelling that can be done with modern computers. The double ring structure is just a little too subtle to see in twentieth century photographs, and the mathematical analysis would have been a bit too tricky to do by hand.
NGC 6781 is not the only ring-like planetary nebula discovered to be bipolar, but it is one of the more prominent examples. (Messier 57, also known as the Ring Nebula, is the most prominent example of something ring-like that is actually bipolar [6].) This discussion about the shapes of planetary nebulae may not be a very deep science topic compared to what I have presented in some of the other episodes in my podcast series, but identifying bipolar nebulae like NGC 6781 and Messier 57 that look like rings is actually important if astronomers want to figure out whether bipolar nebulae are more likely to contain binary star systems and also important just for understanding how bipolar nebulae form. If bipolar planetary nebulae like NGC 6781 continued to be misidentified as spherical, that would just cause a lot of confusion and lead to a lot of weird science papers.
While NGC 6781 has been a target of interest for professional astronomers, it's also a rather popular planetary nebula with amateur astronomers as well. Aquila, unlike many constellations, actually sort of looks like what it's named after, an eagle. NGC 6781 is located about one-third of the distance from Delta Aquilae, which is in the center of the constellation, to Zeta Aquilae, which is the second star from the tip of the right wing. The nebula is slightly under 2 arcminutes wide or a couple of times the size of Jupiter [7]. Try finding this nebula on a night when the Moon isn't up and, of course, in a location with no city lights. It should be visible when using a medium-magnification eyepiece in a medium-sized telescope, like a 10 to 15 cm (4 to 6 inch) telescope [8, 9]. When the nebula is viewed through a much larger telescope, like a 36 cm (14 inch) telescope, it's possible to see structure in the ring [9]. If you've spotted this nebula before or if you can find it after listening to this podcast, let me know.