The Helix Nebula (NGC7293) is a great example of what is called a “planetary nebula” (PN), and is one of the closest PNs to Earth at about 700 light-years. “Planetary Nebula” is a misnomer that derives from their typically round appearance, which might be mistaken for a planet when viewed through primitive telescopes. They are actually emission nebulae resulting from a star shedding its outer layers as it approaches its end of life. Most PNs have a similar appearance, but this image is the best example from my small selection, partly because of its large angular size, and partly because it was captured with better equipment and under better sky conditions in Australia. But just so you can see the similarity, here are some other PNs.
The Ring Nebula (M57):
The Cat’s Eye Nebula (NGC6543):
And the Dumbbell Nebula (M27):
These examples are not all processed the same way, which detracts from the true similarity in their appearance. In the case of the Cat’s Eye Nebula, the small, bright portion in the center actually looks most like an eye, but it is too small to see clearly in this image. I should add that the Cat’s Eye image is also a composite of very different exposures because that center portion is actually orders of magnitude brighter that the outer portions. This too is characteristic of PNs, that they have relatively bright centers and multiple rings of ever less-bright nebulosity. In all PNs there is a nucleus, the star that originally created the nebula, but it is not always visible, and they eventually become dwarf stars. The majority of PNs are significantly distorted from a true spherical shape. The reasons for this are not well understood.
Getting back to the Helix Nebula, this image is composed of about 16 hours of exposures in H-alpha and O-III. There is a very small amount of S-II in the nebula as well, but the image is so weak that it adds nothing to the final results, so I didn’t include it. I’ve actually been working on the processing of this image for a long time, and have gone through many variations. As with the other PNs, it is a challenge to bring out the fainter outer rings of nebulosity, but the best solution for that is just to collect more data. Instead, I focused on enhancing the lines that seem to radiate from the center, which makes it look like the iris of an eye. These lines are actually very subtle differences in the density of hydrogen and oxygen. Of course, they are not perfectly symmetrical – just like the lines of an iris.
Like supernova remnants, planetary nebulae expand and become more diffuse over thousands of years. But in most cases they also become dimmer because as their nucleus star shrinks it produces less of the ultraviolet radiation that ionizes the gases, and so they stop emitting light.