By Jett Peters
Today, I would like to highlight a target that is very rarely imaged, but for wider field setups has a very striking look. This target is Sh2-216 in the constellation Perseus. This target is not only interesting from a looks perspective, but it also has scientific importance. It is the closest planetary nebula to Earth and is the largest planetary nebula in the night sky.
Planetary nebulae have a bit of a misleading name, as they have nothing to do with planets. Instead, they are formed during the final stages of a moderate-sized star’s life. One day, billions of years from now, our Sun will undergo this process. It begins when the star exhausts its hydrogen fuel in its core. Without fusion to counteract gravity, the core contracts and becomes denser, increasing its temperature. This leads to the ignition of helium fusion, which produces enough outward pressure to temporarily counteract and overcome gravitational collapse. As a result, the star expands and becomes a red giant. In this state it is less dense and is cooler, causing it to peak in the red.
Eventually, as the star depletes its nuclear fuel, it can no longer sustain the outward pressure needed to resist gravity. This causes its outer layers to be thrown off into space, forming an expanding shell of ionized gas. The remaining core of the star, now a white dwarf, emits intense ultraviolet radiation that excites the surrounding gas, causing it to glow.
This is the story that led to the formation of Sh2-216 which is only 400 light years from Earth. To image this nebula you will need a relatively wide field telescope and narrowband filters. Due to its distance, it is very large, but quite faint. It is characterized, like most planetary nebulae, with hydrogen alpha and oxygen three emission. My image of Sh2-216 can be seen below. It is still a work in progress, but I hope this may inspire you to image this target before it goes down for the year.
