Planetary Nebulae

The following citations listed in this article are from Planetary nebula - The central stars | Brittanica and Planetary Nebula | National School's Observatory.

    After spending time as red (super)giants, stars enter their next phase: planetary nebulae. According to previously cited sources, a planetary nebula is "a class of bright nebulae that are expanding shells of luminous gas expelled by dying stars." Usually, nebulae are irregularly shaped; they're blobby and each one is unique. Planetary nebulae are all somewhat round and relatively look similar to each other. In the late 1700s, these stars were mistaken for planetary disks, and were given their name because of the mistake. Planetaries, as scientists tend to call them for short, are very important to the interstellar medium, as they supply gas to their surroundings.

Image Credits: ESO Views the Ghost of a Dying Star - Planetary Nebula ESO 378-1

    During this stage of life, a star will shed its outer layers and create a glowing shell, which is actually just a shell of extremely hot gas. Think of it like a very shy person opening up and making new friends: they let down their guard and their true self shines through. Weird and kind of corny, I know, but a star lessens the distance between its core and the space around it during this phase. Planetary nebulae are very unstable and tend to "pulsate." This continuous movement creates stellar winds, which tear off the outer layers of the star. 

    The loose parts of the planetary are scattered over time, and cluster together with other pieces of debris and dead star, eventually forming clouds. Later in time, these clouds will restart the star life cycle, birthing new protostars. According to previously cited sources, "Planetary nebula[e] last just a few tens of thousands of years." This stage of a star's life is extremely short compared to the other billions-of-years-long stages it's been through. High-mass stars don't typically go through this phase; they tend to go straight from red supergiants to supernovas.

Image Credits: XMM-Newton Views Born-Again Planetary Nebula Abell 78

    Take a good look at the previous image and compare it to the image before it. Planetary nebulae vary in color. That's what makes them so beautiful! You can see the inner parts of the star under the glowing shell. In this image, the shell appears green, but stars are actually incapable of being green, due to black-body spectrum. After this stage of a low-mass star's life, a white dwarf will be left behind, which will be further explained in the next entry.

NOTE: While I was studying this topic, it was kind of hard to find information that made sense to me. I tried to nitpick through the articles to find facts that sounded familiar, but this entry was very hard to support with professional evidence.