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Western Slope Skies - What Kind of Planet is a Planetary Nebula?

The Ring, the Dumbbell, the Eskimo, the Helix. These lyrical names are assigned to deep sky objects known as planetary nebulae. Hubble Space Telescope images of these display some of the most beautiful objects in the night sky.



In 1782, William Herschelobserved an object that was unlike any he had observed before. He theorized that it was a planet, but was puzzled by its relatively low brightness, i.e. a disk, like a planet, but faint, like a nebula. Later he called these types of objects ‘planetary nebulae.’  In fact, these objects have nothing to do with planets, but are the last vestiges of red giant stars at the end of their life. Regardless, the term ‘planetary nebulae’ remains to this day.

All stars are in a constant battle. Gravity, due to the immense mass, is collapsing the star. Pressure, due to the internal nuclear fusion reaction, is expanding the star. With certain exceptions, gravity always wins. However, for a few thousands of years out of a lifetime measured in billions of years, pressure holds the advantage.

Stars ranging in mass from that of our Sun to 8 times the Sun eventually reach a stage where they have fused much of the hydrogen into helium. Without going into more complex details, these stars then begin to expel the outer gas layers. Eventually the inner hot core of the star is exposed, emitting ultraviolet energy that ionizes the elements in the outer layers. Depending upon the elements, we observe many different colors in planetary nebulae, for example, red from hydrogen and green from doubly-ionized oxygen.

The release of energy causes the inner core to begin to cool. The core will finally reach a point where it is not hot enough to emit UV energy. The planetary nebula disappears and only the core, which is now a white dwarf, remains slightly visible. One cubic centimeter of a white dwarf has a mass of about one ton… one million times denser than water.

While many planetary nebulae appear spherical to us, there are others that are quite asymmetric. The reasons for this are not yet well understood. Thankfully, the wide diversity in shape and color provides us wonderful views and images of these objects.

Art Trevena

Western Slope Skies is produced by members of the Black Canyon Astronomical Society. This episode was written and recorded by Bryan Cashion.

Additional resources:
Phil Plait’s crash course in Planetary Nebulae