Planetary Nebula – The Fate of Our Sun

It’s “very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet”. This notation was penned in January 1779 by French Astronomer Antoine Darquier de Pellepoix. He was describing an object he thought he discovered, finding later however that it had been previously discovered by Charles Messier (Messier 57, the Ring Nebula).  But that isn’t the story, the story is Pellepoix’s use of “resembles a fading planet” in his description. Pellepoix was the first to describe a “planetary” nebula and it stuck, earning a place in the language of astronomical classification.

Just to be clear, a planetary nebula is not a planet.  It is an emission nebula that often takes on a spherical shape, resembling a planet. That’s also what makes planetary nebulae so special to observe and image. They can be spectacular to view in larger telescopes or with smaller scopes equipped with specialized, image intensifying eyepieces. They are beautiful objects to photograph.

As my title indicates, our Sun will produce a planetary nebula, but don’t hold your breath. This won’t begin for another several billion years, once our Sun has become a bloated red giant and is going through late in life spasms. All the planetary nebulae out there evolve from stars like our Sun, and those up to 8 times the mass of our Sun.

So, let’s fast-forward a few billion years and check out our Sun.

Unfortunately, Earth is toast. At this point in the Sun’s life, it is going through convulsions as hydrogen fusion gives way to helium fusion. This transitional period produces erratic fusion pressure within the Sun’s core and gravity begins its slow march to victory. The Sun’s core shrinks due to gravity winning over lower outward fusion pressure. Layers above the core have expanded outward as the core shrinks, and our Sun has become a much bigger albeit cooler star. It has become a red giant. The inner planets, including Earth get enveloped and broiled by the Sun’s chromosphere. Toast.  So, we will have to observe from a safer distance, perhaps Mars or an asteroid.

Late in its red giant phase things get even more chaotic, with pulsating, explosive bursts of fusion, then gravitational core constrictions. During this process multiple layers of the Sun get blasted away at escape velocity. A final massive explosion occurs, sending more of the Sun outward, and its core collapses until it is mostly carbon and oxygen atoms all scrunched together. The atom’s electrons provide sufficient pressure, called degeneracy pressure, to keep it from further collapse-because it is a solar mass star. No longer capable of fusion it’s now a white dwarf, about the size of Earth.  Beyond 8 solar masses however the collapse would continue. That’s another story.

Intense radiation coming from the white dwarf causes all the layers blown away to glow and a planetary nebula is born.

What’s in the Sky?

April 16; 45 minutes before sunrise; east:  Jupiter, Venus, Mars, and Saturn make a pretty line-up