Will Our Sun Become a Planetary Nebula?
Among the most beautiful celestial objects we can see and image in our telescopes are the colorful planetary nebulae, for examples see the Nebulae gallery. These are not planets of course, the name is a hold-over from the hazier views of astronomers centuries ago. The prevailing theory for their formation is that stars of a certain size range will eventually become planetary nebulae as their nuclear fusion fuel runs low. It has been thought that in the fullness of time our sun, sol – a G-type main sequence star – is destined to become a red giant engulfing the inner planets including earth. The subsequent physics are pretty well understood. As nuclear energy release diminishes, the core of the red giant collapses into a super-dense white dwarf, followed by expulsion of the outer layers of gases away from the surface, ergo a planetary nebula forms.
Writing these lines led me to do a little review, and I was startled to read that the thinking is changing. This may not be our future glorious doom after all. Sol is now being reclassified as a yellow dwarf star (G-type stars with mass range 80-100% of sol), which may be too small to drive the physics needed to enter the red giant/nebula sequence. At least our sun is big within its peer group, though I prefer to associate “dwarf’ with JRR Tolkien rather than our sun. The thinking now, though still debated of course, is that G-type stars need to be at least one-fifth heavier than our sun in order to drive the planetary nebula process. The proposal is that most planetary nebula we see in our galaxy actually formed from binary star systems. Think about this next time you observe a planetary nebula in your telescope.
Oh well… where would humanity be without classifying and theorizing? Ask Linnaeus, the 18th century scientist who developed the classification mania whose influence still grips the way natural sciences operate.