What's a Planetary Nebula? Hint: There are no planets!
A question that astronomers get asked a lot by the public is: “What are planetary nebulae?” And then, the next question is inevitably: “Why are they called planetary nebulae, then?” Let’s see if we can get the facts straight on this confusing celestial phenomenon.
The Butterfly Nebula (Credit: NASA, ESA and the Hubble SM4 ERO Team
A long time ago, people looked up at the night sky and saw stars. Those were dots of light in the sky. Then they noticed that some wandered the sky in different patterns than the rest of the stars—these became known as the planets. But there were other things up there in the sky, things that weren’t points, but were cloudy smudges (prior to electricity, it was much easier to see these things in the night sky with the naked eye). These were called nebulae. Once the telescope was invented, more and more of these smeary, cloudy objects were discovered. The problem was, people didn’t know what they were looking at. Just like how they once called the planets, “wandering stars,” they called every hazy looking object nebulae. It wasn’t until the early 20th century, just a hundred years ago, that we understood that some of these smudges were actually galaxies outside of our own.
This made astronomers scramble to figure out what in the universe all these other things were that they called nebulae. Scientists like Vesto Slipher, Edwin Hubble, and Henry Norris Russell obsessed over these things, looking at the types of light they emitted or absorbed, and over time, they figured out that there were several different types of nebulae, including planetary nebulae.
Except…planetary nebulae have nothing to do with planets. They were initially called that because as early as the 18th century people thought they resembled the round shape of planets when they observed them through telescopes. Even after we learned there is nothing planetary about them, however, the old name stuck, confusing people for centuries to come.
So, what ARE they, then? A planetary nebula is formed when a low-to-medium mass star (one that is the size of our sun up to about 8 times the size of the sun) starts to die. First, the star expands to become a red giant. Then, gradually, the star’s gassy atmosphere will dissipate or be cast off, until just its core remains as what’s called a white dwarf. That core lets off ultraviolet radiation that ionizes the material that was cast off, causing it to glow (the auroras we see on Earth are ionized particles as well—the radiation strikes particles and splits electrons from atoms, making a glow). This creates the beautiful glowing nebulae that we have enjoyed viewing from Earth for centuries. Our own sun will some day create a planetary nebula, billions of years in the future.
Images of four different planetary nebulae, shown in a combination of X-ray and visible light (Credit: X-ray: NASA/CXC/RIT/J.Kastner et al.; Optical: NASA/STScI)
It’s strange to think about, but planetary nebulae are rather short-lived phenomena. They last several tens of thousands of years, rather than the billions of years that measure other life events of a star. But that is still a lot longer than human history.
There are other types of nebulae as well, including dark nebulae and emission nebulae (which planetary nebulae are a type of). Perhaps I’ll write about those in a future post!