Now that we’re finally talking about white dwarfs, we’re getting into the really cool stuff.
In my last post, we explored planetary nebulae, and we left off with a question: where does the fast wind that forms planetary nebulae come from? Well, remember that planetary nebulae are formed from the atmospheres of medium-mass stars, but there’s still the stellar interior to worry about.
White dwarfs are objects comparable in size to our own Earth. They are the remains of medium-mass stars like our own sun. Often, you can see a white dwarf at the center of a planetary nebula with a large telescope. Together, they form what’s left of a star after it loses stability completely.
Meet the planetary nebula, one of the universe’s most gorgeous phenomena.
If you’ve ever looked through a telescope, you may have seen one of these before. Through a small telescope, one might look like a little planet—hence the name. But make no mistake, these nebulae have nothing to do with planets, and everything to do with stars.
Then, as the star runs out of helium fuel in its core, it expands into a giant a second time. This is the last time a medium-mass star will expand. It’s also the end of the line for the fuel in its core, since it can’t get hot enough to fuse carbon.
At this point, the star is so big that gravity at the surface is too weak to hold onto its atmosphere, especially in the face of the superwind of radiation pressure from the still-collapsing core.
The result is a planetary nebula…but what exactly is a planetary nebula? What is it made of? Why does it look the way it does?
When we talk about star death, we’re not really talking about death. We’re talking about the end of a functioning star. Astronomers tend to personify cosmic objects like stars, saying that they are born and die, when it’s more like they transition into something new.
With stars in particular, there’s two main courses their “life cycles,” such that they are, can take: one for massive stars and one for low-mass stars.
We can further subdivide low-mass star “deaths” into those of red dwarfs—like our nearest stellar neighbor, Proxima Centauri—and those of medium-mass stars, like the sun.
But before we dive into the final stages of these stellar life cycles, let’s review what kinds of stars we’re talking about here…