Man, do I love me some planetary nebulae. Here’s one for you:
That’s IC (for Index Catalog, a supplement to the original New General Catalog of astronomical objects) 4593, a nebula in Hercules, and it’s one of four nebulae images just released from Hubble. What you are seeing here is the death of a star like the Sun. I have written about them before (here and here, with some more general info here), so go read those to get the intro.
I studied planetary nebulae (or PNe) for years, and they fascinate me. The fantastic shapes are caused by the collision of winds emitted by the central stars as they turn into red giants and evolve into small, hot, white dwarfs. Normally, red giants don’t spin rapidly, so you might expect the winds to expand spherically (if the star spun quickly, the centripetal force would flatten out the wind into an oblate spheroid, like a basketball someone sat on). But actually a spherical PN is rare! Somehow, these stars are getting some kind of injection of angular momentum, and one (IMO) very good hypothesis is that the stars consume planets as they expand. As the star vaporizes the planets, the angular momentum from the planet’s orbital energy spins the star up. The winds flatten out, and you get those incredible shapes that remind me of squids and flowers.
But if you look at IC 4593, you’ll see two bright red knots of gas with long streamers; one is at the upper left and the other at the lower right. A cursory examination indicates many interesting things: they are on opposite sides of the star; the streamers point directly back to the star; the knots are mostly nitrogen (indicated by the red color in these images); they are moving rapidly compared to the rest of the gas (this can be seen by taking spectra); they are not at as high a temperature as the rest of the gas.
What are they?
They’re called FLIERs, for Fast Low-Ionization Emission Regions. Fast is obvious enough. Low ionization means that they are not hot, and not getting pelted by UV radiation from the star. Emission means they are glowing. They’re seen in lots of PNe.
I’ll cut to the chase and say they aren’t well-understood, at least as far as I can tell by looking at the literature. Most likely some are caused by some weird feature of the wind collision where gas is squeezed out under higher pressure, like a watermelon seed when you squeeze it between your fingers. But this idea doesn’t explain all the features of FLIERs seen in other PNe, and their origin is still a mystery.
An odd thing that crops up in the papers I’ve read is that the knots may not actually be moving all that quickly. The speed is inferred from the Doppler shift in the spectra, and some astronomers are wondering if perhaps that’s not showing the speed of the knot itself, but maybe is due to gas flowing around them, or from some other combination of effects.
Observations from Hubble and other ‘scopes will keep coming in, and the models will keep getting refined. Studying these objects will turn up insights into how stars like the Sun die, which in turn will tell us how they change as they age, which has a direct impact on us. Maybe not for a while yet – maybe a billion or five years – but any knowledge on this topic is a good thing.
Plus, they’re pretty. That counts too.
