What is a planetary nebula?
What is a planetary nebula, and what do we know about their beautiful structures?
Planetary nebulae are formed during the later stages of a star’s life (the Sun, for example) as instability at the core causes the outer layers of the star to be shed.
As the stellar wind pushes material away from the star, it can excite and interact with surrounding material, creating beautiful, ethereal structures.
One of the first objects I looked at through a telescope was the Ring Nebula. There was something about the neatly coloured doughnut that appealed, though I soon found that seeing more than a smudge is difficult – even through a large telescope.
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In images, though, especially those that flowed from the Hubble Space Telescope, the complexity of these fragile objects became obvious.
In addition to rings, there are planetary nebulae that look like egg timers, nebulae shaped like butterfly wings and even a singular object that goes by the name of the Red Rectangle.
One planetary nebula known as the Boomerang Nebula is the coldest place in the Universe.
Close up, there are substructures – ribs, stripes and more – and it’s these smaller scale features that are addressed in a new study concentrating on hourglass-shaped nebulae, several of which seem to display faint filaments that the paper describes as a ‘spiderweb’ structure.
The Hubble image of the Matryoshka Nebula (below) shows how complicated such objects can be.
A bright central source sits in the middle of the hourglass, the sides of which seem made of almost-transparent gossamer, with a texture that hints at unresolved structure.
Material is believed to be flowing away from the central source and the whole thing sits in a larger, oval nebula consisting of long, curving arcs of glowing cooler gas.
Whatever is sculpting these structures, it produces symmetry, both north-south through the hourglass lobes and east-west in the spiderweb structure.
The beauty of these objects is transitory, however. Something like the Ring Nebula will exist for only a few tens of thousands of years.
The key insight of the model in the paper, though, is that the planetary nebulae we see today don’t exist in a pristine environment.
The process of atmosphere loss is erratic and takes time, so the star exists surrounded by its own debris.
Some of this material, the authors say, must be falling back onto the star and their detection of what seems to be an accretion disk around the central source suggests that there is enough material to make a difference.
When gas is moving, it can become shocked and excited, and it seems like this process can weave the spiderweb structures that we observe.
If the inside of the hourglass is a region where material flows outwards, its edge is a place where material can flow back.
This process is likely to be somewhat chaotic and what appeared to be a precious, permanent jewel in the sky is revealed to be a dramatically dynamic system, as changeable as the pattern of waves on the surface of the sea.
Planetary nebulae in pictures
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Chris Lintott was reading… Fallback in Bipolar Planetary Nebulae by Willem A Baan et al. Read it online at arxiv.org.
This article originally appeared in the October 2021 issue of BBC Sky at Night Magazine.