A planetary nebula is one of the most beautiful objects in the universe. Formed from the decaying remnants of a mid-sized star like a sun, no two are alike. Cosmically ephemeral, they last for only about 10,000 years – a blink of a cosmic eye. And yet they are vitally important, as their processed elements spread and intermingle with the interstellar medium in preparation for forming a new generation of stars. So studying them is important for understanding stellar evolution. But unlike their stellar brethren, since no two are alike, it’s hard to efficiently pick them out of astronomical deep-sky surveys. Thankfully, a research team has recently developed a method for doing just that, and their work could open up the door to fully understanding the great circle of stellar life.
Out with a Whimper
When stars like our sun finally kick the bucket, they don’t do it in a neat and tidy fashion. Instead, over the course of a million years or so they slowly turn themselves inside out, ejecting their outer layers into the surrounding solar system. Ragged gasp by ragged gasp, the star sheds its layers, leaving behind only a blazing hot core. This core, now properly called a white dwarf, has a temperature of around a million degrees and emits copious amounts of X-ray radiation.
This radiation strikes the gas surrounding the now-dead star. That gas is mostly hydrogen and helium, just like everything else in the universe, but also contains bits and pieces of heavier elements and molecules like carbon, oxygen, and even water. Energized by the intense radiation blasting off the white dwarf, the elements absorb that energy and re-emit it in all sorts of colorful wavelengths. In case you were wondering, this is exactly how fluorescent light bulbs work but on a much bigger and messier scale.
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Over time the white dwarf will cool down and no longer be able to sustain lighting up the entire nebula surrounding it, at which point it the nebula will fade from view. This happens roughly 10,000 years after the initial exposure of the core.
This is what we call a planetary nebula (I won’t get into the history of the name because it basically makes no sense and we’re just going to have to live with it). Every single planetary nebula is unique because the physics of forming them – from ejecting layer upon layer of a star’s material – is so complex that it can never be exactly repeated. Even though planetary nebulae don’t last long, they are surprisingly common, because the stars they come from are themselves relatively common. So ultimately we see them all over the place, twinkling like Christmas ornaments in the deep sky.
The Circle of Stellar Life
Finding, categorizing, and understanding planetary nebulae are critically important for wrapping our astronomical heads around the full evolution of stars within a galaxy. This is because planetary nebulae form the material for new generations of stars. Through slow dispersion of the dust and gases in the nebulae, and sometimes even violent explosions due to extreme radiation and winds, the material makes its way into interstellar space. There it mixes and mingles with the general galactic milieu and eventually finds its way into a new baby stellar system, and the cycle continues.
What’s more, we need to understand planetary nebulae because they give us a picture into how stars like our sun die. In our surveys we see all sorts of planetary nebulae. Sometimes we see beautiful helical or spiral structures. Sometimes we see spheres or ovals. And sometimes we just see a bunch of tattered rags that can barely call themselves a nebula. How do such intricate and disparate patterns emerge? How can two stars that are seemingly very similar give rise to radically different planetary nebulae? We don’t know.
And that’s not the end of the questions. How critical are planetary nebulae to enriching the interstellar medium? Compared to say supernova. How quickly can material disperse and find its way embedded into some new generation of stars?
These are all very good questions, all without any very good answers
A Few Good Pixels
The proper response to any set of questions like this is usually more data. We need a lot of observations of a lot of planetary nebulae to try to build up a decent statistical database so we can start comparing and contrasting in a solid scientific way. But there’s a problem that appears if we want to start developing massive surveys to pick out thousands upon thousands of planetary nebulae in the sky. The problem is that no two nebulae are alike, so it’s very hard to come up with a simple classification scheme that picks out planetary nebulae from some other random bits of space stuff.
Even more frustratingly, at the scale and resolution of most sky surveys, planetary nebulae are just a few fuzzy pixels across. How can you possibly tell one from another? This is where the new research comes in. A team of astronomers performed an enormous number of simulations and simulated observations of planetary nebulae, in addition to other sources that they might be confused with like galaxies and quasars.
They then chopped up this data in as many different ways as possible, seeing how planetary nebulae looked at certain wavelengths compared to others. They identified a key series of tests that allowed them to filter out almost any other contaminant, leaving only a population of clean (still fuzzy) planetary nebulae. With this technique future automated sky surveys could easily incorporate planetary nebulae into their catalogs, perhaps helping to answer some of the questions of how exactly the circle of seller life goes round and round in the galaxy.