The smoking guns of dying stars

VY Canis Majoris, the largest known star in our galaxy, with it's huge smoky clouds of gas and dust being lost into interstellar space. Credit: NASA, ESA, and R. Humphreys (University of Minnesota)

The ancient Greeks once believed that the heavens were immutable. A vast starry vista, eternally unchanging above our heads. But we now know this to be untrue in the slightest. A lot has changed since then, however, and over the past few hundred years, astronomers have unfurled ever increasing knowledge of the lives of stars. Look out across a starry night sky and you can see stars in all stages of their lives, from brightly burning newborn stars, to ageing giants in their final gasping breaths. As stars near the ends of their lives, they begin to shine a rich red colour as they expand and dramatically increase in size.

As red giants, these stars are burning the last of their fuel, and as they do so they begin to sputter and gasp. Just as a candle flame does when it starts to burn out, red giant stars flicker – though for these, the largest stars in the universe, those flickers can take thousands of years each. These final bursts of energy are known as thermal pulses, and they mark the star’s final days. During the last couple of million years of its life, a red giant star will lose incredble amounts of material to interstellar space. Even when not caught mid-pulse, these stars lose several times the mass of our planet every year. And the puzzle of exactly how they do so has been recently been unravelled by a team of astronomers led by researchers at the University of Sydney.

All stars emit a stellar wind – a steady stream of particles being constantly accelerated outwards by the star. Even the Sun emits its own solar wind (with a speed measured at 535 kilometres per second as I write this). The wind from red giant stars is slightly different. As the star loses more mass, stellar material cools and condenses into dust. This dust then catches starlight which is so intense near to the star that it actually pushes that dust outwards. This effect, known as radiation pressure, causes the tiny reflective dust grains to act like minute sails and accelerates them away. Each dust grain is tiny compared to the kind of dust we find on our bookshelves, much more like fine smoke than any dust we’d recognise. Instead of being called stardust, it could easily be called starsmoke!

“The winds that stream from the upper atmosphere of the red giant stars are responsible for removing massive amounts of matter. The grains that we have discovered here will come as a real shock to the accepted wisdom in the field. They are both much larger and much closer to the stellar surface than anyone expected.” said Barnaby Norris, a PhD student at the University of Sydney, and lead author on the research published earlier this year in Nature.

This stardust emitted by these smoky smouldering old stars is transparent, not unlike finely powdered glass. The implications of this stardust being detected to close to an ageing star could help us to better understand the processes that occur in stars as they die, and how they manage to lose such prodigous amounts of mass so rapidly. As Norris said, “Hopefully our findings will help to illuminate a key step in the grand cycle as matter is expelled from stars into the galaxy only to seed new generations of stellar and planetary birth.”

On a final poetic note, all of the chemical elements essential to life are created within stars, before being seeded back into the cosmos when stars die. This means that this condensing stardust contains the fundamental raw materials needed for life to eventually form. As Carl Sagan so often used to muse, we are literally made of stardust.

The Cat's Eye nebula, a star which has now ended its life and is casting its outer layers into the cosmos. The concentric rings show material lost as stellar wind during the star's final thermal pulses.. Credit: ESA, NASA, HEIC and The Hubble Heritage Team (STScI/AURA)