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Dying Stars Leave Dusty Trails

By David Reneke

News from the space and astronomy communities around the world.

Stars get pretty sloppy towards the end of their lives and the nuclear fuels start to run out. The star pulsates, expanding and contracting like a fish just out of the water. Each pulse belches out globs of gas into space that eventually get recycled into a new generation of stars and planets. The universe thought about recycling long before we came along.

But there’s a puzzle – accounting for all that lost material is difficult. Like trying to see a wisp of smoke next to a stadium spotlight, observing these tenuous sheets of stellar material swirling just over the surface of the star is considerably challenging.

But by using an innovative technique to image starlight scattering off interstellar grains, astronomers have finally succeeded in seeing ripples of dust flowing off dying stars.

The three study stars, W Hydra, R Doradus and R Leonis, are what astronomers call red giants – stars that are no longer fusing hydrogen in their cores but have moved on to forming heavier elements. Each is completely enveloped by a very thin dust shell, most likely made up of minerals, allowing atoms to start sticking together to form more complex compounds.

Minerals like these will go on to seed asteroids and possibly rocky planets like the Earth in the continual cycle of death and rebirth playing out in the galaxy. It’s probably been going on since time immemorial.

Astronomers used the 8-metre Very Large Telescope in the Chilean Atacama Desert and a suite of clever tools to tease out the subtle reflections off these dust shells. The trick to seeing light bouncing off interstellar dust particles involves taking advantage of one of the polarisation of light waves.

When light bounces off a dust grain, the waves oscillate in roughly the same direction. Astronomers call this light polarised. It’s simply the orientation of those waves, but it says so much. Just like the sky gets brighter and dimmer as you turn your sunglasses, looking at such a star through differently oriented polarising filters will reveal a halo of polarised light surrounding it.

The different orientations expose different segments of the halo, and a thin ring of scattered light begins to reveal itself around these three stars. These new observations represent a milestone in our understanding of not only a star’s end game but also the production of interstellar dust that follows.

Like giant smokestacks, red giant stars expel sooty minerals into space, and they are propagated by strong stellar winds. Results such as these can help tie together the death of one generation of stars with the birth of another.

Unravelling the mysteries of grain formation in space takes us one step closer to piecing together the many steps that lead from stellar death to the creation of rocky planets like our own.

Cosmic Collision Heading Our Way
Like an episode from a yet-to-be-made cosmic disaster movie, we might be witnessing the final stages in the formation of our galaxy. A giant cloud of hydrogen gas, accelerating at more than 240 km/s2, is closing in very fast on the Milky Way, and is likely setting off a huge burst of star formation.

At this rate the cloud will collide with interstellar gas in the Milky Way’s disc in less than 40 million years, condensing into tens of thousands of bright, massive stars that will explode as supernovae within a couple of million years.

“Its comet-like shape indicates it’s already hitting gas in our galaxy’s outskirts,” said Felix J. Lockman of the National Radio Astronomy Observatory in Virginia, USA. “It is also feeling a tidal force from the gravity of the Milky Way and may be in the process of being torn apart. Our galaxy will get a rain of gas from this cloud, and then in about 20–40 million years the cloud’s core will smash into the Milky Way’s plane.” A sobering thought for future generations to ponder.

Many clouds of hydrogen surround the Milky Way but astronomers didn’t start spotting them until half a century ago, well after the advent of radio telescopes, which are able to detect cold, neutral hydrogen gas. The early observations were not accurate enough to determine the clouds’ distances, masses or directionality.

The hydrogen cloud is named Smith’s Cloud after Dutch astronomy student Gail Smith, who discovered it in 1963. Astronomers have found that Smith’s Cloud has a mass of at least one million solar masses and measures 9800 light years long by 3300 light years wide.

Curious about the cloud’s elongated shape, astronomers led by Lockman took tens of thousands of radio brightness measurements. The data reveal that the cloud is just 8000 light years away from the Milky Way’s central plane, making it the closest one known. Its elongated shape is apparently due to the tidal effects of the Milky Way.

Once again the universe shows its mettle and reminds us who’s in charge. Hang on, it’s going to be a bumpy ride!

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at