Although galaxies are composed primarily of empty vacuum, the space between stars is filled with mind-bogglingly huge clouds of gas and dust. The dust is a fine mix of microscopic particles of various carbon and silicon compounds, and is a major component of some the most breathtaking nebulae in the sky. But despite the amount of time astronomers have spent studying these dust clouds, they have never been quite sure where it comes from. That might be set to change, though, as new observations of the fragmented remains of a supernova appear to confirm a leading theory: Dust in space is the hot ash of a supernova explosion, formed from the shredded outer layers of the exploding star
The current theory about how most of the dust in a galaxy is formed looks like this: The final years of a really big star’s life are turbulent. The hydrogen reserves in its core are long since depleted, and nuclear reactions at its core keep changing as it finds new ways to burn the ashes of the previous reaction. These newly forged elements form shells of material growing outwards from the core, and the star becomes more and more enriched with different types of materials. Every time the core reactions change, the temperature and pressure adjust, causing the entire star to blow up or reduce, becoming hotter and colder, larger and smaller. The star grows so huge during these oscillations, the outermost layers of the star end up so far from the core, that the surface gravity becomes very weak and entire layers of superheated plasma simply fly away into space, propelled by the solar wind. This ejected material spreads out in concentric spheres of hot gas, which we can see from Earth as a planetary nebula. But when the star finally burns through the last possible fuel, when the core is filled with iron, the reactions stop. With no energy to keep the star inflated, all those quadrillions of gigatons of material collapse inwards, smash into collide at the centre, and bounce, leading to the sudden violent detonation that is a supernova. Enormous amounts of hot enriched gas blast out into space at an appreciable fraction of the speed of light, catching up to and then smashing into the previously ejected shells of gas. These collision regions are hot and dense, allowing for all kinds of chemistry to occur, leading to the formation of organic molecules, many of which condense into microscopic grains of dust. This dust continues to spread outwards and eventually settles into the clouds we see through our telescopes.
It’s a good theory with no competing alternatives that make sense, and it is widely enough accepted that it’s taught as fact by astronomy professors. The problem is that, until recently, astronomers had never actually seen it happen. There was never any direct evidence to prove that it was true. But new observations of SN 1987a taken through the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile might be about to change that. In 1987, a blue supergiant star in one of the Milky Way’s satellite galaxies exploded. It was the first supernova to be recorded that year, and was the brightest to be observed since the 17th century. It was close enough, at 160,000 light years, that it was visible to the naked eye for southern hemisphere observers who knew where to look. Within days, every free telescope on the planet was swung around to capture as much of the event as possible. Even today, astronomers still keep it under close observation. Naturally, those astronomers who study galactic dust were excited about this opportunity to finally test their theories, but the results of their observations were not promising. Infrared telescopes were used to study the region of the explosion for 500 days to try and spot any sign that carbon, silicon and oxygen had bonded in the fires of the blast to form dust. Although they did find some, it was far too little to prove anything.
But fast forward a few decades to the year 2013, when astronomer Remy Indebetouw, of the National Radio Astronomy Observatory (NRAO) and the University of Virginia, turned his attention to the area with the modern ALMA instrument. ALMA is both extremely sensitive and capable of very high resolution images. His observations, collected over several years and taken a quarter century after the supernova, has detected a large mass of the dust that was always predicted, as well as quantities of carbon monoxide and silicon monoxide gas.
“We have found a remarkably large dust mass concentrated in the central part of the ejecta from a relatively young and nearby supernova,” said Indebetouw. “This is the first time we’ve been able to really image where the dust has formed, which is important in understanding the evolution of galaxies.”
“SN 1987A is a special place since it hasn’t mixed with the surrounding environment, so what we see there was made there,” he added. “The new ALMA results, which are the first of their kind, reveal a supernova remnant chock full of material that simply did not exist a few decades ago.”
The initial glow of the supernova faded a few months after the supernova exploded, but the space around it is still filled with glowing shells of material ejected in the millennia before the final blast. These shells, which appear as rings from a distance, were brightly illuminated when they were first lit up by the intense ultraviolet light of the supernova. In recent years they have begun brightening again as the shockwave reaches them, compressing and heating them up. Worryingly for the theory, a portion of this shockwave will rebound back towards the star, passing through the dust on its way. “At some point, this rebound shockwave will slam into these billowing clumps of freshly minted dust,” said Indebetouw. “It’s likely that some fraction of the dust will be blasted apart at that point. It’s hard to predict exactly how much — maybe only a little, possibly a half or two thirds.”
In other words, although it’s now been proven that supernovae create large amounts of dust, we still don’t know if this dust survives long enough to make it out into deep space and participate in the evolution of stars and galaxies. Fortunately, we have a ringside seat to watch what happens, and only have to wait for the next interaction in the continuing saga of SN 1987a.
Indebetouw’s results were published in a paper in the Astrophysical Journal Letters, and can be read here: arXiv:1312.4086 [astro-ph.SR]