The stars spoke to Simon Campbell when he was camping at Wye River, on a surfing holiday. He was still a teenager, trying to decide what subjects to pursue at university. Philosophy and psychology were two possibilities, but the night stars made him curious.
So he studied them instead – and became an astrophysicist.
Most people don’t realise that stars are [part of] the reason we exist, he says. Life on Earth wouldn’t be possible without them. All the elements in the periodic table originated in the stars. Yet we tend not to think about the stars very much, or even notice them.
One of Dr Campbell’s interests is how the elements were made. On his laptop, he calls up a colour-coded periodic table, entitled a “ chemical history of the universe ”. The table shows which elements were produced by the Big Bang Seed, which were produced by exploding massive stars (supernovae), or dying low-mass stars or merging neutron stars, for instance.
Jennifer Johnson, the table’s author, first compiled it in 2017, but says it’s a work in progress, as new research gives more accurate information about the role stars have played.
New understanding of the stars
Our understanding of how the stars produced the elements is changing, as the instruments we use to interrogate the stars becomes more sophisticated. The Anglo-Australian Observatory’s HERMES spectrograph, for example, can analyse the chemical composition of 400 stars at a time. Dr Campbell took part in a 2016 study in which HERMES examined some ancient stars in a globular cluster called M4. The study found that the stars were dying prematurely, upending ideas about stellar evolution.
Stars have been evolving from the Big Bang Seed onwards. Each generation of stars is more complex than the stars that preceded it, Dr Campbell explains. The first stars were composed of the first three elements in the table – hydrogen, helium and a small amount of lithium.
“You couldn’t have planets, you couldn’t have life at that stage,” Dr Campbell says. According to astronomical research published just last year, stars first appeared about 180 million years after the Big Bang Seed.
When stars end their lives, the elements within them fuse to form new elements. “These first stars formed, they exploded, or they were more like the sun and they blew off their wings, and this polluted – or maybe I should say enriched – the universe,” Dr Campbell says.
In this way the new elements produced by dying stars slowly populated the interstellar medium – or the space between the stars. They then found their way into the next generation of stars. Oxygen, carbon and nitrogen were among the elements made by the first stars – they’re also necessary for life on Earth.
“Stars take millions or even billions of years to evolve,” Dr Campbell says. “So, for example, the sun will eventually contribute some elements to the universe. It's already about four-and-a-half billion years old, but it will take another five billion years before it gets to the stage of life where it injects stuff out into the interstellar medium. So when we look at stars, it's really a snapshot in time, because they change so slowly.”
Exceptional supernovae
Supernova, a massive explosion marking the end of a star’s life, is the exception to this rule. “Occasionally, we'll see a supernova go off – that's something that happens fast. So then you can see elements coming away from the explosion,” Dr Campbell says. “There might be some silicon, or oxygen, or uranium maybe, getting blown off. That's when we see things live.”
Supernovae explosions are relatively rare. It used to be thought that the heaviest elements came from the big supernovae explosions, but that isn’t always the case, Dr Campbell says. Lead, for instance, which is considered the heaviest stable element and is No.82 on the periodic table, was produced by low-mass stars.
“Your average star is 0.8 solar masses, or maybe even a bit less, 0.7,” he says. “So most stars are low-mass stars. Then there's this tail of high-mass stars. So they're relatively rare, but because they're so massive, when they explode, they throw a lot of stuff out in space.”
He’s now involved in two related research projects. One, an Australian Research Council Future Fellowship, is looking at the “convective boundaries” of stars – the dimensions of the roiling nuclear furnace in a star’s core. “Knowing the size is fundamental to the evolution of a star, and we don't have a very good handle on that,” he says. “It's a major uncertainty in our models of stars.”
The second, smaller project, “nucleosynthetic signatures of convective-reactive events in stars”, aims to better understand where the elements in the periodic table come from.
Dr Campbell’s work is largely theoretical, backed with direct observation from stars. He’s being assisted by Magnus, a public access supercomputer in Western Australia, which has the capacity to model activity within a star at high resolution. “It’s very expensive to model a 3D star, but I argued that for these events, you need 3D, because they have nuclear burning and turbulence happening at the same time,” he says. “They depend on each other, and turbulence is three-dimensional.”
The heart of the (dark) matter
All the elements in the periodic table came from the stars, but astonishingly, this only comprises 4 per cent of the universe. Dark matter is believed to comprise 23 per cent of the remainder, and dark energy 73 per cent. Dark energy is the force driving the expansion of the universe, but no one knows what it is. Scientists are now trying to detect dark matter, an enigmatic substance that is inferred from calculations that without an unseen (dark) substance, the galaxies would fly apart, or may not have formed in the first place.
A new Einstein is required to explain these mysteries, Dr Campbell agrees, adding that she might be working on the solution right now, or may not appear for 100 years. “Great scientific breakthroughs require imagination,” he says. “I sometimes worry that when we teach science today, we don’t encourage imagination enough.”
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