The ghosts of dead stars

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Supernova remnant G278.94+1.35, dubbed ‘Diprotodon’, captured by CSIRO’s ASKAP radio telescope. Credit: Sanja Lazarević

Supernova remnants are some of the most visually impressive objects in space. Astronomer Kovi Rose offers us a unique window into these violent and powerful celestial events.

Something explosive always seems to be happening in space. We often see headlines in the news about dramatic events like a flaring star, a gravitational wave from colliding neutron stars, or the latest supernova erupting in a galaxy far, far away.

The stories normally tend to focus on the peak periods of these energetic events, which generate in a week roughly a trillion-trillion times as much energy as we generated on Earth last year. But what remains after a star’s collapse – a supernova remnant, as astronomers call it – is both spectacular and scientifically interesting.

The end of a star

Stars are endlessly collapsing under gravity. This immense pressure drives a fusion reaction, where hydrogen particles join together into heavier elements. The energy produced by this fusion reaction pushes outwards, stopping the star from collapsing in on itself. However, when a star starts to run out of fuel for its fusion engine, the balance breaks down and things get interesting.

For stars roughly the size of our Sun, there is no big explosion as they reach their final years. Instead, when they run out of fuel, they gently shrink into a glowing lump of carbon and oxygen called a white dwarf. White dwarfs don’t collapse entirely under the force of gravity, because the electrons in the remaining atoms are strong enough to push back. This is thanks to a quirky quantum effect called electron pressure.

A white dwarf can produce a supernova, but only under very specific circumstances, when the white dwarf is orbiting another star. When a white dwarf gets too close to the other star – which could even be another white dwarf – its gravitational influence will start to pull in material from the other star. This breaks the balance between gravity and those simmering electrons, ultimately causing the white dwarf to explode!

Bigger stars do end their lives in a supernova, and usually without any outside help. These stars – with more than 8 times the mass of our Sun – live fast and die young. They burn through their nuclear fuel faster than their smaller cousins, with lifetimes of millions (not billions) of years. These stars start by fusing hydrogen into helium in the core. As that runs out, they start fusing helium atoms together instead. And so it continues up the periodic table. The heavier the element, the faster the star runs out of fuel – with carbon and oxygen burning for mere years and months, respectively. But this can’t go on forever.

Once the core is made of iron, the fusion process grinds to a halt. With no new energy keeping the star inflated, its layers suddenly collapse. The rush of material inwards hits the remaining iron core and produces a shockwave that moves outwards at speeds nearing a quarter of the speed of light. These aptly named core-collapse supernovae usually leave their densely packed remains behind in the form of a neutron star – or, depending on how massive they were, a black hole.

Tuning the radio

For both classes of supernova, the stellar matter from the explosion is launched out across space at thousands, or even tens of thousands, of kilometres per second. Moving at these speeds, the leading front of the supernova can take tens of thousands of years to slow down, usually after spreading out across several light-years of space (one light-year is about 9.5 trillion kilometres) and sweeping up any additional material they encounter along the way. This is a supernova remnant: an interstellar bubble created by the wake of one of nature’s most energetic explosions.

This powerful blast wave contains fast-moving electrons that interact with nearby material in a fascinating way. The space around a supernova is filled with magnetised matter, and because of the special relationship between electricity and magnetism, the electrons curve rather than flying straight. As their paths change, the electrons are forced to slow down. Some of their energy is converted into light – but not always as light our eyes can see.

Visible light is just one window into the full spectrum of electromagnetic waves. It has a short wavelength of a few hundred nanometres; for context, the average width of a single human hair is nearly 100,000 nanometres. Most of the light in supernova ‘bubbles’ has much less energy, with a wavelength of tens of centimetres or even metres. This particular type of light is called radio.

Radio astronomers have built just the right instruments to detect this kind of light emitted by supernovae. From the initial blast to the giant bubble-like structures they create as the explosion moves out through space, radio telescopes can detect these explosive supernova ‘bubbles’ expanding and eventually slowing down as they become a remnant.

We also see the brightness and energy of the light changing depending on how much material the shockwave sweeps up as it expands, or how strongly magnetised the surrounding material is. By studying the radio light generated by supernova remnants, we can learn when and how they formed, as well as what kind of dense objects the explosion left behind.

Australia’s view

Radio astronomy has a long, continuous history in Australia. We were one of the first countries in the world to use radio instruments to study celestial objects. The American radio engineer Karl Jansky, widely considered the founder of radio astronomy, first detected radio emission in 1933 from a dense region somewhere in the Milky Way. However, in 1954, CSIRO astronomers in Sydney figured out that the source of Jansky’s detection was located right at the centre of our galaxy.

As the field of radio astronomy developed, astronomers and engineers began exploring different types of telescopes that could be used to study a range of objects in the sky. Depending on the design of the instrument, we can use them to detect point-like radio sources – like the centres of distant galaxies – or diffuse clouds and filaments, like the boundaries of a supernova remnant. And using advanced image-processing techniques and modern telescopes like CSIRO’s ASKAP radio telescope, we can create images that show the beauty of the radio sky at both small and large scales.

Exploring our galaxy

Supernova remnants are stunning markers of the explosive history of our galaxy. And luckily for astronomers, we’ve already discovered hundreds of them. Observations of that white road of stars that runs across the sky, the Milky Way, have revealed a foamy sea of interstellar bubbles created by ancient supernovae.

The shapes of supernova remnants reflect the circumstances of their formation and their encounters with neighbouring objects, including cosmic clouds of gas and dust. Some appear symmetrical, while others take on distorted forms, moulded by interactions with nearby material or overlapping with other expanding bubbles. In fact, our whole solar system sits near the centre of a ‘superbubble’ – a vast cavity containing most of the stars visible to the naked eye. Scientists reckon the superbubble was carved out by the cumulative explosions of multiple supernovae over millions of years.

Radio astronomers estimate that as many as 1,500 supernova remnants may be still hiding in our galaxy undiscovered. New observations with highly sensitive radio instruments like ASKAP and the upcoming SKA telescopes will help us uncover these elusive interstellar bubbles, and reveal more details about the energetic processes that shaped the Milky Way.

Kovi Rose is an astrophysics PhD candidate at the University of Sydney who studies the radio light from nearby dwarf stars and distant supernovae.

Originally published by Cosmos as The ghosts of dead stars