Supernovae are some of the most violent events in our galaxy. They, along with comets and planets, are one of the few astronomical events seen and speculated about since the most ancient times. Like comets, their sudden appearance in the night sky was often a source of alarm and panic.
Though modern science has unlocked many of their secrets, there are still many open questions about what exactly happens when a giant star dies. Catching a supernova in its first moments is hard. We thus have little understanding of what happens in a star just before it starts to explode.
This is one reason why the nearby supergiant Betelgeuse gets so much attention. Not only do astronomers expect the star to undergo a supernova sometime in the next few thousand years, it is also close enough for us to get a clear view of what happens. The essay that follows gives some idea of what we might expect to see when it does.
The new star was brighter than any other, outshining even Venus. Records tell us it was as bright as the Moon, even during the day, and at night cast shadows in the streets. It was, astrologers and priests agreed, a sign from God. But what did it mean? Did it foretell plague and famine, as priests across the Middle East argued? Or was it a symbol of God’s favour, as the Chinese emperor claimed?
Whatever the meaning, the new star gradually faded away. After dramatically shining through the summer of the year 1006 AD, the star was lost altogether several years later. This strange event was not without precedent. Ancient astronomers even had a name for such visitors— guest stars. The ancients knew that the night sky changed, albeit rarely, and watched the skies carefully for heavenly signs.
Thanks to their detailed observations, we now have a record of the night sky going back thousands of years. Modern astronomers have searched through these records, assigning scientific explanations to the strange sightings of the past. Guest stars, like that of 1006 AD, are most commonly supernova, the final catastrophic moments of giant stars.
Stars spend most of their lives in a delicate balance. The tremendous weight of the star pulls inwards, trying to crush everything together in a single central point. As it does, this creates an enormous pressure in the core of the star. This pressure is so intense that individual atoms are forced together, triggering nuclear explosions. The shockwaves from these explosions push the star apart again, counterbalancing its weight.
When stars are born they are formed mostly of hydrogen atoms. Hydrogen is the lightest and simplest of all the elements and is by far the most common substance in the Universe. But over time, due to the constant nuclear reactions in the star’s core, the star slowly converts its supply of hydrogen into heavier elements, like helium and lithium.
This situation can’t last forever. Eventually the star starts to run out of hydrogen. What happens next depends on the size of the star. The bigger and heavier the star is, the larger the pressure trying to crush it together. If the star is big enough, it can force another round of nuclear reactions, colliding bigger and bigger atoms together. But smaller stars, like our own Sun, can’t do this.
Instead, as the nuclear reactions gradually cease, the star cools. The outer layers expand, forming a red giant - big but cool. They slowly escape into space, forming glowing clouds of gas, like those seen in the beautiful Cat’s Eye Nebula. The inner core remains behind and, over billions of years, fades away. When stars like our Sun die, they go quietly.
Our Sun is a minnow compared to some of the beasts out there. Betelgeuse, a red supergiant star, and one of the brightest stars in the night sky, is eleven times bigger. Rho Cassiopeiae, three and a half thousand light years from Earth, is forty times bigger than the Sun, and perhaps half a million times brighter. Eta Carinae, a star so big it probably shouldn’t even exist, is two hundred times bigger than the Sun and a staggering five million times brighter.
These stars are the true giants of our galaxy. When they die, often after a short and chaotic life, they don’t go quietly. Indeed, for a few brief moments they may outshine every other star in the Milky Way combined.
When a supergiant star exhausts its hydrogen supplies, it shifts to burning other elements. Lighter elements, like helium and lithium go first, but as these fuels run out the star switches to heavier and heavier elements. Each step up to heavier elements releases less energy in nuclear reactions, providing less energy to fight against gravity, and causing the star to gradually collapse inwards.
The end is reached when the star starts trying to burn iron. Iron has the most stable nuclear properties of all the elements, and releases almost no energy to balance the weight of the star. The result is catastrophic. Within a few hours the entire star collapses. Material rapidly flows into the centre of the star, causing a gigantic thermonuclear explosion. The shockwave blows away the outer layers of the star, releasing huge amounts of energy as it does so.
From Earth, the first sign of a distant supernova is a sudden blast of neutrinos. These superlight particles originate in the core of the collapsing star, and carry enough energy to ignite the rest of the supernova. Minutes to hours later the light from this explosion reaches Earth. For a few weeks the dying star may outshine its entire galaxy, and, depending on how close it is to Earth, might shine as bright as the full moon.
With the light comes a blast of intense gamma radiation. The supernova of 1006 AD, now known to have occurred around seven thousand light years from Earth, left traces of this radiation buried in the polar ice sheets. Fortunately that supernova was at a safe distance — scientists have calculated that a really close supernova could be hazardous to life on Earth.
Such nearby supernova are luckily rare, but evidence suggests one may have happened 450 million years ago. If it really did happen, then the radiation blast would have destroyed the ozone layer in seconds, leaving the planet dangerously exposed to the Sun’s radiation. Within years, 80% of life on Earth could have died.
After the initial explosion, the core of the old star survives. Depending on the size of the star, it may form an ultra dense remnant known as a neutron star, or, alternatively, a black hole. The outer layers of the star, blown off into space, create an expanding bubble of debris. These bubbles survive for tens of thousands of years, creating spectacular visions in the night sky.
Though a supernova might seem like the ultimate form of violent destruction, they play a vital role in seeding the Universe with life. The force of the explosion is one of the only places where heavier elements, including gold, mercury and lead, can be created. Without these powerful explosions, life as we know it on Earth could not exist.
The shockwaves from supernova also help trigger waves of star birth. When such shockwaves pass through clouds of gas, they often trigger gravitational disturbances. Long after the dying star is gone its ashes coalesce into new stars, creating a cosmic cycle of life and death.
The supernova of 1006AD, seen across the world and remembered as a guest star, was probably the brightest seen in recorded history. Astronomers now think the explosion was caused by colliding white dwarf stars — themselves the remnants of earlier dying stars. Supernova didn’t turn out to be signs from the gods, as the ancients thought. But perhaps they are a sign of something else — a reminder of a cosmic dance on a magnificent scale.
So much reporting around health, science and space exploration is unrealistic, hyperbolic and misleading. These are complicated topics, and there are often no easy or straight forward answers. Instead what is needed is analysis, discussion and an exploration of the possible ways forward.
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