By Emily Levesque, University of Washington
Supernovae are extremely energetic events, outshining entire galaxies and blasting many suns worth of mass into the vacuum of space. Our eyes see the visible light produced by supernovae, but fully understanding them requires observations across the entire electromagnetic spectrum. A supernova begins with a collapse, but it highlights the extreme physics going on during the deaths of these enormous stars.
Life of a Star
Imagine a star that’s at least eight times as massive as our Sun. Stars like this lead very different lives, following the “live fast, die young” philosophy: they only live for about 10 million years, as opposed to the 10 billion that our Sun will live. During their lives, these stars maintain a careful balance between the inward press of their own gravity and the outward push of energy produced by the nuclear fusion going on in their cores.
When a star first forms, its core will fuse hydrogen into helium. Once the core has exhausted its supply of hydrogen, it changes gears and begins fusing helium into carbon instead. This continues through heavier and heavier elements: carbon into oxygen, oxygen into silicon, and then silicon into iron. At this point, the star is now in trouble.
How a Supernova Explosion Occurs
Fusing all of these elements have produced energy, but fusing iron takes energy, so the delicate balancing act that has supported the star against its own gravity becomes disrupted, gravity wins, and in less than a second that iron core implodes. The outer layers of the star tumble after it and then bounce off that imploded core, producing a rebound shock that blasts the star’s material out into space.
That bounce and shock are what we see as a supernova explosion, and what remains from the supernova is either the compact core of the star—a tiny dense husk known as a neutron star—or the infamous end point of stellar death known as a black hole. The sudden appearance and slow fade are typical of supernova light curves.
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The Guest Star
The painting, drawn on the ceiling of an Ancestral Puebloan dwelling in Chaco Canyon, New Mexico, is thought to depict a supernova that happened nearly 1,000 years ago, in July of the year 1054. The astronomer who recorded this would have been treated to a shocking sight: a brilliant new star appearing near the horizon around dawn, so bright that it was comparable to the light of the nearby crescent moon and easily visible in the daytime sky.
Records of the supernova also exist from Japanese, Arabic, and Chinese astronomers, who referred to it as a ‘guest star’—it appeared suddenly in the sky, stayed bright and easily visible in the daytime for several weeks, and then faded away in the night sky over a couple of years.
Back in 1054, the telescope hadn’t been invented yet, so astronomers were left to rely on their own eyes and whatever notes they made. To the observers in 1054, this astonishing new star would simply have faded away, seeming to disappear into nothingness. It wasn’t until the mid-1700s that astronomers would identify what looked like a faint nebulous remnant of gas at that same location in the sky.
The object was eventually dubbed the Crab Nebula since sketches depicting the bright filaments of gas resembled leggy sea creatures. By the early-1900s, astronomers had begun to spot subtle changes in the nebula’s shape and to connect it with tales of the spectacular guest star of 1054. The guest star proved to be the luminous final moments of an enormous star dying 6,500 light-years away.
Tycho’s Supernova
In November of 1572, a supernova appeared near the constellation Cassiopeia and famously caught the notice of Tycho Brahe. Brahe is something of a larger-than-life figure to science historians, and it’s not hard to see why.
By 1572, Brahe had enjoyed the upbringing of a Danish nobleman, explored early interests ranging from alchemy to botany, bucked tradition by marrying a woman of common birth, built an observatory and alchemy lab at a nearby abbey, and famously lost part of his nose in a swordfight. Still, the supernova stood out as a landmark event in his life, captivating his imagination and cementing his desire to devote his scientific career to the study of astronomy.
As with the supernova in 1054, the remnant of the supernova in 1572 has since been spotted, around 9,000 light-years away, and photographed by modern telescopes capable of detecting the x-rays emitted by the expanding shock wave of the supernova. Today, it is commonly referred to as Tycho’s Supernova.
Kepler’s Supernova
Another supernova happened just 32 short years later, in 1604. Tycho Brahe, unfortunately, missed this one, having died just three years earlier, but the supernova was faithfully observed by his colleague, Johannes Kepler.
Like Brahe, Kepler identified what looked like a suddenly-appearing bright new star, this time in the constellation Ophiuchus, that outshone any other star in the sky and was visible in the daytime for weeks. Like the 1572 event, this 1604 discovery eventually came to be known as Kepler’s Supernova, with modern-day telescopes finding and studying the supernova remnants.
You can see the beautiful soap-bubble remnants of all three of these supernovae: from 1054, 1572, and 1604. Today, astronomers believe that these three supernovae were caused by a very different kind of explosion than the one that produced the Crab Nebula.
Common Questions about Supernova Explosion
Highly massive stars live shorter than the smaller ones, dying in a very dramatic way. At first, when the stars are born, they fuse hydrogen into helium. When the supply of hydrogen runs out, the core then fuses helium into carbon. This process continues through heavier elements such as oxygen, silicon, as well as iron. Once the core starts fusing iron, the star loses its balance and explodes, leaving behind a supernova explosion.
This supernova explosion was discovered by Tycho Brahe, hence the name.
In 1604, Johannes Kepler observed a supernova. He identified a suddenly appearing new star in the constellation Ophiuchus. Brighter and shinier than any other star in the sky, it was visible during the day for weeks. The newly discovered supernova was later known as Kepler’s Supernova.
