A view of the Crab Nebula composed of data from five observatories. Image: NASA
Supernovae, or stellar explosions signaling the death of a star, have always fascinated astronomers.
When the pandemic began in late 2019, Betelgeuse, a red giant star in the familiar constellation of Orion, suddenly darkened, sending astronomers scrambling for their telescopes. Many speculated that Betelgeuse was about to go supernova. But the dimming had a simpler explanation: condensation of dust in front of the star as seen from Earth.
Having put that false alarm behind them, astronomers are excitedly poring over a new study describing the first-ever detection of an electron-capture supernova. Designated SN2018zd, it was discovered in a galaxy 90 million lightyears away by amateur astronomer Koichi Itagaki in 2018, validating a 40-year-old theory about the existence of electron-capture supernovae.
Supernovae are stars in their death throes that become extremely hot red-giants, incredibly dense white-dwarfs, even denser neutron stars or finally blink out of existence as black holes, depending on how massive they are. The longevity of a star depends on its mass as it burns its hydrogen fuel to produce energy, which prevents it from collapsing in on itself due to the pull of gravity. The bigger the star, the quicker it dies, as more mass means more heat, and the hotter it gets the faster it uses up its fuel.
Temperatures of millions of degrees and high densities deep inside the star force four hydrogen atoms to fuse into one helium atom. In the process, 0.7% of the mass is converted into energy. (Albert Einstein famously summed this up in his equation, that energy is equal to mass times the speed of light squared). In the Sun, for instance, 600 million tonnes of hydrogen are converted to helium every second.
At some point, the star’s hydrogen reserves are fully converted to helium and it bloats into a red giant, allowing gravity to take over and ignite new nuclear reactions that convert the helium again into carbon. This continues until all the carbon is turned into oxygen and, eventually, into iron. The fusion of all elements lighter than iron releases more energy than it consumes, but elements from iron on the periodic table can’t be fused without consuming more energy. This is called the iron peak.
At this stage, the star’s iron-rich nucleus can no longer produce energy. The star then explodes outwards in a supernova, its brightness spreading millions of lightyears out into space[footnote]1 lightyear is 9.46 trillion km[/footnote] and its core gets cloaked by an expanding nebula of ionised gas and dust.
Supernovae are classified as ‘thermonuclear’ and ‘iron-core collapse’. Thermonuclear supernovae are produced by collapsing low-mass stars that have up to about eight-times the Sun’s mass. Iron-core collapse supernovae are produced by massive stars with more than ten-times the Sun’s mass, and whose remnants are incredibly dense neutron stars.
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In the early 1980s, however, Japanese astrophysicist Kenichi Nomoto theorised that there could also be another type of supernova apart from these two: the electron-capture supernovae.
Some stars stop fusion when their cores are made of oxygen, neon and magnesium. They do not have enough mass to produce iron and their core elements cannot fuse into heavier elements to prolong their lives. Instead, a process known as electron-capture smashes some of the electrons in the oxygen-neon-magnesium core into their atomic nuclei. With the electrons removed, the stellar core crumbles under its own weight to start an electron-capture supernova, with a neutron star at its centre.
Proof of electron capture supernovae remained elusive, however – until 2018, when Itagaki discovered SN2018zd, which Nomoto has described as “a wonderful case of the combination of observations and theory.”
Kenichi Nomoto theorised the existence of a third type of supernova in the 1980s, and called its recent discovery “a wonderful case of the combination of observations and theory.”
This significant discovery also points to another electron capture supernova that has been hiding in plain sight for a thousand years: SN 1054, which Asian sky-watchers recorded in 1054 AD! Astronomers have considered it to be a prime example of an electron-capture supernova. It was so bright at the time that it could be seen in daylight for weeks, and at night for a couple of years, before it faded into the Crab Nebula. Its extraordinary luminosity was probably due to ejecta from the supernova colliding with material cast off by the parent star – one of several characteristics that ties in neatly with the recently spotted SN2018zd.
Some five billion years ago, in one of the arms of the Milky Way galaxy, the shock waves from an exploding supernova spread heavy elements from the dead star into interstellar space. As the cloud of gas and dust eventually condensed, the resulting nebula began to spin faster and faster[footnote]To conserve momentum[/footnote], flattening itself into a disk.
Through millions of years, gravity pulled the bulk of this mass towards its centre until the ever-increasing heat and density eventually burst into thermonuclear brilliance: the Sun was born. Around it, scores of elements in the gas and dust formed clumps – proto-planets and their moons that would become the Solar System.
Prakash Chandra is a science writer.