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Have you ever thought about the energetic explosions that occasionally occur โ both within our own galaxy and in galaxies all across the Universe โ and wondered, โwhere did all that energy come from?โ Throughout human history, what seemed to be new stars have populated our sky at random:
suddenly appearing as if from nowhere,
brightening and reaching a peak magnitude,
and then slowly fading away to become invisible to the eye once again.
Since the advent of the telescope, weโve learned that these events, collectively referred to as supernovae, are common, and most frequently correspond to the cataclysmic death of a massive star as its central core collapses.
Even though these supernova events can shine brilliantly, with typical intrinsic brightnesses that are about the equivalent of ten billion suns, that only represents a tiny amount of the energy liberated during a core-collapse event: about 1% of the total energy generated. A whopping 99% of that supernova eventโs energy is practically invisible: emitted in the form of neutrinos, a ghostly species of particle that interacts so rarely, the theorist who invented it quipped, โI have done a terrible thing. I have postulated a particle that cannot be detected.โ
And yet, weโre certain that neutrinos are indeed the culprit behind where a core-collapse supernovaโs energy goes. Hereโs the science behind this remarkable story.
This cutaway showcases the various regions of the surface and interior of the Sun, including the core, which is the only location where nuclear fusion occurs. As time goes on and hydrogen is consumed, the helium-containing region in the core expands and the maximum temperature increases, causing the Sun to โcross the main sequenceโ as its energy output increases. The balance between the inward-pulling gravity and the outward-pushing gas pressure, only slightly augmented by radiation pressure (and mostly in higher-mass stars), determines the size and stability of a star, while the coreโs size, temperature, and element abundance determines the rate and species of fusion inside.
Credit : Wikimedia Commons/KelvinSong
Inside of every star, a set of forces fight against one another. On one hand, thereโs simply gravitation: the attractive force between all particles with mass and/or energy, which serves to draw everything together. In a star, that serves to pull all the layers of the star inwards: towards the center. However, stars generally donโt collapse, because thereโs something else pushing back against gravity: pressure.
While most people assume (and many even teach) that itโs radiation pressure pushing back โ pressure generated by the high-energy photons produced from nuclear fusion reactions โ thatโs actually a misconception . Radiation pressure, in most stars (including the Sun), plays only a minuscule role in pushing back against the inwards force of gravity, especially during the hydrogen fusion phase thatโs dominant in our Sun today.
Instead, itโs gas pressure, or the kinetic energy of the individual quanta making up the starโs interior, that push back against the gravitational force. This gas pressure isnโt exactly the way gas pressure works here on Earth, where itโs a gas of neutral atoms. Instead, in a starโs interior, the โgasโ thatโs present is a quantum mechanical gas: a gas of electrons, as nearly all of the atoms inside of a star are fully ionized, and split apart into free electrons and atomic nuclei.
The anatomy of the Sun, including the inner core, which is the only place where fusion occurs. Even at the incredible temperatures of 15 million K, the maximum achieved in the Sun, the Sun produces less energy-per-unit-volume than a typical human body. The Sunโs volume, however, is large enough to contain over 10ยฒโธ full-grown humans, which is why even a low rate of energy production can lead to such an astronomical total energy output. It takes approximately 50 million years for the Sun to go from having no fusion in its core, during the protostar phase, until it reaches this equilibrium state: where fusion provides 100% of the energy for the star, where gravitational contraction ceases.
Credit : NASA/Jenny Mottar
The starโs core โ the innermost part of the star โ is where nuclear fusion actually occurs. In the early stages of a starโs life, the majority of fusion that occurs is hydrogen fusion: where a net of four hydrogen nuclei come together to form a helium nucleus, while the gas (hot electrons) bounce around between them, preventing that core from gravitationally contracting.
Eventually, however, the core begins to run out of hydrogen, as it converts into helium. When this occurs, the material within the core begins to redistribute, with the innermost core becoming denser and hotter. If the overall star is massive enough, temperatures will then reach high enough so that helium fusion can begin: where three helium nuclei come together to form carbon, while hydrogen fuses into helium in a shell surrounding it.
This process continues as long as your star is sufficiently massive to reach high enough core temperatures, moving on to:
carbon fusion after helium,
neon fusion after carbon,
oxygen fusion after neon,
silicon fusion after oxygen,
until the innermost core becomes filled with nuclei like iron, nickel, and cobalt. Those are the most stable elements on the periodic table, so once the core becomes filled with those elements, no further fusion reactions can occur.
Artistโs illustration (left) of the interior of a massive star in the final stages, pre-supernova, of silicon-burning. (Silicon-burning is where iron, nickel, and cobalt form in the core.) A Chandra image (right) of the Cassiopeia A supernova remnant today shows elements like iron (blue), sulfur (green), and magnesium (red). Ejected stellar material can glow due to heat in the infrared for tens of thousands of years, and the ejecta from supernovae can be asymmetric and can have segregated elements within it, as shown here. In the right environment, this asymmetric material can be unevenly incorporated into future generations of stars.
Credits : NASA/CXC/M.Weiss (illustration, left) NASA/CXC/GSFC/U. Hwang & J. Laming (image, right)
This leads to something really interesting: while the core was fully supported by gas pressure at lower temperatures and for light elements, that radiation pressure becomes more and more important at higher temperatures and for heavier elements. Therefore, when nuclear fusion ceases in the core โ because youโve run out of the nuclear fuel that enables those fusion reactions โ the core contracts and heats up. This is why you can progress from one type of fusion to another, up and up the periodic table, as long as there are more stable elements to create and, hence, more energy to release via Einsteinโs famed E = mcยฒ .
When you reach iron in the core, however, there is no more radiation, and so gravitation starts to win. The core contracts, causing it to heat up, but with only the (insufficient) gas pressure to fight gravity, contraction continues. This makes the core heat up further and become denser: a dense collection of heavy nuclei, amidst a hot gas of electrons.
This might not seem like a recipe for disaster, but it actually is. What happens when you have extreme densities, extreme temperatures, and lots of atomic nuclei together with electrons?
This illustration shows 5 of the main types of radioactive decays: alpha decay, where a nucleus emits an alpha particle (2 protons and 2 neutrons), beta decay, where a nucleus emits an electron, gamma decay, where a nucleus emits a photon, positron emission (also known as beta-plus decay), where a nucleus emits a positron, and electron capture (also known as inverse beta decay), where a nucleus absorbs an electron. These decays can change the atomic and/or mass number of the nucleus, but certain overall conservation laws, like energy, momentum, and charge conservation, must still be obeyed. Electron capture is the dominant process in a core-collapse supernova, where a massive star dies and becomes a neutron star or black hole.
Credit : CNX Chemistry, OpenStax/Wikimedia Commons
Very efficiently, you begin to trigger a particle physics process known as electron capture : converting the protons and electrons in that region into neutrons, with a neutrino (specifically, an electron neutrino) produced as a by-product. It might seem like a relatively benign process, especially considering that various nuclear reactions (involving fusion) have been occurring within these stellar cores for extremely long periods of time, but thereโs nothing benign about electron capture at all. It actually turns out to be a destabilizing process that leads, very rapidly, to the destruction of the star entirely.
Why is that?
Because the only thing that was holding the starโs core up against gravitational collapse was the electrons that were present. As the core contracts and electron capture begins, the electron density drops, and so the gas pressure drops, causing the core to contract even faster, as thereโs less of a force opposing gravitational collapse. The increased speed leads to higher temperatures, greater densities, and faster electron capture, as protons and electrons fuse together into neutrons. In the span of mere seconds, the core has contracted down from somewhere around the size of a planet to somewhere around just 10-12 km in radius.
Stars that are at least 8 solar masses at birth can build up extremely heavy elements in their core, igniting carbon fusion and then subsequent stages of fusion, eventually undergoing core collapse (and the electron capture process) until a neutron star is formed. At sufficiently high masses, those neutrons may collapse further to a black hole, and other, exotic fates are also possible depending on the specific configuration of the predecessor system.
Credit : H. Suzuki, Progress of Theoretical and Experimental Physics, 2024
This process turns the core โ previously composed of protons, neutrons, and electrons โ into a solid ball of neutrons: one thatโs denser than a uranium nucleus, with a full solar mass or more of matter locked up in something thatโs merely the size of a single large mountain on Earth. And sure, with the core contracting so rapidly like that, the surrounding matter falls onto it, undergoes an enormous flash of nuclear fusion, and then gets blown outwards entirely. As the core collapses into what becomes a neutron star, the rest of the (now-former) starโs outer layers experience a flash fusion reaction, rebound off of that core, and explode outwards.
Thatโs where the puzzle comes from. Consider the following.
We know how nuclear fusion works, including the process of electron capture.
We know how mass converts into energy: via Einsteinโs E = mcยฒ .
We know how gravitational potential energy works, and that energy must be conserved when a collection of matter goes from a large, diffuse distribution to a small, compact distribution.
And we know how to measure the light from a variety of events, such as supernovae, across the electromagnetic spectrum.
When we measure the light from a supernova, whether from the nearby Universe or the far reaches of the cosmos, we can infer the amount of energy thatโs produced by a supernova event in the form of all forms of light, and compare it to the energy thatโs theoretically required by the physics of the event.
The two largest, brightest galaxies in the M81 Group, M81 (right) and M82 (left), are shown in the same frame in these 2013 and 2014 photos. In 2014, M82 experienced a supernova, visible in the 2014 (blue) image just above the galactic center.
Credit : Simon in the Lakes
Not only donโt they match, but the mismatch is huge: the energy required by the physics of the event is a full 100 times greater than the energy thatโs emitted across all forms of light. Everything that a telescope can see from a supernova makes up only 1% of the total energy emitted by it.
So whereโs the rest?
The answer is simple: in the form of neutrinos. Remember that neutrinos are light: extremely low in mass. Whereas a proton or neutron is about 2000 times heavier than an electron, an electron is at least 4,000,000 times heavier than a neutrino is. When you have an event like electron capture, what happens is a proton and an electron combine to make a neutron and a neutrino (specifically, an electron neutrino).
But because the newly-produced neutron is heavy, and thatโs typically exacerbated by virtue of the neutron being bound together with other nuclear particles as well, nearly all of the kinetic energy (i.e., the energy of motion) from the electronโs capture goes into the neutrino, rather than the neutron. When you have a heavy product and a light product in a reaction that needs to conserve both energy and momentum, itโs the light product that inevitably carries most of the energy away, even though the heavy and light product typically have equal-and-opposite momenta.
In the inner regions of a star that undergoes a core-collapse supernova, a neutron star begins to form in the core, while the outer layers crash against it and undergo their own runaway fusion reactions. Neutrons, neutrinos, radiation, and extraordinary amounts of energy are produced, with neutrinos and antineutrinos carrying the majority of the core-collapse supernovaโs energy away. Whether the remnant becomes a neutron star or black hole, ultimately, depends on how much mass remains in the core during this process.
Credit : TeraScale Supernova Initiative/Oak Ridge National Lab
And then, because neutrinos hardly interact with matter at all โ it would take about a light-yearโs worth of solid lead to have a 50/50 shot of a neutrino interacting once โ the neutrinos just stream out of that ball of neutrons, unimpeded, during the supernova process. This was summarized very nicely by physicist Hideyuki Suzuki , who wrote:
โthe mean free path [how far they can travel before interacting with another particle] of neutrinos is much longer than those of other particles. As a result, neutrinos carry the energy and drive the evolution of the core.โ
Whereas the observed energy of supernova explosions (in light, and also in the motion of the material composing a supernova remnant) is enormous, at about 10 44 J (about the energy produced by the Sun over a timescale of a billion years), the total energy produced by a core collapse supernova is more like 10 46 J . That mismatch, therefore, leads to the expectation that 99% of the energy from core-collapse supernovae must be carried in the form of neutrinos.
Webbโs NIRCam (Near-Infrared Camera) captured this detailed image of the remnant of SN 1987A, which has been annotated to highlight key structures. At the center, material ejected from the supernova forms a keyhole shape. Just to its left and right are faint crescents newly discovered by Webb. Beyond them an equatorial ring, formed from material ejected tens of thousands of years before the supernova explosion, contains bright hot spots. Exterior to that is diffuse emission and two faint outer rings.
Credit : NASA, ESA, CSA, Mikako Matsuura (Cardiff University), Richard Arendt (NASA-GSFC, UMBC), Claes Fransson (Stockholm University), Josefin Larsson (KTH); Processing: Alyssa Pagan (STScI)
We finally got the opportunity to test this directly from an unexpected source: when a core-collapse supernova went off in the Large Magellanic Cloud: a nearby galaxy located just 165,000 light-years away. Known as SN 1987A , it is the closest supernova to be observed by humans since the year 1604, and at peak brightness, despite being in a different galaxy, it reached the threshold that it could have even been seen by the naked eye. In the subsequent 39 years, its remnant has been observed many times, including by an array of space telescope like Hubble and Chandra, that have tracked its expansion and evolution, leading to an extraordinary leap in knowledge about what occurs in the aftermath of core-collapse supernovae.
However, about four hours before the light arrived, neutrinos arrived in a variety of detectors on Earth, with no other explanation and no advance warning. Those neutrinos, corresponding to a โburstโ over a span of around 10 seconds, represent the neutrinos emitted as part of the electron capture process from the core-collapse supernova. When we infer the mass of the core remnant and the total neutrino flux, and we combine that with the energy of the collected neutrinos that we measured, we find that it does, in fact, add up to the other 99% of the energy we were expecting to be emitted in these stellar cataclysms.
Three different detectors observed the neutrinos from SN 1987A, with KamiokaNDE the most robust and successful. The transformation from a nucleon decay experiment to a neutrino detector experiment would pave the way for the developing science of neutrino astronomy. The light from the supernova would not arrive until hours later.
Credit : Riya and Astroriya/Wikimedia Commons
Although there are some details that are slightly different from the picture that was painted here โ the majority of supernova neutrinos that are produced come from interactions of those neutrinos with the neutron star cores, producing neutrinos of all three species rather than just the electron neutrinos produced by the electron capture process โ the fact is that core-collapse supernovae are unique as the strongest source of all types of neutrinos with around 10 MeV of kinetic energy apiece: about 20 times the rest mass energy of the electron that was captured during the process that led to their creation.
Our neutrino detector capabilities have advanced so significantly since those first astrophysical neutrinos were detected back in 1987 that if a core-collapse supernova were to occur within the Milky Way today, instead of the dozens of neutrinos we detected back in 1987, weโd see millions of neutrinos. Thanks to facilities like Hyper Kamiokande and the IceCube Neutrino Observatory , the very next core-collapse supernova that occurs anywhere within the Local Group of galaxies will immediately become the most information-rich event, in terms of neutrinos, that humanity has ever seen or measured. Despite the impressiveness of a supernova, only 1% of all of the energy it produces appears in the form of light of any type. The rest is all in neutrinos: the most elusive, yet still directly detectable particle, in all of existence.
This article 99% of a core-collapse supernovaโs energy is invisible is featured on Big Think .
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