Cosmic events are the universe’s most extreme phenomena, spanning everything from the death of a single star in a supernova to the collision of entire galaxies over billions of years. Some release more energy in seconds than our Sun will produce in its entire lifetime. Others send ripples through spacetime itself, detectable only with the most sensitive instruments ever built. What ties them together is scale: each reshapes its corner of the cosmos and, in some cases, leaves traces on Earth that scientists can measure millions of years after the fact.
Supernovae and How Stars Die
When a massive star exhausts its nuclear fuel, it can no longer support its own weight. The core collapses inward in a fraction of a second, and the resulting rebound blows the outer layers of the star into space in a core-collapse supernova. Despite decades of study, several aspects of the mechanism that drives these explosions remain uncertain, and researchers continue to investigate the role of turbulence and non-spherical structure in three-dimensional simulations.1Europe PMC. The mechanism(s) of core-collapse supernovae These events are violent enough to forge elements heavier than iron and scatter them across interstellar space, seeding future generations of stars and planets with the raw materials for rocky worlds and, eventually, life.
Not all supernovae work this way. Type Ia supernovae involve white dwarfs, the dense remnants of smaller stars, rather than massive ones. Spectroscopic and photometric evidence indicates that these are thermonuclear explosions of white dwarfs that have accumulated too much material from a companion star or collided with another white dwarf. Their relatively uniform brightness makes them useful as “standard candles” for measuring cosmic distances, helping astronomers determine the expansion rate of the universe.2PubMed. Type Ia supernovae: their origin and possible applications in cosmology One proposed trigger involves head-on collisions of white dwarfs in triple-star systems, where simulations show that collisions of typical white dwarfs produce nickel-56 yields spanning the wide range observed in the majority of Type Ia events.3The Astrophysical Journal Letters. Head-on Collisions of White Dwarfs in Triple Systems Could Explain Type Ia Supernovae
At the most extreme end sit pair-instability supernovae. These involve very massive stars, over roughly 100 times the mass of the Sun, where conditions in the core convert high-energy photons into pairs of matter and antimatter particles, robbing the star of the pressure it needs to hold itself up. If enough energy is generated, the entire star is blown apart in a single explosive episode, leaving behind no remnant at all.4arXiv. Pair-instability evolution and explosions in massive stars Unlike core-collapse events, the entire stellar mass is ejected.5ResearchGate. Pair-Instability Supernovae No confirmed observation of a pair-instability supernova has been made yet, though several candidate events have been proposed.
Gamma-Ray Bursts and Hypernovae
Gamma-ray bursts are the most energetic explosions observed in the universe, releasing in seconds the energy our Sun will emit over its entire lifetime. The “long” variety, lasting more than a couple of seconds, has been linked directly to a special class of supernovae. The connection was established through several well-observed cases in which a gamma-ray burst was accompanied by an unusually powerful supernova. These explosions are thought to result from the prompt collapse to a black hole of the core of a massive star, around 40 times the Sun’s mass, that had already shed its outer layers of hydrogen and helium. The accompanying supernovae showed strong oxygen lines, classifying them as Type Ic, and their energies were much larger than those of typical supernovae, earning them the label “hypernovae.”6New Astronomy Reviews. Hypernovae, gamma-ray bursts, and first stars
What determines whether a collapsing massive star produces a standard supernova, a hypernova, or a hypernova with an accompanying gamma-ray burst? Variations in the angular momentum of the star at the moment of collapse appear to be a key factor, potentially explaining the diversity among these events and their possible association with gamma-ray bursts.7The Astrophysical Journal. The Proto-Neutron Star Phase of the Collapsar Model and the Route to Long-Soft Gamma-Ray Bursts and Hypernovae In other words, how fast the star’s core is spinning when it dies shapes the character of the explosion. A rapidly rotating core can funnel jets of material outward at close to the speed of light, producing the tightly focused beam of gamma rays we detect from across the observable universe.
When Spacetime Itself Shakes
On September 14, 2015, the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) simultaneously detected a transient signal that swept upward in frequency from 35 to 250 Hz. It matched the waveform predicted by general relativity for two black holes spiraling into each other and merging. The signal was so clean that the false alarm rate was estimated to be less than one event per 203,000 years. It was the first direct detection of gravitational waves and the first observation of a binary black hole merger.8PubMed. Observation of Gravitational Waves from a Binary Black Hole Merger
Gravitational waves are ripples in the fabric of spacetime, produced whenever massive objects accelerate. Every orbiting pair of stars produces them, but only the most violent events generate waves strong enough for instruments to detect. Since that first detection, dozens more have followed, including mergers of neutron stars. The 2017 detection of a neutron-star merger, GW170817, was accompanied by signals across the electromagnetic spectrum, marking the dawn of “multimessenger” astronomy, where the same event is observed through completely different channels.
Simulations of neutron-star mergers consistent with GW170817 show that these collisions eject material rich in heavy elements forged by rapid neutron capture, producing a visible afterglow called a kilonova. Some of that ejected material moves at more than 60 percent the speed of light, giving rise to broadband radio and X-ray emission on timescales of years after the merger.9The Astrophysical Journal. GRMHD Simulations of Neutron-star Mergers with Weak Interactions: r-process Nucleosynthesis and Electromagnetic Signatures of Dynamical Ejecta These events are now thought to be a major source of elements like gold, platinum, and uranium in the universe.
Gravitational waves also carry information from the very earliest moments after the Big Bang. Primordial gravitational waves, if detected through their imprint on the polarization of the cosmic microwave background, would provide direct evidence for inflation, the brief period of exponential expansion thought to have occurred in the universe’s first fraction of a second.10International Journal of Modern Physics A. The Polarization of the Cosmic Microwave Background Due to Primordial Gravitational Waves That search remains ongoing, with current instruments setting increasingly tight limits but no confirmed detection yet.
Fast Radio Bursts
First discovered in 2007 in archival data from a radio telescope, fast radio bursts are millisecond-long pulses of radio waves arriving from distant galaxies. Some occur just once; others repeat. Their brevity and immense distances imply staggering energy output, yet their origin has been one of the hottest puzzles in astrophysics. The leading explanation ties them to magnetars, neutron stars with extraordinarily strong magnetic fields. Persistent radio sources associated with some repeating fast radio bursts are attributed to synchrotron emission from relativistic charged particles in a magnetar wind nebula, powered by the star’s spin-down or internal magnetic field decay.11The Astrophysical Journal. Persistent Radio Sources Associated with Fast Radio Bursts: Implications from Magnetar Progenitors
This connection was strengthened in 2020, when a magnetar inside our own galaxy produced a burst bright enough to have been detectable as a fast radio burst if it had occurred in another galaxy. Still, not every fast radio burst neatly fits the magnetar model, and researchers continue to debate whether multiple mechanisms might be at play. Some one-off bursts, for instance, could result from catastrophic events like the collapse of a magnetar into a black hole rather than from repeated magnetic activity.
Stars Torn Apart by Black Holes
Stars that orbit too close to a supermassive black hole can be ripped apart by tidal forces, producing a luminous flare called a tidal disruption event. These events provide windows into the centers of galaxies at scales that are otherwise impossible to observe outside our own galactic neighborhood. They offer a unique opportunity to study supermassive black holes under feeding conditions that change dramatically over weeks to months, regularly reaching extreme mass inflow rates.12arXiv. Tidal Disruption Events
About half the star’s mass typically falls onto the black hole, while the other half is flung outward. The infalling material forms a temporary accretion disk that can outshine the entire host galaxy for months. Since the first candidates were identified in the 1990s, sky surveys have been detecting them at an accelerating rate, now finding dozens per year. Each one reveals information about the black hole’s mass, spin, and the stellar environment at the galactic center. Some tidal disruption events also launch powerful jets of material, making them visible at radio wavelengths long after the initial optical flare has faded.
Cosmic Rays and the Neutrino Connection
Cosmic rays are not rays at all but high-energy particles, mostly protons and atomic nuclei, that slam into Earth’s atmosphere from all directions. Most originate from supernova remnants within our galaxy, but the most extreme, with energies above about a billion billion electron volts, must come from far more powerful sources. One proposed mechanism, known as “espresso” acceleration, suggests that these ultra-high-energy cosmic rays are produced when lower-energy galactic cosmic rays penetrate the relativistic jets of powerful active galactic nuclei sideways, receiving a one-shot energy boost by a factor related to the square of the jet’s speed. For powerful blazars with high enough jet speeds, this process could accelerate particles to more than 10²⁰ electron volts.13The Astrophysical Journal Letters. “ESPRESSO” Acceleration of Ultra-high-energy Cosmic Rays Detailed three-dimensional simulations of this process have corroborated the picture, finding results consistent with observed properties of ultra-high-energy cosmic rays in terms of their energy distribution, chemical composition, and arrival directions.14The Astrophysical Journal. Bottom-up Acceleration of Ultra-high-energy Cosmic Rays in the Jets of Active Galactic Nuclei
Neutrinos provide a complementary view. Because they barely interact with matter, neutrinos travel in straight lines from their sources, undeflected by magnetic fields. In September 2017, the IceCube detector at the South Pole registered a neutrino with an energy of about 290 trillion electron volts. Its arrival direction was consistent with the location of a known blazar, TXS 0506+056, which was observed to be in a flaring state at the time, suggesting that blazars may be a source of high-energy neutrinos.15PubMed. Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A This was the first roughly three-sigma association between a high-energy neutrino source and an astrophysical object.16The Astrophysical Journal. A Multimessenger Picture of the Flaring Blazar TXS 0506+056: Implications for High-energy Neutrino Emission and Cosmic-Ray Acceleration More recently, two additional blazars at distances greater than redshift one have been found to be correlated with IceCube neutrino events, strengthening the case that these jets are particle accelerators of extraordinary power.17The Astrophysical Journal. Leptohadronic Multimessenger Modeling of Two High-redshift (z > 1) Neutrino Emission Blazar Candidates
Solar Storms and Space Weather
Not all consequential cosmic events happen far away. The Sun regularly produces eruptions that can directly affect Earth. Coronal mass ejections, or CMEs, are vast clouds of magnetized plasma hurled from the Sun’s surface. When they hit Earth’s magnetic field, the resulting geomagnetic storm can disrupt power grids, satellite operations, and radio communications. An extreme geomagnetic storm comparable to the Carrington event of 1859 could have a significant impact on modern infrastructure, particularly power grids.18Journal of Space Weather and Space Climate. Comparison of the modelled geoelectric fields of the Carrington and Halloween storms
In May 2024, the Sun produced the largest geomagnetic storm in two decades. It was driven not by a single eruption but by a pileup of multiple CMEs from the same hyperactive solar region. Four halo CMEs launched on May 8–9 interacted during their transit to Earth, amplifying the resulting magnetic field to unusual levels. Researchers described it as a “perfect storm” scenario, a combination of circumstances producing an event of unusual magnitude.19The Astrophysical Journal Letters. A Pileup of Coronal Mass Ejections Produced the Largest Geomagnetic Storm in Two Decades Analysis of heavy ions in the solar wind during such storms has revealed that increases in the average charge state of elements like oxygen, magnesium, silicon, and iron correlate with storm intensity, giving forecasters another tool for gauging severity.20The Astrophysical Journal. The State of Solar Wind Heavy Ions in Interplanetary Coronal Mass Ejection–Driven Geomagnetic Storms
Historical records preserved in unexpected places reveal even more extreme solar events. Tree rings record spikes in carbon-14 produced when intense bursts of radiation strike Earth’s atmosphere. One such spike, dated to AD 993–994, shows a rapid increase of about 9 percent in carbon-14 content within a single year, an event about 0.6 times the size of a similar spike in AD 775.21Nature Communications. Another rapid event in the carbon-14 content of tree rings These so-called Miyake events, named after the researcher who identified them, far exceeded any solar storm in the modern record. If an event of that magnitude occurred today, the consequences for electronics and power infrastructure could be severe.
Asteroid Impacts and Planetary Defense
Of all cosmic events, asteroid impacts are the ones most directly connected to everyday human survival. Large impacts have shaped Earth’s geological and biological history, with the most famous example being the asteroid that contributed to the extinction of non-avian dinosaurs roughly 66 million years ago. The environmental effects of major impacts extend far beyond the crater itself. The ejecta plumes from large impacts may produce enough nitric oxide from shock-heated air to damage the ozone layer.22Reviews of Geophysics. Environmental perturbations caused by the impacts of asteroids and comets Combined with dust blocking sunlight, global fires, and acid rain, a sufficiently large impact can trigger planet-wide ecological collapse.
For the first time in history, humanity has demonstrated the ability to do something about this threat. In September 2022, NASA’s DART spacecraft deliberately crashed into Dimorphos, a small asteroid orbiting a larger one, and successfully altered its orbit. The mission demonstrated that kinetic impactor technology is a viable technique to potentially defend Earth if necessary.23Nature. Successful kinetic impact into an asteroid for planetary defence Follow-up analysis confirmed the capability of successfully conducting such deflection missions.24The Planetary Science Journal. Measurability of the Heliocentric Momentum Enhancement from a Kinetic Impact: The Double Asteroid Redirection Test (DART) Mission The key limitation is time: deflecting an asteroid by a tiny amount years in advance can shift its path enough to miss Earth, but the same nudge delivered months before impact would barely matter. Early detection remains the critical bottleneck.
Traces of Distant Explosions on Earth
Supernovae happening tens of light-years away might sound academic, but they leave physical evidence on our planet. Iron-60, a radioactive isotope with a half-life of about 2.6 million years, is predominantly produced in massive stars and ejected in supernova explosions. A highly significant increase in iron-60 concentration was found in a deep-sea manganese crust, dated to about 2.8 million years ago, compatible with a supernova at a distance of a few tens of parsecs.25PubMed. 60Fe anomaly in a deep-sea manganese crust and implications for a nearby supernova source Broader surveys of deep-sea archives from all major oceans confirmed that the signal is global, extended in time, and of interstellar origin from multiple events, with iron-60 influxes dated to about 1.5–3.2 million years ago and again at 6.5–8.7 million years ago.26PubMed Central. Recent near-Earth supernovae probed by global deposition of interstellar radioactive (60)Fe Even more recently, a continuous interstellar iron-60 influx has been detected on Earth over the past roughly 33,000 years, possibly a late echo of those same million-year-old supernovae, with iron-60-bearing dust particles still drifting through interstellar space.27PubMed Central. 60Fe deposition during the late Pleistocene and the Holocene echoes past supernova activity
How dangerous are nearby supernovae to life on Earth? Modeling suggests the answer is “less than you might fear.” Ionizing radiation from a supernova triggers chemical reactions in the stratosphere that break down ozone, but for an event at the distances inferred from the iron-60 record, the maximum global ozone depletion is on the order of about 10 percent, comparable to the ozone depletion caused by human-made chemicals. Most of that depletion occurs at high latitudes. Researchers concluded that it is unlikely such ozone changes would have a major impact on the biosphere.28Communications Earth & Environment. Earth’s atmosphere protects the biosphere from nearby supernovae Earth’s atmosphere, it turns out, is a remarkably effective shield. A supernova would need to occur much closer, perhaps within about 25 light-years, to pose a serious threat to complex life.
Cosmic Rays and Earth’s Climate Over Deep Time
Beyond iron-60, another long-running debate concerns whether the flux of galactic cosmic rays influences Earth’s climate over timescales of tens of thousands to hundreds of thousands of years. The idea is that cosmic rays help seed cloud formation in the lower atmosphere, and that variations in cosmic ray intensity, driven by changes in the Sun’s magnetic activity or by nearby astrophysical events, could modulate global cloud cover and therefore temperature. Analysis of cave formations, which grow during warm and wet periods, found that speleothem growth occurred during four periods of low cosmic-ray flux over the past 200,000 years, including the current warm period, with growth absent during episodes of high cosmic-ray flux such as the Laschamp geomagnetic excursion.29Journal of Atmospheric and Solar-Terrestrial Physics. Evidence for a link between the flux of galactic cosmic rays and Earth’s climate during the past 200,000 years
The cosmic-ray-climate link remains contested. The correlation exists, but correlation across geological time is tricky: many other factors change simultaneously, and the physical mechanism connecting cosmic rays to cloud formation has been difficult to confirm experimentally at atmospheric scales. Most climate scientists consider greenhouse gases and orbital variations to be far stronger drivers of glacial-interglacial cycles, with cosmic rays potentially playing a minor modulating role at best. The question is not settled, but neither is it the kind of finding that rewrites the textbook on why ice ages happen.