Novae, supernovae, and kilonovae—as well as the poorly-understood phenomenon of hypernovae—are critical events in the cosmos, responsible for shaping the birth of star systems and rocky planets like our own. They also play a pivotal role in the evolution of galaxies, the distribution of elements, and the intricate tapestry of the universe’s story. They can also produce beautiful nebula remnants, and while none have been spotted since the 1600s, a naked-eye-visible supernova in our own galaxy is surely a spectacular event to witness, with the brightest of these appearing obvious to the eye even in broad daylight.
Though Novae, supernovae, kilonovae, and hypernovae signal the end of life for certain stars, they are distinct phenomena, arising from different scenarios and exhibiting diverse energies, luminosities, and aftermaths.
Supernovae, hypernovae, and kilonovae all produce extraordinary amounts of gamma and cosmic rays. Combined with the cosmic rays emitted by our own Sun, these produce the carbon-14 used for radioactively dating fossils as well as mutations in DNA. It’s possible that supernova explosions caused the DNA mutations that led to the evolution of humans—or, in fact, all complex life on Earth. The chemical structure of DNA and other organic molecules themselves may be a consequence of cosmic ray bombardment too.
Supernovae and hypernovae are extremely rare, happening on average about once per century in a typical galaxy (though our Milky Way has not seen one in several hundred years). On the other hand, novae are fairly common occurrences that happen a few times a decade in our galaxy. The frequency of kilonovae is not known, but given that they are the products of rare, close-together supernova remnants, a kilonova is likely thousands of times rarer than a supernova.
What’s a Nova?
A nova, Latin for “new”, is ironically not the birth of a new star but rather the sudden rekindling of a dying one. Unlike the profound death knells of supernovae, novae are more akin to a phoenix-like resurgence, characterized by a sudden flare-up and increased brightness.
The stage for a nova is typically set in binary star systems—pairs of stars locked in a gravitational tango. One of these stars is usually a white dwarf, the smoldering ember left behind after a star similar in size to our Sun exhausts its nuclear fuel. Orbiting this white dwarf is a smaller, less massive companion star that’s still burning hydrogen, having outlived its formerly more luminous companion. Over time, as these stars pirouette around each other, the powerful gravitational pull of the white dwarf can siphon off the outer layers of its companion, drawing a stream of hydrogen-rich material onto itself.
This material accumulates on the white dwarf’s surface, forming a dense, hot layer. Over time, as more and more material is gathered, the pressure and temperature of this layer increase until they reach a critical point. When this threshold is met, a runaway nuclear fusion reaction ignites, causing the accumulated hydrogen to fuse into helium in a rapid, explosive manner. This sudden burst of energy results in the brilliant flare-up that we observe as a nova.
Though dramatic, novae are not fatal for the stars involved. The white dwarf survives its explosive outburst, and the cycle can repeat itself after a period of time once it accumulates enough material from its partner again. This can occur multiple times over thousands or millions of years, giving astronomers numerous opportunities to witness this cosmic dance of rebirth.
However, while novae are less energetic than supernovae, they are by no means gentle events. They can release as much energy as our Sun does over thousands of years in just a few days or weeks. The debris from the explosion can also be ejected into space at speeds of several thousand kilometers per second, contributing to the interstellar medium and the ongoing cycle of stellar evolution.
What is a Supernova?
While novae mark a star’s dramatic outburst, supernovae (plural of supernova) signify the explosive and often terminal end to a star’s lifecycle. These cosmic detonations are of such scale and luminosity that, for a short while, a single exploding star can outshine its entire galaxy. Such intense events not only spread the heavier elements across the cosmos, laying down the raw material for future stars and planets, but also influence the very evolution of galaxies.
All supernovae are extremely important to the formation of stars, planets, and living things. The energy and radiation from supernovae can trigger star formation in nearby gas clouds or even halt it by dispersing the gas, while the intense heat and pressure during the explosion synthesize elements heavier than iron, including gold, silver, and uranium.
What is a Hypernova?
Hypernovae are a little-understood class of extreme supernovae. The exact conditions leading to a hypernova remain an area of intense study. However, it’s widely believed that they result from the explosive deaths of particularly massive blue giant stars, potentially those with over 30 times the mass of our Sun. These stars have unique properties. Such stars often exhibit higher rotation rates, which can influence their evolution and eventual demise. The abundance of elements heavier than helium (often referred to as “metallicity” in astrophysics) can also play a role in the star’s lifecycle and its propensity to produce a hypernova.
When these behemoths explode, they release an amount of energy that is 10 to 100 times greater than that of a standard core-collapse supernova. The mechanics behind such immense energy release are still debated, but it’s hypothesized that, during the explosion, relativistic jets—beams of matter moving close to the speed of light—form and pierce through the star’s outer layers.
Hypernovae are closely associated with long-duration gamma-ray bursts (GRBs), which are extremely energetic flashes of gamma rays that can last from a few seconds to several minutes. While all hypernovae might not produce GRBs, a significant portion of long-duration GRBs are believed to originate from hypernovae. Hypernovae also show distinct spectral characteristics compared to typical supernovae, including broader light curves and peculiar element abundances.
Just like their supernova counterparts, hypernovae are responsible for synthesizing elements. Given their immense energy, it’s believed that they might play a unique role in the production of certain heavy elements like kilonovae. Hypernovae are also likely sites where black holes are born.
What is a Kilonova?
Among the vast pantheon of explosive astronomical events, kilonovae have recently emerged as one of the most intriguing and scientifically captivating. More energetic than a typical nova yet somewhat subtler than a classic supernova, kilonovae are borne from the fiery union of two neutron stars, or occasionally a neutron star and a black hole. They not only offer a visual spectacle but are also the forges of some of the rarest elements in the universe.
Neutron stars, the incredibly dense remnants left behind by core-collapse supernovae, sometimes find themselves paired in binary systems. Bound by gravity, these neutron stars spiral towards each other over aeons, radiating gravitational waves — ripples in the fabric of spacetime. When they finally converge in a cataclysmic collision, a kilonova is born.
The energy released during this monumental merger is phenomenal. Within moments, massive amounts of material are ejected at near-relativistic speeds. This ejected matter, rich in neutrons, becomes the site of rapid neutron capture (r-process) nucleosynthesis. A short gamma-ray burst, one of the universe’s most energetic phenomena, may accompany the collision, beaming intense radiation out into space.
The r-process nucleosynthesis occurring in the aftermath of the merger is responsible for creating many of the heavy elements we find in the universe, and the initial explosion of kilonovae can produce elements as heavy as fermium-257 (atomic number 100), though any elements heavier than uranium and trace amounts of plutonium, of course, will decay radioactively long before they settle into a star or planet. Kilonovae are the primary producers of gold, platinum, uranium, and thorium in the Universe, not supernovae, as was previously thought.