TO HUMAN eyes, the night sky is serene, save for the moon and a few wandering planets. Peer into space with a telescope that can scan the entire sky in days, however, and it appears as a great cosmic fireworks display – a riot of bangs and flashes radiating across the electromagnetic spectrum, from radio waves to gamma rays.
Astronomers are eager to catch these fleeting astrophysical phenomena, known as transients, because they can reveal a great deal about the universe, from what matter does in the most extreme conditions to how the cosmos evolved. And we are about to see a lot more of them thanks to the Vera C. Rubin Observatory in Chile, which is expected to detect 10 million transients a night when it switches on next year.
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The trouble is that, for the untrained observer, it can be difficult to make sense of transients. For starters, similar-looking outbursts can have different origins and vice versa, such that a single source can give rise to a variety of astrophysical signatures. That alone can make it tricky to keep track of what’s what.
Here, then, is a concise primer that also serves as a guide to the causes and consequences of these ephemeral illuminations – and how, in some cases, they challenge our understanding of the physics that governs the universe.
Stellar flares
A living star’s electromagnetic eruptions
Let’s start with an easy one. A stellar flare is an explosion in the atmosphere of a star that results in an intense flash of radiation across the electromagnetic spectrum. When our sun flares, for instance, we see a sudden burst of brightness before it quickly returns to quiescence. Something similar goes on in stars of various sizes, temperatures and luminosities.
We know a fair bit about what causes this in our sun. It comes down to its magnetic field, which is carried by the roiling gases that make up every star. These gases are in constant motion due to convection in the outer layers of the sun and our star’s rotation, so the magnetic field lines are constantly being stretched and tangled. When the lines come into contact, they merge, releasing huge amounts of energy. At the sun’s surface, this quickly heats the surrounding atmosphere and accelerates particles, resulting in sudden outbursts.
Sometimes the excess energy can cause some of the material of the sun to bubble up in a coronal mass ejection. In extreme cases, these reach Earth and interact with its magnetic field, which can endanger satellites and even our ground-based infrastructure. That is why astronomers are constantly monitoring the sun’s activity to warn of incoming solar storms.
We are also keen to see flares on other stars, partly so we can see the consequences for any planets orbiting them, which would have huge implications for their habitability. One idea is that stellar flares can expose nascent life on nearby worlds to fatal levels of radiation. But recent research on an exoplanet called Proxima b suggests that flares can make conditions more conducive to life, by changing the atmosphere such that it can retain heat where otherwise it would be too cold.
Classical novae
The fizzling corpses of low-mass stars
Classical novae are bright outbursts from white dwarfs – the exposed, inert cores that remain after low-mass stars (ones of eight solar masses or less) blow out their outer layers at the end of their lives.
Having run through their nuclear fuel, and unable to ignite fusion reactions in the carbon and oxygen that remains, these cores cool and dim as they radiate away heat from a lifetime of nuclear reactions. Their electrons are so tightly packed that they exert a quantum mechanical pressure known as degeneracy pressure, which prevents the white dwarfs from collapsing in on themselves due to gravity. If a white dwarf is alone, then it will limp on, slowly cooling and dimming. If it has a companion, however, things can get explosive.
In binary systems, where two stars orbit a common centre of mass, one star reaches the white dwarf stage while the other is still actively fusing elements in its core. If the two are close enough, the white dwarf’s gravitational field can suck matter away from its companion’s surface, which gets deposited on the white dwarf. Typically hydrogen gas, this material heats up as it amasses until the temperature gets so high that nuclear fusion reactions ignite, releasing all their energy at once. The result is a brief surge in luminosity, where the white dwarf becomes tens of thousands of times as bright as our sun.
And it doesn’t have to be a one-off. We think that once all the hydrogen is used up, the cycle starts again, leading to recurrent novae. We observe these rarely – in 2021, for instance, we spotted one called RS Ophiuchi, having already seen it erupt in 2006 and 1985. But we think that many novae recur, albeit typically on much longer timescales.
Thermonuclear supernovae
Runaway fusion in a greedy white dwarf
Just like classical novae, the sources of these explosions, often referred to as Type Ia supernovae, are white dwarfs in binary systems. Here, however, as the white dwarf accumulates matter from its companion, its mass increases to the point where degeneracy pressure can no longer resist gravity. This limit is 1.4 times the mass of our sun and is known as the Chandrasekhar limit, after the physicist who calculated it.
But before the star collapses in on itself, the temperature in its core gets high enough that the carbon it contains can finally ignite into nuclear fusion reactions. This sparks a thermonuclear runaway: the energy produced further increases the temperature, which increases the reaction rate of carbon fusion and so on. Blowing up from its core, the star is obliterated in an explosion roughly 5 billion times brighter than the sun.
The fact that only white dwarfs close to the Chandrasekhar limit can explode in this way makes them more or less uniformly bright. This is why we call them “standard candles”: if we see one, we know how far away it is from how bright it appears. We have used them to measure the expansion of the universe, and to discover that it is expanding at an accelerating rate, which cosmologists explain by invoking a mysterious entity called dark energy.
But standard candles may not be quite as standard as they first appear. We have found that their brightness can vary depending on the progenitor system, as well as what kind of galaxy they are in. That matters because even tiny inaccuracies in our measurements of cosmic expansion would have big implications for what dark energy might be.
Core-collapse supernovae
The dramatic finales of massive stars
This class of supernovae is distinct from the thermonuclear variety, and indeed classical novae, in that they are explosions from stars that haven’t become white dwarfs. It is a big cosmic coincidence that they are roughly as bright and energetic as Type Ia supernovae, because they are very different beasts. They are, essentially, the collapse and rebound of a massive star.
The very biggest stars – those of eight solar masses and above – don’t become white dwarfs. The temperatures in their cores are so high that they can fuse increasingly heavy elements until they form iron, which cannot be combined with any other element to release energy. During the star’s life, the nuclear reactions in its core provide enough outward pressure to counteract the mass of its layers. But once the core runs out of fuel, it can no longer support the mass of the layers surrounding it and the star collapses in on itself.
The collapse becomes an explosion because all this material changes direction. We don’t quite understand how that happens, but the idea is that the gas in the core becomes so compact that it can compress no more, so that material falling inward from the outer layers bounces off it to create a shock wave. Matter then bursts out from the star at more than 10,000 kilometres a second, releasing a huge amount of energy, which triggers further nuclear reactions. This is the main source of elements heavier than iron in the universe, which enrich the interstellar medium and drive the evolution of the surrounding space.
What remains is some of the mass from the inner layers of the star, which forms either an extremely dense ball of neutrons – a neutron star – or a black hole. These are the most exotic forms of matter in the universe, where the laws of general relativity (Albert Einstein’s theory of gravity) and quantum mechanics (which describes the subatomic world) collide. Hence, they lie beyond our current grasp of physics.
Kilonovae
Big blasts from colliding neutron stars
Named for the fact that these bursts are roughly 1000 times brighter than a classical nova (see “Classical novae”), kilonovae are produced in a different way to novae and supernovae.
Neutron stars, themselves the extremely dense remnants of some core-collapse supernovae (see “Core-collapse supernovae”), are an exotic form of matter held together by the degeneracy pressure imparted by neutrons. When two neutron stars orbit a common centre of mass, the system releases energy in the form of ripples in space-time called gravitational waves. Eventually, the two neutron stars collide and we see a powerful flash in the optical, infrared and gamma parts of the electromagnetic spectrum.
A lot of what we know about these kilonovae comes from a gravitational wave detection known as GW170817. When it was spotted in 2017, facilities that together observe across the electromagnetic spectrum turned to look. This gave us an exquisite dataset that confirmed some long-held hypotheses regarding kilonovae. Firstly, it supported the idea that neutron star mergers produce short and intense bursts of gamma rays. Secondly, it demonstrated that these mergers are the site of a kind of element formation in which neutrons are absorbed into an atomic nucleus, producing heavy metals like platinum and gold.
The remnant left behind after the merger is either a black hole or a larger neutron star. The details of the merger are anyone’s guess, though, because we don’t have a good grasp of how pressure, density, temperature and composition are related in a gas of neutrons compressed to the point where they can be packed no tighter. This “equation of state” in neutron stars remains one of the biggest open questions in astrophysics.
Fast radio bursts
Mysterious pulses from far far away
First spotted in archival data in 2007, fast radio bursts (FRBs) are incredibly powerful millisecond-duration pulses of radio waves from distant galaxies. Initially, they had astronomers scratching their heads: what kind of event could release as much energy in a fraction of a second as the sun will radiate in 100,000 years?
We spotted a second burst in 2012, and since then the catalogue of FRBs has itself exploded, with more than 675 sightings as of July 2023. As a result, our understanding of them has come on in leaps and bounds.
FRBs are so short-lived and intensely bright that the objects producing them must be incredibly compact. We also know that the radio emission is polarised, which implies the source must have a very strong magnetic field. But the key piece of the puzzle was the discovery of “rٱ”, where more than one burst is observed from the same source. This tells us that whatever produces FRBs isn’t always destroyed in the process. All of which has led to the widely held idea that FRBs are outbursts from strongly magnetised young neutron stars called magnetars.
Astronomers have also been figuring out how to use these detections to probe distant reaches of space. Each FRB pulse arrives at Earth at a broad band of radio frequencies, and from the time delay between the high and low-frequency signals it is possible to infer some of the properties of the space through which they have passed. Since, in most cases, the sources of FRBs are in very distant galaxies, the information they encode could tell us about the vast voids between galaxies. That might include insights about the strength of the magnetic fields there and whether they were present in the early universe, which would force cosmologists to rethink the role of magnetism in the evolution of the cosmos.
Gamma-ray bursts
The brightest flashes in the universe
Gamma rays are the highest energy form of light there is, and gamma ray bursts (GRBs) are the brightest and most energetic transient events we have ever seen. They can last from milliseconds to minutes, though they often show an afterglow in X-ray, optical and radio emissions, which has allowed astronomers to investigate the sources that produce them.
We have discovered that there are two distinct populations. Long GRBs, which have a duration of more than 2 seconds and up to a minute, are thought to be produced by core-collapse supernovae (see “Core-collapse supernovae”). The collapse forms a black hole, whipping up the star’s remnants into powerful jets. Short GRBs, those of less than 2 seconds, meanwhile, are associated with the merger of compact objects like neutron stars and black holes. This was confirmed in 2017 when the gravitational wave signal GW170817 was attributed to the collision of two neutron stars and follow-up observations revealed that, in these circumstances, a short GRB is produced alongside a kilonova (see “KDzԴDZ”).
Gamma ray bursts continue to serve up surprises, though. In 2022, astronomers spotted the most powerful long GRB we have ever seen and dubbed it “the BOAT” – the brightest of all time – on the basis that it is probably the brightest signal to hit Earth since human civilisation began.
Observations of the afterglow suggest that it may not have come from a supernova, as we would expect, because it wasn’t clear whether there was even one in the vicinity at all. This year, we also spotted a long GRB that seems to have come from the collision of two stars, throwing our understanding of these bursts further into question.
Tidal disruption events
Stars torn apart by hungry black holes
These luminous flares, which tend to last a few months, happen when a star passes too close to a supermassive black hole – the gravitational behemoths that lie at the centre of every galaxy, each with a mass millions to billions of times that of our sun.
For most of us, our experience of tidal forces is limited to seeing the sea level at the beach rise and fall. This happens because Earth and its moon interact gravitationally. As you can imagine, the gravitational pull of a supermassive black hole is much more formidable: when an unfortunate star orbits too close, the tidal forces it experiences are sufficient to completely rip it apart, in some cases launching jets of hot matter and radiation that we see from Earth.
We have only seen about 125 of these events, and it isn’t entirely clear how the jets are generated. But the idea is that, as the star stretches and tears, some of its material coalesces into a disc around the black hole – and the black hole’s gravity, in turn, causes this material to spin and heat up. Thanks to the magnetic fields and the conservation of angular momentum, the material in the disk accretes onto the black hole, producing bright flares. In the right conditions, jets can form.
Tidal disruption events can tell us about supermassive black holes, from their mass to the physics that produces jets. But my personal favourite is that each is a specific instance of how a black hole consumes matter and grows over time. In other words, they are windows into a black hole’s diet.
Active galactic nuclei
Supercharged jets that outshine entire galaxies
Staking a claim as the most dramatic of all the transients, active galactic nuclei (AGNs) are essentially gigantic accretion events around supermassive black holes that can emit so much radiation, again in the form of jets, that they outshine the entire host galaxy. When these jets are pointed directly at Earth, we call them blazars. When they are only slightly angled toward us, they are known as quasars.
AGNs produce such powerful jets because accretion is one of the most efficient ways of transforming gravitational potential energy into other forms of energy, such as heat and radiation. As gas and dust swirls ever closer to the supermassive black hole, their gravitational potential energy is transformed into heat, which is radiated away as electromagnetic waves.
What makes AGNs so dazzling, however, is that accretion can also trigger what we observe as jets of radio waves. These jets can reach many times further than the outer reaches of their host galaxy.
But astronomers are interested in active galactic nuclei primarily because they play a crucial role in the evolution of galaxies. As the supermassive black holes that produce them cycle through periods of activity and quiescence, they produce a series of feedback effects in the gas and dust in the host galaxy. These, in turn, drive episodes of star formation, energise the interstellar medium and increase the black hole’s mass – all with huge consequences for the way their host galaxies grow. Observations of AGNs from the distant, and therefore early, universe also provide clues to the mystery of how supermassive black holes form, which remains one of cosmology’s biggest mysteries.
María Arias is a radio astronomer at Leiden University in the Netherlands
