The universe is exploding.
Or parts of it are. The night sky may seem calm, even serene, but that masks events of a catastrophic and nearly unimaginable scale. Across the galaxy and even the cosmos itself, immense outbursts of energy occur that could easily vaporize our planet. Happily, space is vast, and the terrible distance between these events and us diminishes what we see to a faint glow—usually. It’s very rare for our Earth to be touched directly by such explosions, but it does happen, though with generally minimal effect.
As Douglas Adams wrote in The Hitchhiker’s Guide to the Galaxy, “Don’t panic.”
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That said, some of these events are maybe a cause for some small amount of concern. How worried you should be about any given cosmic paroxysm depends upon how powerful it is—and thus how nearby it must be to pose a planetary hazard. With that in mind, let’s peruse the rogues’ gallery of astrophysical assaults, laying them out in order of destructive potential.
Locally, the sun is capable of some epic tantrums. Solar storms are eruptions that release some of the energy stored in the sun’s strong magnetic field. These can create intense, localized explosions called solar flares, or much more powerful and dangerous coronal mass ejections, which spread out over a far larger volume. Both generate blasts of high-energy subatomic particles that can slam into Earth’s magnetic field, causing widespread problems such as blackouts, loss of satellites, and more.
They also create spectacular auroras, so at least there’s an upside.
How much energy is released in such an event? Over the course of a few hours in 2003, the most powerful solar flare ever directly measured released as much energy as the entire sun does in about one fifth of a second. That may not sound like much, but it’s the equivalent to simultaneously detonating about 17 billion one-megaton nuclear bombs. While that’s not nearly enough to, say, melt the world, it’s still a phenomenal event.
Happily, solar storms at this scale are rare and can miss Earth even when they happen, but we should still take them seriously.
Mind you, too, that these are the first and therefore smallest explosions on our list. Things get pretty apocalyptic from here.
When the sun finally runs out of nuclear fuel and dies, some seven or so billion years from now (mark your calendars), it will undergo a series of short-lived eruptions and eventually fade away, cooling over many billions more years. But some stars are much more tempestuous.
A white dwarf is the core of a star that was once like the sun, now exposed to space. This object is massive and tiny, giving it Herculean gravity. If it happens to orbit a normal star, it can siphon off material that then piles up on the white dwarf’s surface. If enough matter accumulates, it can be squeezed so tightly that it ignites nuclear fusion. The result is a seriously powerful explosion called a nova. It can emit as much energy when it explodes as the sun does over many centuries.
I’ll note that there are also recurrent novae, which blow up like this again and again, some even millions of times.
While novae are powerful, some stars really go out with a bang. A supernova occurs when a massive star ends its life. The stellar core collapses, unleashing a truly staggering amount of energy that causes the star to explode; in an instant, several octillion tons of matter are flung outward at an appreciable fraction of the speed of light. The energy released in total can be millions of times that of a nova, enough to outshine an entire galaxy of billions of stars.
A novalike situation can become a supernova as well: if enough material piles up on the surface of a white dwarf, the ensuing explosion can be so large it tears the star itself apart, coincidentally blowing up with an energy roughly the same as a core-collapse supernova.
Being near a supernova can ruin your day. It’s not entirely clear how close one has to be to deleteriously affect our planet, but around 160 light-years of space between us is probably a good estimate. This happens pretty rarely, but close calls have happened: radioactive elements have been found in material from the seafloor that can only be created in a supernova, meaning that some millions of years ago, at least one star flipped its lid close enough to Earth to spray us with a few hundred tons of mildly radioactive material. That’s not really dangerous—we’re still here, after all—but it’s sobering to think something so powerful can reach out to touch us from more than a quadrillion kilometers away.
We’ve been touched by a cosmic monster more recently, too. In December 2004 astronomers were shocked when a huge blast of energy swept over Earth, jostling our planet’s magnetic field and increasing the ionization of its atmosphere. The culprit was a magnetar, a super-magnetized neutron star located about 50,000 light-years away. For reasons still not wholly understood, these ultradense objects—each about the mass of our star squeezed into a bizarre, city-sized ball of degenerate quantum matter—undergo starquakes in which the material on the surface shifts a bit like in an earthly tremor. That shift may only be a centimeter or so, but the enormous mass and ridiculously strong gravity (billions of times that of Earth’s!) release vast amounts of energy, mostly in the form of gamma rays and x-rays. I’m glad this particular starquake occurred halfway across our galaxy—and that only a handful of such objects are known in the Milky Way, most of which are more quiescent.
Black holes make this list as well, in a surprising variety of ways. Core-collapse supernovae can make stellar-mass black holes, and the birth announcement is loud: a gamma-ray burst is the impulsive release of supernova-level energy, but instead of blasting away in all directions, the burst is concentrated into a pair of tightly focused beams. These are more dangerous at farther away than the previous items on our list; closer than 6,000 light-years or so gets dicey. They are so intense they can be seen from incredible distances; one, called GRB 080319B, erupted 7.5 billion light-years from Earth. Yet even from so far away, it was briefly visible to the naked eye!
Sometimes stars wander too close to a large black hole, and the gravity can rip them apart. Called tidal disruption events, these also erupt with energies around that of a supernova. Astronomers have found quite a few of these, all (so far) happily in very distant galaxies.
But it’s hard to beat when two supermassive black holes collide. This can happen when two large galaxies collide; their central monsters can eventually merge, creating a larger black hole. When that happens, a substantial fraction of their mass, about 10 percent, is instantaneously converted into energy. This amount of energy is way, way beyond what the human mind can grasp; the blast from the merger of two one-billion-solar-mass black holes emits as much energy as the sun does in three billion trillion years. That number stopped me cold; my brain started panicking and running around in circles in my skull. That’s far more than the energy emitted by every single star in the entire universe. It’s literally the most powerful bang since the big one.
Why haven’t we seen one of these explosions? All that energy is emitted in invisible gravitational waves, ripples in the fabric of the universe. These giant waves weaken with distance, and such mergers are sufficiently rare that they tend to occur billions of light-years away.
It all seems rather violent, doesn’t it? But take hope! There’s a cycle to the universe. When stars explode, they create and scatter heavy elements such as iron, calcium, and more, all of which are necessary for life as we know it, and in fact, your blood and bones exist because of these explosions. Supernovae can also compress interstellar gas around them, helping form new stars; black hole eruptions can do the same.
These cosmic explosions may ostentatiously announce a catastrophic ending, but they can also usher in a new beginning. I find that rather heartening.
