Ah, autumn. Leaves are falling, days are shortening, and there’s a chill in the air.

But is that just a result of the approaching winter as Earth’s northern axis begins to tip away from the sun? Or is it caused by some other cosmic horror, a cold finger of dread running down your spine as you contemplate the legion of nightmares lurking in the universe all around us?

Space is a scary place. Certainly, many objects in it appear scary to our human brains, and we interpret their shapes as witches and skulls and fiery jack-o’-lanterns. These are in reality relatively harmless illusions whose specific structures only remind us of dreaded visions.

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But there are actual terrifying objects in space, entities operating on a scale that crush our minds and reduce our human experience to a mote of dust floating in the dark.

I’m an astronomer, and I’ve studied and written about many of these objects for decades. What, you might wonder, in that vast cosmos, scares me?

There is, in fact, quite a list.

(But first, an aside: While cosmophobia—the dread of space—is not an officially recognized medical condition, I know some folks for whom this is a very real fear. I by no means am trying to enflame anyone’s actual terror, so let me state clearly: while real, the objects and events below are either so far away from us that even the most powerful has less effect on us than a whisper in a hurricane or are so rare that the odds of them occurring in a human lifetime—or even over eons—is so small as to be essentially zero. After all, how many people whom you know have been killed by a supermassive black hole merger?)

So here, let me show you what gives me a preternatural frisson down my back.

Wolf-Rayet Stars

Even the tiniest of stars is humbling on a human scale. TRAPPIST-1, an ultracool red dwarf barely massive enough to ignite nuclear fusion in its core, still releases the equivalent energy as the detonation of more than 50 million one-megaton nuclear bombs every second.

Now imagine the most luminous stars.

These are called Wolf-Rayet stars, and they’re so massive that the pressures and temperatures in their cores are almost beyond comprehension. They fuse hydrogen and helium into heavier elements at such fantastic rates that they blow out truly soul-searing amounts of energy.

R136a1 is just such a star, the most massive star known, at several hundred times the sun’s bulk. It is something like seven million times more luminous than the sun, a number that leaves me somewhat mentally paralyzed. Replace our sun with R136a1, and I fear Earth wouldn’t last long as a solid object. Happily, the star is actually in another galaxy, so distant it’s not even visible to the naked eye. And while there are many Wolf-Rayet stars in our own Milky Way galaxy, they are all still too far to damage us and in fact are fun to study, producing spectacular and gorgeous displays as they live—dangerously.

Magnetars

Neutron stars are boggling objects all by themselves. A neutron star is the leftover core of a massive star that exploded as a supernova, and its every property reads like a comic book headline. Containing the mass of a star squeezed down into a ball the size of a city, they are so dense a single cubic centimeter weighs several hundred million tons, equivalent to all the cars in the U.S. compressed down to the size of a sugar cube. The gravity on the surface can be several billion times Earth’s, and a typical neutron star’s magnetic field is billions of times stronger than Earth’s as well.

But there is a subclass of neutron stars that are more powerful yet: the aptly-named magnetars, with magnetic fields that can be more than a quadrillion times stronger than that of our own planet. Their magnetic field is tied to their crusts, the thin shell of “normal” matter surrounding the more exotic interior. If there is a starquake, a sudden shift in the neutron star crust, the magnetic field gets shaken violently. This creates a truly enormous explosion of energy that barrels into the galaxy at large like a tsunami.

In December 2004 such a wave swept over Earth, causing some havoc. It hit with such force that it compressed our planet’s magnetosphere, overloaded satellites and even partially ionized our atmosphere. While the damage was minor, this was all caused by the magnetar SGR 1806-20, which is 50,000 light-years away, halfway across our galaxy.

Happily, magnetars are rare, and only a few known are closer to us, all less powerful than SGR 1806-20. I’d hate to think there’s another one like it out there but nearer to us, just waiting to blow off some magnetic steam.

Short Gamma-ray Bursts

Speaking of neutron stars, if one can be a problem, what happens if there are two of them orbiting each other?

Nothing good.

Over time, the orbit of the pair can decay, the duo slowly approaching one another. Eventually the inevitability of gravity brings them close enough to suddenly merge, sending out a tremendous blast of matter and energy that can outshine the sun by a factor of many billions, though just for the briefest fraction of a second. Because of the high-energy photons they fling out and their milliseconds-long duration, these are called short gamma-ray bursts.

Such explosions are rare because binary neutron stars are rare. While no burst has ever been seen in our galaxy, a handful of local binary systems are known. The good news is they’re likely too far away to damage us in the eventuality of their merging. Those don’t concern me.

What does, however, is the possibility of a pair of neutron stars much closer to us, hidden from our view because of their extreme age, cooling over the eons until they are cold enough to be invisible to our telescopes. Massive, dark and in a mad whirl around each other, they will announce their demise in an eruption of energy powerful enough to dwarf the effects of a magnetar.

The odds of this are extremely low, so I don’t think there are any that close. I’m pretty sure. Probably.

Merging Supermassive Black Holes

But if I had to pick one thing out of the cosmic menagerie that gives me astronomical heebie-jeebies, it’s merging supermassive black holes.

These are black holes on a galactic scale, with millions or billions of times the mass of the sun. They can come in pairs, coupled after two galaxies collide, the monsters in their hearts brought together in the merger. Over billions of years, they slide ever closer together until finally they eat each other to form a single, even more Brobdingnagian beast.

But not all their mass gets glommed together to form the final black hole boss. Instead a significant fraction is converted into energy in the form of gravitational waves, a literal shaking of the fabric of the universe, sending out incomprehensibly powerful ripples like an angry god shaking the crumbs off an Olympian tablecloth.

How powerful? The merger’s gravitational roar in a fraction of second is equivalent to the amount of energy our sun emits in over a billion trillion years. It’s more powerful than the combined might of every single star in the observable universe.

Amazingly, the explosion is entirely dark; not a single photon of light is generated—all the energy goes into invisible gravitational waves. Also, these happen, on average, billions of light-years from us, so the energy is dissipated to far less than that of the beat of a bat’s wing on Earth. The gravitational burst is immense, but the vast distance between us wins.

Still, the very idea of such a thing freezes my brain, locking it into a mental feedback loop that’s the equivalent of running around in a tight circle flapping my arms wildly and screaming. Nothing should be that energetic. Nothing. Yet, as far as we know, the universe makes these all the time.

That’s what scares an astronomer.

So as you go trick-or-treating this holiday, be safe. Watch out for traffic and other costumed celebrants—but also keep a part of your attention on the sky. It’s pretty scary up there, too.