54 terms · zero jargon
Every field has its secret language. Here's the universe's, translated — so the next documentary, headline, or fact on this site hits harder.
Things out there
The nearest big galaxy — a trillion stars, 2.5 million light-years away, and the farthest thing visible to your naked eye. It's approaching at 110 km/s and will merge with the Milky Way in about 4.5 billion years. No collision panic: stars almost never touch.
It's coming. Slowly. Politely.
Missions & machines
The NASA program that landed twelve humans on the Moon between 1969 and 1972, using computers less powerful than a modern calculator. No one has been back since — every Moon visitor took off during a 40-month window half a century ago.
Twelve people have walked on another world. So far.
Things out there
A rocky leftover from the solar system's construction, most orbiting between Mars and Jupiter. They range from pebbles to Ceres, which is 940 km wide. NASA tracks the ones whose paths come near Earth — and in 2022 deliberately crashed a spacecraft into one to nudge it.
Building rubble from 4.6 billion years ago.
Distances & units
The average distance from Earth to the Sun: about 150 million kilometres. Handy for solar-system distances — Jupiter is 5 AU out, Neptune 30 AU, and Voyager 1 has passed 165 AU. Beyond that, astronomers switch to light-years.
The solar system's tape measure.
Things that happen
The northern and southern lights: charged particles from the Sun, funnelled by Earth's magnetic field toward the poles, slam into the atmosphere and make it glow green, red, and purple. Other planets have them too — Jupiter's are bigger than Earth.
The sky glowing because Earth's shield took a hit.
Big ideas
The moment, 13.8 billion years ago, when the universe began expanding from an unimaginably hot, dense state. It wasn't an explosion in space — it was space itself starting to stretch, everywhere at once. The expansion never stopped; it's happening right now.
Not an explosion in space — an expansion of space.
Things out there
A region where so much mass is packed so tightly that nothing — not even light — can escape once it falls in. Not a vacuum cleaner: orbit it from a distance and it behaves like any other mass. Get close and physics gets strange.
Time itself runs slower next to one.
Things out there
A chunk of ancient ice and dust, usually a few kilometres wide, left over from the solar system's formation. When one swings near the Sun, its ice turns to gas and forms the famous tail — which always points away from the Sun, pushed by solar wind.
Dirty snowballs older than the planets.
Things that happen
The oldest light in existence: a faint glow left over from when the universe was just 380,000 years old, now stretched into microwaves filling the entire sky. Old analog TVs picked it up — a little of the static between channels was the Big Bang's afterglow.
Baby photos of the universe, visible in every direction.
Big ideas
The name for whatever is making the universe's expansion speed up instead of slow down — discovered only in 1998. It appears to make up about 68% of everything. 'Dark' here mostly means 'we have no idea'.
68% of everything is a complete mystery.
Big ideas
Invisible stuff that outweighs normal matter five to one. We can't see it, but galaxies spin so fast they'd fly apart without its gravity holding them together. Nobody knows what it is — one of the biggest open questions in science.
It holds your galaxy together, and we can't find it.
Things out there
A world round enough to be a planet but that hasn't cleared its orbital neighbourhood — Pluto, Ceres, Eris and friends. The 2006 reclassification of Pluto wasn't a demotion so much as the realization that the outer solar system is full of Plutos.
Pluto has company — that's why it was reclassified.
Things that happen
One body casting its shadow on another. A solar eclipse: the Moon blocks the Sun. A lunar eclipse: Earth's shadow darkens the Moon. Total solar eclipses are a cosmic coincidence — the Moon is 400× smaller than the Sun but also 400× closer, so they match almost exactly.
The luckiest coincidence in the solar system.
Big ideas
The speed needed to leave a body's gravity behind without further thrust: 11.2 km/s from Earth, 2.4 km/s from the Moon, and more than the speed of light from inside a black hole — which is exactly why nothing escapes one.
Earth's exit fee: 40,000 km/h.
Big ideas
The invisible boundary around a black hole: the point of no return. Cross it and no amount of speed or fuel gets you back out, because escape would require moving faster than light. The 2019 'photo of a black hole' shows the glowing gas just outside one.
The universe's only true one-way door.
Things out there
Any planet orbiting a star other than the Sun. We've confirmed nearly 6,000 so far, from lava worlds to planets where it may rain glass. The first was only found in the 1990s — before that, we honestly didn't know if other planets existed.
Some sit at the right distance for liquid water.
Missions & machines
SpaceX's workhorse rocket and the machine that made reusable spaceflight normal: its first stage flies to the edge of space, then returns and lands upright on a ship or pad — some boosters have flown more than twenty times. It cut launch costs dramatically.
Rockets used to be single-use. This one commutes.
Big ideas
Physicist Enrico Fermi's lunchtime question: given billions of Sun-like stars and planets, and billions of years of head start, the galaxy should be full of civilizations. So — where is everybody? Every proposed answer, from 'life is rare' to 'they're quiet', changes how you see the night sky.
'Where is everybody?' — still unanswered after 75 years.
Things out there
A city of stars — anywhere from millions to trillions of them — held together by gravity, along with gas, dust, and a lot of dark matter. Our home galaxy is the Milky Way. Galaxies come in spirals, blobs, and everything in between.
You live in one, in a quiet suburb of it.
Things that happen
Ripples in spacetime itself, launched by cataclysms like black holes colliding. Predicted by Einstein in 1916, first detected in 2015 by instruments that can sense a stretch smaller than a proton's width across 4 km. It's a whole new way of 'hearing' the universe.
We can now hear spacetime ringing like a bell.
Big ideas
Einstein's version: mass bends spacetime, and things move along those curves. It holds you to Earth, the Moon to us, and galaxies together — and it also bends light and slows time. GPS satellites must correct for it daily or maps would drift by kilometres.
Not a force pulling you down — curved spacetime pushing you along.
Big ideas
The 'Goldilocks' band of distance around a star where a planet could keep liquid water — not so close it boils, not so far it freezes. Being in the zone doesn't guarantee life (Venus and Mars technically flank ours), but it's where we look first.
The universe's most valuable real estate.
Missions & machines
The telescope that changed everything, orbiting since 1990. It measured the universe's age, proved galaxies host black holes, and took the Deep Field: pointing at a 'blank' speck of sky and finding thousands of galaxies in it. Still working after three decades.
Pointed at nothing, found everything.
Big ideas
The 'empty' space between stars — which isn't empty: a whisper-thin soup of gas, dust and cosmic rays, a few atoms per cubic centimetre. For comparison, the best vacuum chambers on Earth are far more crowded. Voyager 1 is flying through it right now.
Even nothing is something.
Missions & machines
A football-field-sized laboratory orbiting 400 km up at 28,000 km/h, continuously inhabited since November 2000 — meaning no one under ~25 has lived a day when all humans were on Earth. Sixteen sunrises a day, visible to your naked eye when it passes over.
There are people up there right now.
Missions & machines
The most powerful space telescope ever built, parked 1.5 million km from Earth with a 6.5-metre gold-coated mirror. It sees in infrared, which lets it peer through dust and catch light from the first galaxies — looking more than 13 billion years into the past.
A time machine with a gold mirror.
Distances & units
The internationally recognized boundary of space, 100 km above sea level — where the air becomes too thin for wings to generate lift, and you need orbital speed to stay up. Cross it and you've officially earned astronaut status.
Space starts closer to you than the next city over.
Distances & units
The distance light travels in one year: about 9.46 trillion kilometres. It's a unit of distance, not time — though it does mean looking far away is looking into the past. Proxima Centauri is 4.25 light-years away; you see it as it was 4.25 years ago.
A ruler, not a clock — but it makes telescopes time machines.
Things that happen
A meteor is the streak of light when a bit of space rock burns up in our atmosphere — a 'shooting star'. If a piece survives and lands, that piece is a meteorite. Around 48 tonnes of space material hits Earth's atmosphere every day.
Shooting stars aren't stars — they're often sand-grain sized.
Things out there
Our galaxy: a spiral disc of 100–400 billion stars, about 100,000 light-years across. The milky band you see on dark nights is its disc viewed edge-on from inside. We live in a calm outer arm — prime real estate, far from the chaotic center.
That faint band in the night sky? That's home, seen from inside.
Things out there
A natural object orbiting a planet. Earth has one (a suspiciously large one), Mars has two potato-shaped ones, Saturn has hundreds. Some moons are more interesting than planets: a few hide entire oceans under their icy shells.
The best places to look for alien life might be moons.
Things out there
A vast cloud of gas and dust in space. Some are stellar nurseries where new stars condense; others are the beautiful wreckage of dead stars. The famous 'Pillars of Creation' photo shows one, light-years tall.
Star factories and star graveyards, equally photogenic.
Things out there
The collapsed core of a giant star after a supernova: roughly one and a half Suns of matter crushed into a ball the size of a city. A teaspoon of it would weigh billions of tonnes. Some spin hundreds of times per second.
The densest 'normal' matter in the universe.
Distances & units
The sphere of space whose light has had time to reach us since the Big Bang — about 93 billion light-years across, holding roughly 2 trillion galaxies. It's not the whole universe, just our horizon. Every point in space has its own.
Not the universe's edge — just the edge of what we can see.
Things out there
A theorized shell of trillions of icy bodies surrounding the solar system, stretching perhaps 1.5 light-years out — the source of long-period comets. It's the Sun's true edge: Voyager 1 won't clear it for tens of thousands of years.
The solar system doesn't end at Pluto. Not even close.
Big ideas
Falling, forever, and missing. An orbiting spacecraft is in constant free fall toward Earth, but moving sideways so fast that the ground keeps curving away beneath it. That's also why astronauts float: not because there's no gravity, but because they're falling.
Astronauts aren't weightless — they're falling in style.
Distances & units
About 3.26 light-years — the professional astronomer's distance unit, defined by how much a star appears to shift when Earth moves around the Sun. And yes, Han Solo used it wrong (it measures distance, not time).
The unit Star Wars famously misused.
Things out there
A world big enough that gravity has pulled it into a ball, orbiting a star, and dominant enough to have cleared its neighbourhood of debris. Fail that last test and you're a dwarf planet — ask Pluto.
There are more planets than stars in our galaxy.
Things out there
A spinning neutron star that sweeps beams of radiation across space like a lighthouse. Each time the beam crosses Earth, we detect a pulse — some tick more precisely than atomic clocks. The first one found was briefly nicknamed LGM-1: 'little green men'.
Nature's most precise clocks.
Things out there
The blazing core of a distant galaxy where a supermassive black hole is actively feeding. Infalling gas heats up so much it can outshine all the galaxy's stars combined. Quasars are visible across billions of light-years — beacons from the young universe.
A feeding black hole brighter than a trillion suns.
Things out there
What a star like the Sun becomes near the end of its life: the core runs low on hydrogen, and the outer layers swell hundreds of times larger and cool to a red glow. In about 5 billion years the Sun will do this — likely swallowing Mercury and Venus.
The Sun's retirement plan.
Things that happen
As space expands, light travelling through it gets stretched to longer, redder wavelengths. The farther a galaxy, the more its light is stretched — which is how we know the universe is expanding and how we date the most distant objects.
The universe's odometer, written in color.
Things out there
The supermassive black hole at the center of the Milky Way, 26,000 light-years away, weighing about 4 million Suns. Every star in the galaxy — including ours — orbits it. We photographed its glowing ring in 2022.
You're orbiting it right now, at 800,000 km/h.
Missions & machines
The Search for Extraterrestrial Intelligence: scanning the sky for signals that nature can't easily produce — narrow radio tones, laser pulses, engineered patterns. Decades of silence so far, which is itself interesting: it's called the Fermi paradox.
We've been listening for decades. The silence is the plot twist.
Big ideas
The point at a black hole's center where, according to the math, density becomes infinite and known physics stops working. Most physicists take that as a sign we need better physics — quantum gravity — rather than that infinity actually exists.
Where our equations wave a white flag.
Things that happen
A sudden explosion on the Sun that releases the energy of millions of nuclear bombs in minutes, often flinging a cloud of charged particles our way. Strong ones can knock out power grids and satellites — a big one in 1859 set telegraph offices sparking.
Space weather is real, and it has knocked out power grids.
Things that happen
A constant stream of charged particles blowing off the Sun at hundreds of kilometres per second. Earth's magnetic field deflects it (giving us auroras); Mars, which lost its field, had much of its atmosphere stripped away by it.
The Sun is quietly sandblasting every planet.
Things out there
A giant ball of mostly hydrogen, squeezed so hard by its own gravity that atoms fuse in its core and release energy. That's the light and heat. The Sun is a star — an utterly average one, which is somehow more amazing.
Every star you see is a sun. Some have their own planets.
Things that happen
The explosive death of a star, briefly outshining its entire galaxy. Supernovae forge and scatter heavy elements — the iron in your blood and the calcium in your bones were made in one. No supernova, no you.
You are made of exploded stars, literally.
Things that happen
The moving line between day and night on any world — the sunset line, seen from space. On tidally locked exoplanets, the terminator never moves: a permanent ring of eternal sunset, which might be the most livable strip on such worlds.
A permanent sunset you could stand in. (Not the robot.)
Things that happen
When gravity slows a moon's spin until one side permanently faces its planet — which is why humanity never saw the Moon's far side until a spacecraft flew around it in 1959. Many exoplanets are tidally locked to their stars: eternal day on one side, eternal night on the other.
Why the Moon never turns its back on us — or its face away.
Missions & machines
Twin spacecraft launched in 1977 for a grand tour of the outer planets. Both are now in interstellar space — the farthest human-made objects — each carrying a Golden Record of Earth's music, greetings, and sounds, just in case someone ever finds them.
Our message in a bottle, 25 billion km out and counting.
Things out there
The leftover core of a Sun-like star after it sheds its outer layers: an Earth-sized ember of carbon and oxygen, no longer fusing, slowly cooling for billions of years. One teaspoon weighs about a tonne.
A dead star the size of Earth.
Big ideas
The floating astronauts feel isn't the absence of gravity — at the ISS's altitude, Earth's pull is still 90% as strong as on the ground. They float because they're in continuous free fall around the planet, falling and missing forever.
Gravity up there is nearly full strength. They're just falling.
Nothing matches — the universe is big, but this page isn't (yet). Try another word.