1. Astronomy, astrophysics, cosmology — what’s the difference?

Three words get used as if they mean the same thing: astronomy, astrophysics, cosmology. They overlap a lot, but they’re not identical.

• Astronomy is the oldest of the three. It’s about looking at things in space (planets, stars, galaxies, comets, anything that isn’t Earth’s surface or atmosphere) and describing what’s out there.
• Astrophysics asks why those things behave the way they do. It uses physics and chemistry to explain how stars burn, why galaxies spin, how black holes work.
• Cosmology zooms out further still. It treats the entire universe as the thing under study: how it started, how it’s changed, what shape it has, and what eventually happens to it.

In practice, an astronomer and an astrophysicist often share an office. The line is fuzzy. “Astronomy” is the umbrella you reach for when you don’t want to pick a label.

One more bit of vocabulary before we start. Astronomy is what’s called a natural science, which just means a field that figures things out by observing the world and running experiments rather than by pure reasoning. That distinction matters more than it sounds, because for most of human history we tried to work out the cosmos by pure reasoning alone, and we were wrong about almost all of it.

2. The Big Bang

Around 13.8 billion years ago, the entire universe (everything we can now see, plus a lot more we can’t) was packed into a region smaller than an atom. Then it started expanding. It’s still expanding today. That, in one sentence, is the Big Bang.

A few things to clear up, because the name causes confusion.

The Big Bang wasn’t an explosion in space. There was no space yet for it to explode into. Space itself is what got bigger. Picture a balloon with dots drawn on it: as you blow the balloon up, every dot moves away from every other dot, but no dot is “the centre” of the balloon’s surface. That’s roughly how the universe expands. There’s no point you can fly to and say “here’s where it started”, because every point is where it started.

“13.8 billion years ago” is a measurement, not a guess. We know it because we can see the leftover heat from the early universe in every direction in the sky. Astronomers call this the cosmic microwave background, and it is literally a faint glow that fills space. The temperature of that glow, plus the rate at which galaxies are moving apart, gives us the age of the universe. The number has been tightening since the 1960s.

What was “before” the Big Bang? Nobody knows, and there’s a real question about whether the question even makes sense. Time, as far as we can tell, started with the Big Bang. Asking what came before is a bit like asking what’s north of the North Pole. We’ll come back to this in section 6.

In the first second after the Big Bang, the universe went through a few absurd stages. It was so hot that ordinary matter couldn’t exist yet, only a soup of particles. It cooled fast. Within minutes, hydrogen and helium had formed (the lightest two elements on the periodic table). The universe at this point was a glowing fog: hot, opaque, no stars, no galaxies, no structure.

3. From a glowing fog to galaxies, stars, and us

It took about 380,000 years after the Big Bang for the universe to cool from a glowing fog into something transparent, and another few hundred million for the first stars to switch on. The rest of the story (galaxies, planets, dinosaurs, you) is what filled the next 13.8 billion years.

When the fog cleared, the universe finally let light travel in straight lines instead of absorbing it every few millimetres. That moment of clearing is the cosmic microwave background mentioned earlier. Everything we know about the early universe, we know by reading that ancient light.

For a while after the fog cleared, there were no stars. The universe was full of cool hydrogen and helium gas, dark and quiet, slowly clumping together under gravity. The first stars probably switched on somewhere between 100 and 200 million years after the Big Bang. They were enormous, hotter and brighter than anything we have today, and they didn’t live long. When they died as supernovas (stars exploding so violently they briefly outshine entire galaxies), they scattered heavier elements into space: carbon, oxygen, iron, gold.

This matters because everything heavier than helium had to be cooked inside a star before it could exist. The iron in your blood, the calcium in your bones, the gold in a wedding ring: each one of those atoms was forged inside a star that died long before our solar system existed. “Star stuff” is a phrase popularised by Carl Sagan, and it isn’t a metaphor. It’s chemistry.

Galaxies started taking shape within the first billion years. Our own galaxy, the Milky Way, formed around 13 billion years ago. The Sun is a much later arrival, a fairly average star that lit up about 4.6 billion years ago in a cloud of gas and dust on one of the Milky Way’s outer arms. The leftover dust around the newborn Sun clumped into planets, including Earth, about 4.5 billion years ago. Life shows up in the fossil record by 3.5 billion years ago, possibly earlier. Anatomically modern Homo sapiens appears around 300,000 years ago. Written history starts about 5,000 years ago.

If you compressed the entire 13.8-billion-year history of the universe into one calendar year, the dinosaurs would die out on December 30th, all of recorded human history would happen in the last 11 seconds of December 31st, and you reading this would be a fraction of a millisecond after midnight.

4. Looking up: a short history of how we’ve watched the sky

For most of human history, “watching the sky” meant looking up with your eyes. Telescopes are a 400-year-old invention sitting on top of about 4,000 years of careful naked-eye work, which sounds primitive but produced extraordinary results.

4.1. Naked eyes and ancient catalogues

Babylonian astronomers were tracking the planets, predicting eclipses, and keeping detailed records by around 1600 BC. Greek thinkers later worked out the size of the Earth (Eratosthenes got within a few percent of the right answer in the 3rd century BC, using shadows and geometry). Mayan astronomers built observatories aligned to Venus’s cycle. Polynesian navigators crossed thousands of kilometres of open ocean using only the stars.

What they all got wrong was the geometry. Earth was placed in the centre of the universe, and the planets, Sun, and stars were imagined as embedded in transparent spheres rotating around us. This wasn’t a stupid idea (it fits what your eyes show you), but it required ever more complicated patches as observations got better. By the 1500s the model had so many epicycles (small circles within bigger circles, used to fudge the planets’ apparent motion) that it was clearly broken. Nicolaus Copernicus proposed that the Sun, not the Earth, sat at the centre. Nobody had hard evidence yet. That came with Galileo.

4.2. Galileo points a tube at the sky

In 1609, Galileo Galilei pointed an early telescope (originally invented for spotting ships from a distance) at the night sky. Within a few months he had seen four small worlds orbiting Jupiter, which meant not everything in the sky orbits Earth. He saw that Venus has phases like the Moon, which only makes sense if Venus is orbiting the Sun. Mountains and craters showed up on the Moon, meaning it was a place, not a perfect celestial sphere. None of this was popular with the institutions that had staked their authority on the Earth-centred model. Galileo spent his last years under house arrest.

What he started was unstoppable. Once you had a telescope, the sky stopped being a flat backdrop and became three-dimensional. Within a century, Isaac Newton had written down the laws of motion and gravity that explained why the planets move the way they do, which retroactively made the Copernican picture obvious.

4.3. Hubble: the universe is much bigger than we thought

For a long time after Galileo, astronomy got more detailed but not fundamentally weirder. We catalogued stars. We discovered Uranus (1781) and Neptune (1846). We assumed our Milky Way galaxy was basically the entire universe, with some fuzzy clouds called “nebulae” scattered through it.

Then in 1924, the American astronomer Edwin Hubble looked at one of those fuzzy clouds, the Andromeda “nebula”, and measured how far away it was. The answer turned out to be very, very far. So far that it couldn’t be inside our galaxy at all. Andromeda was its own galaxy, and so were all the other “nebulae” of that type. The universe instantly went from one galaxy to billions of galaxies.

A few years later, Hubble noticed something stranger. The further away a galaxy was, the faster it was moving away from us. Not just a little faster, but in a clean, predictable pattern. The only natural explanation was that the universe itself was expanding. Rewind the expansion and everything ends up in one spot at one moment, which is where the Big Bang idea comes from. Hubble didn’t propose the Big Bang himself, but his observations are what forced the theory into the mainstream.

4.4. Telescopes in orbit

Earth’s atmosphere is great for keeping us alive and terrible for astronomy. It blurs starlight, blocks most of the infrared and ultraviolet parts of the spectrum, and lights up at night thanks to cities. So the next leap was putting telescopes above the atmosphere.

The Hubble Space Telescope (named after the same Edwin Hubble) launched in 1990. Its famous deep-field images, in which it stared at an apparently empty patch of sky for days and revealed thousands of distant galaxies, are why most people today know what a galaxy looks like.

The James Webb Space Telescope launched in 2021 and parked itself about a million miles from Earth, far enough out that the Earth’s own heat doesn’t interfere with it. Webb sees in infrared, the kind of light that comes from extremely distant (and therefore extremely old) galaxies whose light has been stretched out by the expansion of space on its long trip to us. Stretching light is called redshift, and it’s how we measure how far away the deepest galaxies are. Webb is currently showing us the earliest galaxies ever observed, some so far back in time that they shouldn’t, by our previous models, exist yet. The models are getting revised in real time.

5. Going up: leaving the planet for the first time

Until October 1957, no human-made object had ever left the atmosphere. By 1969, twelve years later, two humans had walked on the Moon. That kind of acceleration is the part worth remembering. Almost everything we have put in space, we have put there in living memory.

The Soviet Union got out first, with Sputnik, a beach-ball-sized satellite that did almost nothing except beep. The United States, alarmed at being second, poured astonishing amounts of money into NASA, and the two countries spent the next decade taking turns at firsts. First human in orbit (Yuri Gagarin, 1961, Soviet). First American in orbit (John Glenn, 1962). First spacewalk (Alexei Leonov, 1965, Soviet). First time anyone stood on another world (Neil Armstrong and Buzz Aldrin, 1969, American).

The Apollo program took twelve humans to the surface of the Moon and brought them all back. After that, the political pressure ran out, and humans haven’t been back to the Moon since 1972. We’ve been to low Earth orbit a lot, though. The International Space Station has been continuously crewed since November 2000, which means there has been at least one human off this planet for every single day of the 21st century so far.

Robots have done most of the actual exploring. The Voyager probes, launched in 1977, did flybys of Jupiter, Saturn, Uranus, and Neptune, sent back the first close-up photos of those worlds, and then kept going. Both probes are now in interstellar space, the region beyond the Sun’s influence, and they’re still talking to us, faintly. Mars has been visited by a long line of rovers since 1997, the most recent of which (Perseverance) is currently driving around the surface looking for chemical signatures of ancient life. There are landers on Mars, orbiters around every planet in the inner solar system, and probes either on, or buzzing past, comets, asteroids, and a few of the larger moons.

The most recent change is economics, not a new mission. Until the 2010s, getting a kilogram of anything into orbit cost tens of thousands of dollars. Reusable rockets, mostly from SpaceX, have cut that number by roughly a factor of ten, and it’s still falling. That’s why there are now thousands of small satellites in orbit instead of a few hundred, and why companies, universities and even high schools can afford to launch things. A professional astronaut on a government rocket isn’t the only space story anymore.

6. What we still don’t know

Here is the honest summary of where physics stands right now: about 95% of the universe is made of stuff we can’t see, can’t directly measure, and can’t yet explain. The 5% we do understand (everything we’ve covered so far, including every atom in every star and planet and person) is the small visible fraction.

The other 95% has two names. Astronomers call the invisible mass dark matter, because we know it’s there but it doesn’t give off light. We know it’s there because galaxies spin too fast. If the only thing holding a galaxy together were the gravity of its visible stars and gas, the outer parts would fly off. They don’t. Something invisible is providing the extra gravity. What that something is, we don’t yet know. There are candidate particles, but none has been detected directly.

The bigger mystery is Dark energy. It’s causing the expansion of the universe to speed up over time. You would expect gravity to slow expansion down, and for most of the universe’s history it did. Then, about 5 billion years ago, the expansion started accelerating instead. Dark energy is the placeholder name for whatever is pushing. It might be a property of empty space itself, or it might be something stranger. We have a number for how much of it there is (a lot, about 68% of the universe by energy) but no description of what it actually is.

A few more open questions, in no particular order. We don’t know what happened before the Big Bang, or whether “before” is a meaningful word given that time itself probably started then. We don’t know if the universe is finite or infinite. We don’t know how it ends, although the leading guess is heat death: the universe keeps expanding and cooling until nothing interesting can happen anywhere. Other candidates include the big rip (expansion accelerating so hard that it eventually tears galaxies, then stars, then atoms apart) and the big crunch (expansion eventually reversing into a collapse back to a single point). Current measurements favour heat death, but not by much. And we don’t know whether there is life anywhere else in the universe, though we now have telescopes capable of looking at the atmospheres of planets around other stars and checking for chemical signatures of life, which feels like the kind of question we might actually answer this century.

That’s a lot of unknowns. But the trend matters. Every century or so, the cosmos turns out to be about a thousand times bigger and a thousand times stranger than the previous generation thought it was. No obvious reason for that to stop now.