At the beginning, the entire universe fit into a point smaller than an atom: infinitely dense and hot. What we call the Big Bang was not an explosion in space; it was the expansion of space itself, still going on today.
To explain that start, physicists propose a dizzying idea: cosmic inflation. In a fraction of a second almost impossible to picture, the universe grew by an astronomical factor, faster than light. That burst of expansion stretched tiny quantum fluctuations into the seeds of the future galaxies.
In the first fraction of a second the universe expanded and cooled enough to form the first particles. Within its first three minutes the lightest nuclei were forged: hydrogen and helium, the building blocks of everything else.
Three hundred and eighty thousand years later, everything had cooled enough for light to travel freely for the first time. That first glow still surrounds us: we call it the cosmic microwave background, and it is the oldest photograph we have of the universe.
Hydrogen and helium are not enough to build a planet, let alone a person. Carbon, oxygen, iron — every heavy element — was cooked inside stars.
Over millions of years a star fuses hydrogen into helium, then helium into carbon, climbing the periodic table. When a massive star dies, it explodes as a supernova and scatters those elements across the cosmos. The even heavier atoms — gold, platinum — are born in still more violent events: collisions of neutron stars.
A star is, at heart, a fusion reactor held in balance: gravity squeezes it inward while the energy it releases pushes outward. That pulse can last billions of years in a star like the Sun, or just a few in the most massive ones, which live fast and die young in one last dazzling blast.
Those stellar ashes gather into clouds of gas and dust that, given time and gravity, form new stars and planets. The Sun and the Earth formed this way some 4.6 billion years ago, out of material recycled from generations of earlier stars.
Before the Earth there was a spinning disk of gas and dust around the young Sun. Grains collided and stuck together into rocks, then planetesimals and, finally, planets. Ours ended up at just the right distance for water to stay liquid: neither so close that it boiled away, nor so far that it froze.
That is why Carl Sagan's line is not poetry but physics: we are made of star stuff. The calcium in your bones, the iron in your blood, the oxygen you breathe — each was forged in the heart of a star that died long before the Sun existed.
Knowing where we come from changes the scale of things. We are not spectators of the universe: we are one of the ways it found to look at itself.
Every time a telescope catches the light of a distant galaxy, it is reading a postcard from the past — because that light took millions of years to arrive. To look up at the sky is, literally, to look at our origin.
It also changes how we measure time. 13.8 billion years are so vast that the whole of human history — from the first cave paintings to this very text — would take up less than a blink on that scale. And yet, in that blink, matter learned to ask about its own origin.
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