Definition
A meteor shower happens when Earth passes through a cloud of dust left behind by a comet or asteroid, and countless bits of space debris slam into our atmosphere at once and glow. It looks like shooting stars pouring out of one spot in the night sky, like rain.
The Trail of Space Dust a Comet Leaves Behind
Ever driven down a highway and passed through a cloud of dust kicked up by the car ahead? Or through a pile of fallen leaves? Something very similar happens in space. A comet circles the Sun in a long oval orbit. It's like a giant dirty snowball made of ice and rocky dust. When it gets close to the Sun, the heat turns its ice straight into gas. As the gas escapes, the comet sheds a band of sand-sized crumbs along its orbit.
Long after the comet is gone, that band of crumbs stays put and keeps drifting along the same path. Earth circles the Sun once a year, so on about the same dates every year, it plows right through this dust cloud. And it does so at a mind-blowing speed of about 100,000 kilometers per hour (about 62,000 mph).
As Earth cuts through the dust, tons of tiny particles smash into the air. That is the real secret behind a meteor shower, and it's why we get them in the same season every year.
How Does a Tiny Grain of Sand Make Such a Bright Light?
When you see a blazing streak across the night sky, it feels like a huge boulder is falling. But most of the particles behind a meteor shower are only the size of a grain of sand or a tiny pea. The secret to how something so small can look so bright is its speed.
When a space particle hits Earth's atmosphere at dozens of kilometers per second, air molecules high up pound its surface with incredible force. In an instant, the particle heats up to thousands of degrees and vaporizes. The atoms that break off, along with the air around them, get energized and glow. With a bigger chunk, the air piling up in front creates a shock wave that adds even more heat.
So a giant star isn't falling from the sky. It's a speck too small to see, burning up in one last dazzling flash because of a violent crash with the atmosphere.
A Bit More Precisely: Why Do They Seem to Come From One Point?
To be more precise, the meteors in a shower don't actually burst outward from one spot in the sky like fireworks. The space dust is really coming in along parallel paths, all heading the same direction.
So why do the streaks look like they spread out from a single point? It's the same optical illusion of perspective you see when you look down a long stretch of railroad track. The two parallel rails seem to meet at one point far away.
The spot in the sky where the meteors seem to start is called the radiant. Names like the Perseids or the Geminids come from the constellation sitting behind the radiant in the background. They have nothing to do with those constellations themselves.
Common misconceptions
A meteor shower is giant stars in space dying and falling to Earth.
No real star falls. A meteor shower is made of grain-of-sand or pea-sized bits of dust left by a comet (or, rarely, an asteroid) burning up in Earth's atmosphere.
Where you meet it
The Perseids, the classic summer meteor shower, light up the sky around August 12 every year.
The Geminids are a famous show on cold nights in mid-December. They're a rare case, because they come from debris left by an asteroid called Phaethon, not a comet.
In one sentence
A meteor shower happens when Earth passes through the dust trail left by a comet (or rarely an asteroid), and tiny grains of sand hit the atmosphere and burn up in dazzling streaks of light.