How old is the starlight you see tonight?

Light is the fastest thing there is. It covers 299,792.458 kilometres every second. Even so, crossing something as huge as the universe takes it time. Right now, sunlight takes 8m 19s to reach you, so you actually see the Sun as it was a few minutes ago.

The stars are much farther away, so astronomers measure their distances in light-years. A light-year is not a time but a distance: how far light travels in one year, 9.5 trillion kilometres. An airliner flying non-stop at cruising speed would take about 1.2 million years to cover it.

That makes every star distance a clock as well. A star ten light-years away is seen by light that left it ten years ago. So when you look up, you never see the stars as they are now. You see each one as it was when its light set out, and every star shows you a different moment. The night sky is not one picture of the present. It is many pictures of the past, taken at different times and arriving together.

The light from the bright stars is anything from a few years to thousands of years old. Procyon, the little dog's star near Orion, sends light 11.5 years old. Vega's light is 25.0 years old.

Go further out and the light gets older fast. Spica's light left it 250 years ago. Long before anyone who is alive today was even born. Antares, the red heart of the Scorpion, shines with light about 550 years old. Rigel, at Orion's foot, shines with light about 860 years old.

The farther the star, the less sure the number. Close stars are measured precisely, by how they seem to shift as Earth goes round the Sun. Far stars shift too little, so astronomers lean on other methods, and these don't always agree. For Deneb, the brightest star of the Swan, the light is between 1,240 and 2,830 years old. That spread is not carelessness. It is how much is still unknown about one of the most luminous stars you can see.

Somewhere in the sky is a star whose light set out the year you were born, and it changes as you get older. A person in their twenties may look at Fomalhaut, whose light is 25.1 years old. In their thirties, it could be Pollux, at 33.8 years.

A baby born in 2024 could expect to live about 73 years, on the world average. Some stars you can see have light older than that. Regulus, in the Lion, sends light 79 years old. From the southern sky, Gacrux, at the top of the Southern Cross, sends light 88.6 years old. Look at them and you see light that set out more than a lifetime ago.

Seen that way, a clear night holds a lifetime of moments. Each star you can name is a different year of the past, all arriving at once.

It is a common idea that many of the stars you see are long dead. But for almost all of them this is not true. The light of a bright star left it years or centuries ago, but stars live for millions to trillions of years. Even the shortest-lived bright stars almost certainly still shine.

The best-known exception might be Betelgeuse, the red giant at Orion's shoulder. It will one day explode as a supernova. NASA expects it to happen about 100,000 years from now. Some astronomers argue it could come within a few centuries, and others dispute their reasoning; the range runs between 300 and 100,000 years. Its light is between 440 and 880 years old. So on the shortest estimate, Betelgeuse may already have exploded, with the news still on its way, carried by light as the messenger. On the longest, it will shine for ages yet. Nobody knows which.

Even a dead star can keep shining. When a star like the Sun runs out of fuel, it leaves a white dwarf, a core about the size of Earth that cools slowly over billions of years.

Deneb's light set out within recorded history, which goes back about 5,200 years. To see light older than any city, look beyond our galaxy.

From the northern hemisphere, on a dark autumn night, you can find a faint smudge near Cassiopeia. It is the Andromeda Galaxy, a whole galaxy of stars too far away to see one by one, and its light set out 2.5 million years ago. NASA puts it this way: the light you see left when our earliest ancestors figured out stone tools.

Our own species, Homo sapiens, is about 320,000 years old. So when you look at Andromeda, you see light that was already most of the way here before there were any people like us to see it.

Since you opened this page, the light now on its way from Vega has come 0 kilometres closer.

Most of the time we look at the sky as if it were a picture of now. It is closer to a letter from the past, written at many different times and delivered all at once, to everyone who looks up. The person next to you sees the exact same old light, right now.

Sometimes the letter brings news. In 1054, astronomers in China saw a new star blaze up, bright enough to see by day for nearly a month. It was a star exploding, thousands of years earlier, and its remains are the Crab Nebula. The light of that ending reached them on its own long schedule, as all starlight does.