Did you know that light is the fastest thing in the entire universe? It travels at an incredible speed of 300,000 kilometers per second. That's so fast it could circle the Earth 7 times in just one second!
Stars are so far away that their light takes a long time to reach us. When you look at a star in the sky, you don't see it as it is now, but as it was when its light began its journey to Earth.
Every night, you can take little trips back in time just by looking at the sky:
Astronomers use powerful telescopes like Hubble or James Webb to look further into the universe. And the further we look, the further back in time we see! Some galaxies we observe today may no longer exist.
When you observe some very distant stars, their light began its journey to Earth when dinosaurs roamed our planet. This light has traveled for millions of years through space to finally reach your eyes tonight!
The speed of light is represented by the letter c. It's a symbol scientists use to talk about this incredible speed. Nothing can go faster than light. We simply write this as: \(v < c\).
N.B.:
A light-year is a unit of distance, not time! It's the distance light travels in one year, about 9,460 billion kilometers. For example, if a star is 100 light-years away, it means its light takes 100 years to reach us.
| Celestial Object | Distance | Light Travel Time | Era on Earth When Light Left |
|---|---|---|---|
| Moon | 384,400 km | 1.3 seconds | 1 second ago |
| Sun | 150 million km | 8 minutes | 8 minutes ago |
| Sirius | 8.6 light-years | 8.6 years | 8.6 years ago |
| Andromeda Galaxy | 2.5 million light-years | 2.5 million years | About 2.5 million years ago, before modern humans |
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