* Light travels at a finite speed: Light travels at approximately 299,792,458 meters per second (186,282 miles per second).
* Stars are distant: The stars we see are incredibly far away. Even the closest stars are light-years away (a light-year is the distance light travels in one year).
* The time it takes for light to travel: The light we see from a star left that star a certain amount of time ago, equal to the distance to the star divided by the speed of light.
Therefore, the age of the light from a star is the same as the time it took that light to reach Earth.
Examples:
* Proxima Centauri: The closest star to our sun is about 4.24 light-years away. The light we see from Proxima Centauri today left that star 4.24 years ago.
* Sirius: This bright star is about 8.6 light-years away. The light we see from Sirius today left that star 8.6 years ago.
* Betelgeuse: This red supergiant star is about 640 light-years away. The light we see from Betelgeuse today left that star 640 years ago.
Important Note: When we talk about the "age" of starlight, we are referring to the time the light took to travel to Earth. The star itself is likely much older than the light we are observing from it.
