How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

Question

How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

In: Physics

Answer 1

They’re not measuring age directly, they’re measuring distance. The age is mostly incidental, and is brought up in pop-science articles because people really like the geewiz factor of “The light from this star is [x] years old!” Its really only relevant for distant galaxies that are so old that the conditions of the universe were significantly different, several billion years or more. For instance, our neighboring galaxy Andromeda is ~2.5 million light-years away. That sounds far, but 2.5 million years is practically “instant” on astrophysical time scales.