# 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?

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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

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.

for close objects you can use parallax, which is the same way you get depth perception from binocular vision. Only with stars, you take images about six months apart, so the distance between your “eyes” is about 2AU. Basically, how much do the foreground stars move in relation to the background stars.

For more distant objects you can either estimate based on how fast the object is moving away from us(redshift), or you can calculate based on how how much light reaches us vs how much light an object of that type is supposed to make(standard candle).

Once you know how far away something is, you know how long light took to get from there to here, because the speed of light is known.

As the universe expands, light expands with it, redshifting the light. That’s the essence of Hubble’s Law. The older the light, the more its waves will be expanded (and its frequencies redshifted) by its journey through the expanding universe. So astronomers can look at a galaxy’s spectrum, find the emission and absorption peaks of different elements, see how much they’ve shifted, and then apply Hubble’s Law to that shift to figure out how far away that galaxy is.

For close objects, light “age” is known from a distance measurement, as Charyou-Tree said.

To open another line of thinking on the question, have the photons in the light been travelling at, well, the speed of light and so, to them, are they aging for 10 years or 1009 years or are they still in the same instant?

To add to the methods mentioned:

They also use a certain type of stars called Cepheids. These are stars that pulsate perdiodically. Lucky for us, the period of the star and its absolute brightness are linearly connected. So by measuring the light we can get the period, from which we can derive the absolute brightness (that is the brightness of the star when it would be at 10 parsec away from us.) As we know how light diminishes with distance, we can calculate the distance if we know the measured and the absolute brightness.

This follows the following formula:

Measured brightness-absoluye brightness=5*log(d)-5

A next method is using a nearby supernova/ the supernova of the star. Of type Ia to be exact. Because supernovae of that type always emit practically the same amount of light, we again know the absolute brightness. As these things are WAY brighter than regular stars, we can use them across greater distances. (You can look up the ranges of all of the mentioned methods in this thread, I can’t recall them correctly)

Just want to say I have the same question and none of you have answered this from a perspective anyone can understand, probably because you don’t understand it well enough either

Different materials give off different light patterns and colors. So if you analyse light, and you can tell from the pattern that the star is 90% hydrogen, and 10% helium, then you know what color the light should be.

Then you can see how far that light has been red-shifteted or blue-shifted, and you know how long its been traveling because space expansion is happening at an constant acceleration.

So since you know how long its been travling, you mulitply by the speed of light and you have distance.