how do we tell how old a galaxy is?
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At the scale of galaxies, distance and time are tied together. The further away something is, the older it is. It’s why we use the light year measurement for distance even though it’s based on time. Because of this, we can use measurements for distance to figure out age as well.
There are multiple ways to measure distance/time to a galaxy. Astronomers use as many as possible to get a good idea of how old a galaxy is because each of the methods could be wrong or difficult to use in certain scenarios. Some don’t work beyond a certain distance away. A non-exhaustive list of how we tell distance:
One is parallax. Basically, you look at a distant object when the earth is on one side of the sun then take a look at it again when the earth is on the other side. This lets you see the object from 2 different angles and can use geometry to figure out how far away it is. This doesn’t work for really distant objects because it gets harder to measure the difference.
Another is spectroscopy. Basically, taking light from a galaxy and sending it through a prism. Elements will absorb specific wavelengths of light and will leave a fingerprint. Hydrogen for example absorbs specific wavelengths of light. Astronomers take the light from a distant galaxy and figure out where the hydrogen lines are. They tend to be red shifted due to the expansion of the universe. Once they know how much it has redshifted, they can figure out how far away the galaxy was when it emitted the light. This is why the JWST is so important. It’s sensitive enough to pick up the really low energy from the oldest galaxies, something we couldn’t do in the past.
Another is brightness of specific supernova. Some events are very predictable in terms of how bright they are. White dwarves going supernova as they absorb gas from a partner star consistently explode at the same mass so they generate the same amount of light. They can get a decent idea of how far away a galaxy is if they detect such a supernova and measure its brightness.
Another is by observing cepheid variable stars. Like the specific supernova above, they have predictable properties. Like how bright they are and how fast they pulsate. An astronomer can see such stars pulsating at a certain frequency and know how bright it should be. How dim the light is gives us a good idea of how far away it is.
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