Eli5: how do astronomers know the size of objects in space?

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For example, if something would be a few light years wide, would that not mean we would have to use a thing to map it out for that amount of light years in years to get the actual map? Or do they use different methods, like scaling things close to it?

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They are very educated guesses. The mass can be well calculated for anything with a moon or planet orbiting it thanks to Kepler’s equations.

The size and density are estimated based on mass and luminosity (how bright it is). For some things like stars that resemble our sun, this is a good guess. For other things, it is an educated guess.

If you can measure the angular width of an object, and its distance, you can calculate its width in light-years or whatever.

I know one method to measure the size of stars in binary star system (two stars circling each other). So when stars moving around each other, periodically a star covers the other one and to us both look dimmer at that moment (like solar eclipse). Based on how long the eclipse lasts, their size can be estimated.

For other objects that are large enough, with observation from big telescopes we can derive their apparent size if we know distance from us to them.

But all these are oversimplified. For each object is a different method needed depended on what data we can get from it.

They can also use the relative movement from different parts of the Earth’s orbit to estimate distance. See [Parsec](https://en.wikipedia.org/wiki/Parsec)

There are several methods that build on each other. It’s called the “cosmis distance ladder”.

Let’s say we already know the Earth-Sun-distance. Then we can measure the distance to nearby stars using the parallax method: We take a picture of the sky, wait six months, take another picture, and the nearby stars will appear to have shifted vs. the background stars. We can measure the distance to theses stars that way, but this only works for a couple hundred lightyears.

Some stars are called “variables”, because they cyclically become brighter and dimmer. With the information from the last step, we found out that there is a direct relation between such a stars cycle lenght and its absolute brightness. Now knowing this, we can judge the distance to any of these stars, no matter how far away they are, as long as we can see them. If that star is a member of, for example, a star cluster or a nearby galaxy, we now know the distance to that cluster/galaxy.

Another step is the redshift. Using earlier methods, scientists realizet that, on the really large scale, the further a galaxy is away from us, the more its light frequencies are shifted to the “red” end of the spectrum. This allows us to judge the distance of really faraway things, like galaxies close to the other end of the visible universe.

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There are even more methods. Point is, any method in the ladder only works because it is based on the information we got from one or more methods furthed down on the ladder.