How do scientists know how long galaxies are?
In: Mathematics
That’s the funny thing. They don’t really know. It’s mostly estimated. We can make an educated guess using how close stars are able to be to eachother without destroying one another and then just take that measurement on a larger scale.
We can measure the distance to a galaxy via redshift, and use some trigonometry and angle measurements to predict their size. We can also use their brightness to estimate their size. Lastly, we can use the speed of their spin to predict their size.
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