You can make out pixels on your screen when it is close because your screen is just made up of a lot of pixels. Similarly, galaxies are just a bunch of stars clumped together. Now a galaxy is not a phone screen, but it does behave in a similar way. When galaxies are close to us, we see bigger bumps coming from that galaxy due to how the galaxies stars are organized. Just like a screen, when galaxies are farther away, all those stars blend together, and the galaxy will look really smooth, similar to the way the pixels on a screen blend together when you sit back from it. When we know the size of the bumps, because of the way stars are organized in a galaxy, it helps astronomers figure out how many stars that galaxy has.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00142