How to distinguish between distant and close galaxies?

Increasing the visible size keeping the same amount of light emitted

When we look at a distant source of light that spreads in all directions, we see only a portion of the photons emitted by it. As the distance from the source growth, it looks less and less bright since we see less and less of the emitted photons, and its size is usually seems smaller. Galaxies at a higher distance look dimmer, but they do not seem to be very small. The reason is that the light emitted by their stars, went to us for so long that during this time space of the universe has managed to expand several times. We judge of the size of a galaxy by the distance between the photons emitted from the left and right edges of it. That distance is also increased several times, creating the illusion of a size larger than would be seen in a static space.

Distant galaxies look larger

A visually large galaxy does not mean a close galaxy.


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