Question

What are the observations which cause astronomers to conclude that galaxies change over time.

Answer 1

astronomers continuosly study the the patterns of different galaxies (including our milky way galaxy) to study the changes in them. now in order to study the evolution of the galaxy, astronomers generally study the star composition within them at their differen locations. now a star has a fixed evolution scheme, from a main sequence star to a red giant to a supernova explosion to a white dwarf or neutron star ( refer to the H-R diagram of the stars to learn this in details).

Understanding the changes in the lifetime of a main sequence star is a simple matter of nuclear physics, where we can calibrate the turn-off points for various star clusters to give their ages.
This, then, provides a tool to understand how our Galaxy formed, by mapping the positions and characteristics of star clusters with known ages.
When this is done it is found that old clusters form a halo (center) around our Galaxy, young clusters are found in the arms of our spiral galaxy near regions of gas and dust.

therefore it can be concluded that with time the galaxies become more compact and spherical from an initial disk-shaped galaxy. Thus they keep on changing continuosly.