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How can astronomers tell the difference between a group or cluster of stars and an actual galaxy?

Jessica Norris, Mooresville, Indiana
NGC-4163
NASA/ESA/K . McQuinn (University of Minnesota, Minneapolis
Most telling is that, aside from a few local ones, galaxies exhibit large shifts toward the red end of their light spectra, which reveal that they are receding from us at high speeds due to cosmic expansion, whereas clusters in the Milky Way do not.

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