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Images only 2D need spectra for 3D
The previous picture pointed out the problem of perspective in astronomy. Quite often we cannot tell how far away an astronomical object is, and so when we see two apparently related objects such as these two galaxies we cannot be sure as to whether they truly are related or are just chance alignments. The small galaxy to the right of this barred spiral galaxy was once thought to be a companion or satellite galaxy which orbits around the spiral galaxy. However a spectral observation of the companion galaxy revealed that its features were strongly redshifted indicating that the galaxy was moving away from us at a speed much greater than would be expected for a galaxy in a stable orbit. So either this galaxy has been somehow flung out of the spiral galaxy, or it is quite unrelated, and is in fact a distant quasar at the edge of universe explaining its fast speed as just being the expansion speed of the universe at that distance.