The Universe often fools our eyes, offering the false impression that celestial objects are located at the same distance from Earth.
A prime example of this phenomenon can be seen in the spectacular image below captured by NASA's Hubble Space Telescope, where the galaxies NGC 5011B and NGC 5011C are depicted against a starry background.
Located in the constellation of Centaurus, the nature of these galaxies has puzzled astronomers for years.
Indeed, NGC 5011B (on the right) is a spiral galaxy belonging to the Centaurus Cluster of galaxies lying 156 million light-years away from the Earth. Long considered part of the faraway cluster of galaxies as well, NGC 5011C (the bluish galaxy at the center of the image) is a peculiar object, with the faintness typical of a nearby dwarf galaxy, and the size of an early-type spiral.
Astronomers were curious about the appearance of NGC 5011C. If the two galaxies were at approximately the same distance from Earth, they would expect the duo to show signs of interactions between them. However, there was no visual sign of interaction between the two.
To solve the riddle, astronomers studied the velocity at which these galaxies are receding from the Milky Way and determined that NGC 5011C is actually moving away far more slowly than its apparent neighbor. To be sure, its motion is more consistent with that of the nearby Centaurus A group at a distance of 13 million light-years.
Thus, NGC 5011C, with only about ten million times the mass of the sun in its stars, must indeed be a nearby dwarf galaxy rather than a member of the distant Centaurus Cluster as was believed for many years. Problem solved!