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In some photographs of galaxy clusters, why are the arcs of a lensed galaxy blue?

Stephen Peterson, Hurricane, Utah
Gravitational lensing Abell 2218
The extreme mass within galaxy cluster Abell 2218 bends the light of a more distant galaxy.
NASA/ESA/A. Fruchter and the ERO Team (STScI/ST-ECF)

Galaxy clusters contain hundreds of galaxies, a lot of hot gas, and even more invisible dark matter. A cluster’s extreme mass warps space-time, and light from more distant objects follows those bends and lands on scientists’ detectors.

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