How did Robert Trumpler use open clusters to show that space absorbs light?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
In 1930, the Swiss-born U.S. astronomer Robert J. Trumpler utilized his extensive studies of open clusters to demonstrate that space is not perfectly transparent.12
By measuring the angular sizes and distributions of these clusters, Trumpler observed that more distant clusters appeared fainter than would be expected based on their size and distance. He concluded that this decrease in apparent brightness was caused by a tenuous haze of interstellar material—composed of gas and dust—distributed throughout the galactic plane. This material absorbs and scatters light as it travels through space, a phenomenon that results in the reddening of stellar spectra and provides fundamental evidence for the existence of the interstellar medium.123